level measurement

Level Measurement D I C E T

LEVEL MEASUREMENTS


Level is measured at the position of the interface between phases, where the phases are liquid/gas, solid/gas, or immiscible liquid/liquid.

This measurement is often converted to a volumetric or gravimetric quantity.

Level is a vertical measurement taken from the surface, or interface, to a fixed reference point. Normally, the reference point is the bottom of the vessel holding the substance.

Principle of Level


Determining the HEIGHT and/or VOLUME of liquid (or solid) in a tank (or container)

Units of Level:

1- as DISTANCE - inches, meters, etc 2- as % of level span (40%, 90%, etc)

Principle of Level


Level may be measured directly, by defining the position of the interface; or indirectly, by measuring another quantity, such as volume, and inferring the level measurement by converting that quantity to a level measurement.

Direct and Indirect Level Measurement


Direct methods employ physical properties such as fluid motion and buoyancy, as well as optical, thermal, and electrical properties.

The direct measurement of level is possible because of the relative simplicity of this variable compared with other variables.

Direct level measurement does not require compensation for changes in level caused by changes in temperature. Although liquids and gases expand or contract in response to temperature change, direct level measurements show the actual level of the interface.

Sight Glass

Direct Level Measurement


• by lowering a measuring stick into the water and read the level of the water directly on the stick; by mounting the stick permanently in the tank, you can have the reading without handling it

• by use of sight glass

• by use of electrodes that sense presence of the liquid

• by use of float and pointer mechanism

• by use of sonic, ultrasonic, radar, and microwave detection method

• by use of nuclear radiation method



Direct level measurement using float gage

Tape gauge using float and gauge board.



Indirect level measurement involves converting measurements of some other quantity, such as pressure to level.

By determining how much pressure is exerted over a given area at a specific measuring point, the height of the substance above that measuring point can also be determined.

Pressure Gage

Indirect Level Measurement


• by measuring static pressure of liquid at the tank bottom (atmospheric tanks)

• by measuring the differential pressure between the top and bottom of pressurized tank

• by measuring the weight of the tank

• by use of air bubbler mechanism



Temperature can also affect the accuracy of indirect level measurement. Substances have a tendency to expand when heated and contract when cooled. Gases are greatly affected by changes in temperature, while solids are affected very little. Liquids, on the other hand, are affected somewhat more than solids but less than gases. A change in temperature causes a change in density and this affects the amount of force exerted on a pressure sensor.

Because indirect level measurement is sensitive to specific gravity and the effects of temperature, it is necessary to compensate for these factors to ensure accurate measurement.



Specific gravity of commonly used liquids

Material Specific gravity

Kerosene (41 API)Diesel fuel (No. 2d)TurpentineSesame oilLinseed oilSoy bean oilPhenol (Carbolic Acid)WaterHydrochloric acid (31.5%)Ethylene glycolGlycerine (100%)GlucoseFreonMolasses “A”Corn syrupSulfuric Acid (100%)

.82

.82 - .95

.86 - .87

.923

.925 - .936

.927

.95 – 1.081.001.05 @ 68oF1.1251.26 @ 68oF1.35 – 1.441.37 – 1.49 @ 70oF1.40 – 1.461.40 – 1.471.83

Water levels at two temperatures



Point Level Detection - interest is only at a certain level or depth.

Continuous Level Detection – interest is over a range of levels or depth, typically 0 to 100% of an operating range.

Point & Continuous Level Measurement


This is required when it is necessary to know at all times the exact position of the interface in relation to one or more specific reference points.

Continuous Level Measurement


Certain processes require only that the level of a substance be maintained between two points. Frequently these two points are a high level and a low level. When this is required, a point-to-point level measurement system is used. Such a system activates control devices only when predetermined levels are reached.

Surge tank

Point-to-Point Level Measurement


Schematic diagrams of level instrument installations. (a) High-low level detectors driving lamps and an alarm. (b) High-low detectors operating an inlet valve. (c) Full control of two inlet pumps using a continuous detector readout over a full range of capacity, one or more settable high-low trigger positions, and appropriate lamps and alarms.



In single point measurement, level is detected at a predetermined point only.

Typical applications: sounding alarms lighting lamps & indicators prevent overflow, etc.

Single Point Level Measurement


The level range Fluid characteristics: (a) temperature; (b)

pressure; (c) specific gravity; (d) whether the fluid is clean or dirty, contains vapors or solids, etc.

Corrosive effects Whether the fluid has a tendency to “coat”

vessel walls or the measuring device Whether the fluid is turbulent around the

measurement area

General Considerations


IMPORTANCE OF LEVEL MEASUREMENT IN INDUSTRIAL PLANTS

Saves materials and energy and minimizing waste (economy aspect)

Optimizes the production process and ensures consistent product quality (efficiency aspect)

Avoids abnormally high or low material accumulation which could create a hazardous situation (safety aspect)

Principle of Level


USES

Inventory and accounting of product storage

Inventory and distribution of raw material for processes

Proper operation of various unit processes, such as,

Fractionating towersDistillation columnsSteam production (boilers)Naptha hydrotreater unit (NHTU)Process reactors

Principle of Level


In many processes involving liquids contained in vessels, such as distillation columns, reboilers, evaporators, crystallizers, and mixing tanks, the particular level of liquid in each vessel can be of great importance in process operation. A level, which is too high, for example, may upset reaction equilibria, cause damage to equipment, or result in spillage of valuable material. A level that is too low may have equally bad consequences. Combined with such basic considerations, there is the advantage in continuous processing of reducing storage: capacity throughout the process. This reduces the initial cost of equipment, but less storage also accentuates the need for accurate and sensitive level control. Effective measurement and control of level can usually be justified in terms of economy and/or safety. To the operator, knowledge of this variable provides data on the (1) quantity of raw material available for processing, (2) available storage capacity for products being manufactured, and (3) satisfactory or unsatisfactory operation of the process. In the following slides, a few examples are given.

Importance of Level Control in Process Operation


Efficient Operation of Equipment

Control at One Height

In many process applications, the level must be maintained accurately at a predetermined height, irrespective of load conditions on the process. Several examples will serve to illustrate this point. In a steam or vapor generator, such as a boiler, it is desired to maintain the level at a predetermined value in order that two sets of conditions will be present at all times regardless of the output from the generator. The first conditions will be present at all times regardless of the output from the generator. The first condition requires that the quantity of liquid inventory in the vessel be maintained in order to provide feed for the evaporation process. The second condition requires that a vapor volume space be maintained in order to have available storage capacity for the vapor, plus a volume space which will prevent carryover of entrained liquids in the vapor.




Control at One Height

In continuous processes, a correct level head in certain equipment is of considerable importance. In evaporators, for example, the heating medium may be inside a tube bundle which must at all times be covered to an optimum depth, thereby requiring precise level control. Too Iow a level will uncover the heating surface, lowering the efficiency of the process. Too high a level will require a greater heat input as the head pressure increases, which may result in damage to throughput material or unsatisfactory evaporation.




Proportional (Wideband) and Averaging Level Control

In continuous processes, accumulators or storage vessels are introduced between various stages of the process in order to provide storage (inventory). Process upsets or disturbances are absorbed in such accumulators, and only a minimum of them are~ passed on to the next phase of the process. In such process applications, control of the level at a constant height is not always desirable. It is more important that the outflow of the storage vessel does not change suddenly and cause an upset in the subsequent process stage. Any sudden increase in input to the storage vessel should be absorbed in the vessel. To accomplish this, "averaging" liquid-level control is used, wherein a wideband proportional-plus-reset mode of control is incorporated in the level control instrument.





With this type of control, if the uncontrolled input suddenly increases, the wide proportional band permits the level to rise temporarily, with little change in the controller output-regulating outflow from the vessel. If the input remains at its higher value for a period of time, the automatic reset functions to return the level to the set point, and gradually changes the outflow rate. Conversely, if the uncontrolled input suddenly decreases, the level is permitted to drop; the outflow is not similarly affected but is changed at a gradual rate if the decreased input continues.





The size of the vessel required becomes a function of the (1) maximum process upset to be expected in the system and to be absorbed by the accumulator, (2) duration of the upset, and (3) elapsed time allowed before the full value of the continued upset is to be passed along to the next stage of the process. The term "holding time" is often used to describe the function just explained. The size of the vessel also may be limited by physical height considerations, by the change in level height allowable, and by the economics of vessel cost. All these factors become variables for design and instrument engineers to evaluate in determining the size of vessels and accumulators. The actual performance of the liquid-level controller in smoothing out minor upsets and in absorbing them in the accumulator can be further adjusted by proportional-band settings and reset rate adjustments.




Level Control Permits Smaller Vessels

In simple single-capacity systems, the utilization of level measurement and control can be advantageously applied to keep the capacity of processing vessels within practical limits. If level measurement and control are used, the size of a mixing or reaction vessel may be small. It is not necessary that large vessels be used to handle all available liquid to be processed, since the liquid-level control device will feed only the fluid required to keep the liquid concentration or its height at a predetermined value. Large or bulky vessels thus can be eliminated with accompanying economy. This also means that a small amount of process material is under reaction or in process, thereby reducing attendant hazards, potential losses, or spoilage.




Protection of Centrifugal Pumps

Where it is desired to maintain a head pressure against the suction of a centrifugal pump, the level of the liquid in the storage tank must be maintained at an optimum value. If the level drops too low, flashing and cavitation will occur in the pump suction, with resultant erratic pump discharge and extreme wear on pump impellers. If the level rises too high, there may be a loss of accumulator volume in the vessel, thereby affecting the process from an operating viewpoint.



Product Quality Control

Warp sizing in the textile industry is a good example of how close control of the liquid level directly affects product quality. The warp yarn is run through a bath of sizing solution which adds a protective coating to the yarn. The amount of size absorbed by the yarn is a function of the time during which the yarn is in contact with the size solution. As the yarn passes through the size solution in a prescribed path (usually around a large cylinder rotating at a fixed number of revolutions per minute), the time of contact is a function of the level height of the size solution. A variation in solution level, therefore, will change this contact time and thus destroy warp uniformity, later causing breakage of threads on the loom.



Cost Accounting

The flowmeter, weighing scale, and liquid-level gage are the process cost accountant's principal tools for obtaining facts concerning quantities of liquid raw materials and finished products in storage, and of liquids in process. Thousands of liquid-level meters of all types, from the simplest gage stick, often still used for taking inventories at tank farms, to the most sophisticated remote-level indicating gages are in daily use, principally for cost accounting needs.



Constant head for steady Process flow

Often steady process flows, such as the introduction of raw material to a process are maintained by holding a constant head pressure on the feed line. This can be achieved by control of the liquid level in the feed tank whose feed line exist from the bottom of the tank.

Basic flow versus head factor

Level Measurement System Mathematics


gHCAQ 2

where:

Q= quantity ft3/sC = orifice constantA = area of flowg = acceleration of gravity 32.2 ft/sec2

H = height of the liquid, ft.



Relationship of flow to level in Process vessel

In designing process equipment and determining the requirements of liquid level controlling equipment, an engineer must often calculate in advance how a change in liquid inflow will affect the level in the vessel. In any given vessel, a stable quantity of inflow to the vessel equals the outflow. The relationship is given by:



gHCAQQ ooi 2987.0where:

Qi = flow rate into the vessel gal./minQo = flow rate out of the vessel gal/minC = orifice constant or coefficientAo = Orifice area, in2

G = acceleration of gravity, 32.2 ft/s2

H = height of liquid level, in

Variation in the in flow or in the orifice area, with the other variables remaining constant, causes a level change. A control valve substituted for the fixed orifice provides an easy means of varying the orifice area. If the orifice area is concerned, the level will fall until, at the new area, the level stabilizes at a point where its head effect on the orifice causes outflow to equal inflow.



Fluid pressure to level relationship

By determining how much pressure is exerted over a given area at a specific measuring point , the height of the substance above the measuring point can also be determined.

433.0

Ph ( for H2O)

G

Ph

433.0 (for fluids other than H2O)

h = height of the fluidP =pressure indicated on a gage0.433 psi = pressure exerted by one square inch of water, one foot high.G = specific gravity



Capacity versus Level Height in Various Vessels

Many processing and storage vessels where liquid-level measurement is a factor are cylindrically shaped and mounted vertically on end. Thus, the content for any level height can be simply calculated by



Capacity versus Level Height in Various Vessels

Spherical Tanks

Useful relationships for determining the volume of a sector of a sphere are given below:



Volume and Weight Measurement from Level

Liquid-level measurement resolves itself into position measurement, namely, the position (height) of a liquid surface above a datum line. Measurement, however, need not always be expressed in terms of inches, feet, or meters above the datum line but, with a knowledge of the dimensional and contour characteristics of the containing vessel, can be conveniently interpreted (hence calibrated) in terms of the volume of liquid contained-and further, with information concerning the specific gravity of the liquid, can be expressed in terms of the weight of the liquid in the vessel.



Volume Determinations

If the purpose of level measurement is to determine the volume of liquid contained in the vessel, then direct measurement of level height is preferable because

V = A x H



where V = volume of vessel

H = height of level

A = area of vessel


Volume Determinations

Thus, the volume measured is independent of liquid density. If measurement of the pressure due to hydrostatic head must be used because of specific requirements, the relation determines volume

In this case, the volume measurement depends on the density of the liquid.



A X P

V = ----------

D

where V = volume in vessel at given level

D = density of liquid in vessel

P = pressure due to hydrostatic head


Weight Determination

If the purpose of level measurement is to determine the weight of the liquid contained in the vessel, then measurement of the pressure due to hydrostatic head has advantages because



W = H X D X A = A X P

where W = weight of liquid in vessel

H = height of level

D = density of liquid

A = area of vessel

P = pressure due to hydrostatic head


Weight Determination

Thus, the measurement is independent of liquid density. If direct measurement of level height is used, then

And the weight measurement depends on knowing the density of the liquid.



W = A X D X H


Errors in Measurement of Quantities in Storage Tanks

Where highly accurate measurements of the quantities of liquids stored in tanks are required, it is the usual practice to correct the fluid density for temperature changes and neglect the other temperature and pressure effects. Generally, this is an adequate approach, but it is reassuring to know the magnitude of the uncorrected errors. The following equations allow measurements to be easily and completely corrected to any desired reference conditions.




It is assumed that the true volume of a container, empty and at its reference temperature is accurately known. Also, it is assumed that the measuring element is without error. Under these ideal conditions, the remaining errors are:

1. Error for Tanks “Out of Round”. The tank may not be perfectly round (if cylindrical) or geometrically true (if some other shape).

2. Error Due to Pressure Expansion. The tank wall is stretched, owing to hydrostatic pressure.

3. Error Due to Temperature. The fluid is not at its reference temperature.




Corrections for each of these unavoidable conditions are presented as relatively simple, dimensionless expressions in which any consistent data can be substituted. The results are expressed as relative errors e, defined as


observed value – true value

e = ------------------------------------------

true value



The error e is therefore a decimal and can be converted to a percentage by multiplying by 100. It is positive or negative, depending on whether the observed value is high or low.Also included is a method by which the temperature error can be completely and exactly compensated for. The correction allows for expansion of the fluid and the tank at a temperature T1 below the liquid level, and for the tank and float wire (configuration selected as an example) at another temperature T2 above the liquid level. It allows for an average liquid temperature consisting of one or an infinite number of striations, each at a different temperature. The only requirement is that each horizontal level be uniform.



Classification of Level Measuring Devices


Visual Level Sensor


Dipstick


Probably the oldest form of level measurement

A dipstick is essentially a stick or rod that is calibrated to indicate level. The Dipstick is lowered vertically into a tank or vessel until it reaches a reference point. Usually the bottom of the tank is used to ensure that the dipstick is inserted to the correct depth. The dipstick is then withdrawn and the level is read by determining where the interface last made contact with the dipstick. Reading the scale on the dipstick indicates the level measurement. This scale can be marked for point measurement or in units that provide continuous measurement.

Dipstick

Dipstick


Lead Lines


A lead line acts in the same way as a dipstick. A steel measuring tape with a weight attached, the lead line can be used in most places that the dipstick can.

Lead lines

Lead Lines


Lead Lines


Advantages of Lead lines over Dipstick

• Since the lead line can be rolled up into a smaller, compact unit, it is often easier to handle than a dip stick.

• Lead lines can measure much higher levels than would be practical with dip sticks.

Advantages and Limitation:

Advantage

• Dipsticks, sight glasses and floats are very reliable direct measurement devices. When installed and used correctly, these devices provide a high degree of accuracy.

Visual Level Sensor


Limitations:

• It can only be used to measure level on open process systems.

• Safety precautions must be observed when the process is caustic or toxic to personnel.

• It requires that an operator must interrupt his duties to take the measurement.

• Continuous representation of the process level is not possible; level is known only at the time the measurement is taken. These drawbacks limit the application of these means of visual measurement.

Accuracy:

Although the Dipstick and lead line methods of level measurement may seem crude, they are in fact accurate to about 0. I percent with ranges up to about 20 feet.

Visual Level Sensor


Application:

Two applications for which dip sticks are commonly used are to measure the oil level in internal combustion engines and to determine fuel quantities in underground storage tanks. When measuring clear liquids, special chemicals can be applied to the stick or line that will darken or change color when immersed in the liquid. The chemical should not react with the process.

Visual Level Sensor


Principles of Operation:

The sight glass is a transparent tube of glass or plastic mounted outside the vessel and connected to the vessel with pipes. The liquid level in the sight glass matches the level of liquid in the process tank. As the level in the tank rises and falls, the level in the sight glass changes accordingly. Thus, it is possible to measure the level in the tank, by measuring the level in the sight glass.

Sight Glasses and Gage Glass


Sight glasses operate on the principle that equal pressure on the surfaces of two connected columns causes the liquid to seek the same level.

Thus, the level of the liquid in the sight glass will be equal to the level in the vessel. When it is equipped with a scale, the liquid level in the sight glass acts as an indicator for direct reading.

Sight Glasses


Sight Glasses


Low Pressure sight glasses

Visual Level Sensor


Sight Glasses


Temperature Problems

Sight Glasses


In process system that contain a liquid under high pressure a reflex sight glass is used. The design of a reflex sight glass is based upon the optical law of total reflection of light when it passes from a medium of greater reflective power into a medium of lesser reflective power. To facilitate reading the level, groove facets are cut in the inner surface of the glass at appropriate angles, making it possible to eliminate all light from the vacant space (back portion) of the glass. At the same time, light is permitted to pass through the portion of the glass that is covered with the process fluid. A sharp, clear line marks the height of the liquid surface, above which the air or gaseous space has a bright, mirror-like appearance. Light is reflected by the grooves in the glass above the liquid, but not below the liquid surface. As a result, the liquid appears the same color as the background of the chamber, usually black, giving the greatest contrast. When natural light is not sufficient to see the level, a lighted plastic strip is placed in the back side opposite the viewer. This allows the glass to be used in low light areas and at night.

Reflex Sight Glasses


Sight glasses. (a) Detail of a typical type of column, (b) reflex type



Gage glasses function in a similar manner to sight glasses. Gage glasses are typically glass covered ports in a vessel that make it possible to observe the level of the substance in the vessel.

Gage Glass


Gage Glass


Advantages and Limitations

Advantage:

• The simplicity and reliability of sight-glass and gage-glass level measurement devices explains their frequent use for local indication. When level transmitters fail, or are out of service for maintenance, sight glasses and gage glasses allow the process to be measured and controlled by manual means.

Visual Level Sensor


Limitations:

• Tanks are very often inaccessible, which makes viewing the sight glasses difficult.

• Sight glasses are not designed to provide remote indication. Sight glasses are also vulnerable to breakage. This could result in the release of the process product into the environment. If the process is hot, corrosive, or caustic, the results of accidental spills could be serious. Care should be taken to install sight glasses in locations where the risk of breakage is minimal and they should be shielded whenever possible.

Visual Level Sensor


Applications:

• These measurement devices can be adapted to either open or closed tank applications. The closed-tank sight glass is used in both pressurized and atmospheric processes. Common applications of this device on pressurized vessels include boiler drums, evaporators, condensers, stills, tanks, distillation columns, liquid accumulators, traps and other such applications.

• Sight glasses can also be used in both low and high-pressure process systems. The low-pressure gage consists of a clear round tube fitted between service valves that permits the gage to be isolated from the process for repair or replacement. The valves are also equipped with ball checks inserted With-in the valve chambers that automatically shut off now in the event of a serious leak or rupture. The ball checks permit a free passage of fluid when the process level is changing, allowing the level in the tube to change.

Visual Level Sensor


Visual Level Sensor


Float Devices. Magnetic-Type Float

Float Devices. Magnetic-Type Float


Float

A device that rides on the surface of the fluid or liquid within a storage vessel. The float itself must be substantially lesser density than the substance of interest, and it must not corrode or otherwise react with the substance.

Float Devices


Float used for manual “gauging” of level.

Float Devices


Automatic level gauging

Float Devices


Ball float mechanism operating principles

Chain- or tape-type float gage

Float Devices


Float cable and weight Float and spring

Float Devices


Float Devices


The photograph shows the “measurement head” of a spring-reel tape-and-float liquid level transmitter, with the vertical pipe housing the tape on its way to the top of the storage tank where it will turn 180 degrees via two pulleys and attach to the float inside the tank:

The spring reel’s angular position may be measured by a multi-turn potentiometer or a rotary encoder (located inside the “head” unit), then converted to an electronic signal for transmission to a remote display, control, and/or recording system. Such systems are used exclusively for measurement of water and fuel in storage tanks.

Float Devices


If the liquid inside the vessel is subject to turbulence, guide wires may be necessary to keep the float cable in a vertical orientation:

The guide wires are anchored to the floor and roof of the vessel, passing through ring lugs on the float to keep it from straying laterally.

One of the potential disadvantages of a tape-and-float measurement systems is fouling of the tape (and guide wires) if the substance is sticky or unclean.

Float Devices


Installation of float-type level indicator with guide wires on a fixed-roof tank. A. Guide wire anchor. Wires may be anchored to a heavy weight or the bottom of the tank. B. Float guide wires. C. Float having sliding guides which move freely on the guide wires. D. Manhole sufficiently large for float and anchor weight to pass through. E. Flexible joint. F. Pulley housing. G. Vapor seal (if required). H. Float tape. I. Tape conduit. J. Sliding guides. K. Gauge head. L. Guide wire tension adjustment.

Float Devices


Float-operated hydraulic-type gage

Float Devices


The float’s position inside the tube may be readily detected by ultrasonic waves, magnetic sensors or any other applicable means. Locating the float inside a tube eliminates the need for guide wires or a sophisticated tape retraction or tensioning system.

Float Devices


Float Devices



Floats can also be used with magnets to detect and indicate level. This type of measurement system uses the attraction between two magnets to follow the level of a process liquid. The system consists of a magnet enclosed in a float ring or collar. A second magnet, called the magnet follower, is contained in a non-ferrous metal tube. The magnet in the float attracts the magnet inside the tube, and, as the float raises or lowers with the level of the process liquid, it causes the magnet inside the tube to raise and lower also. By sensing the position of this magnet, the level of the, liquid in the tank can be indicated. Magnetic Gage

Magnetic Level Gauges

Magnetic-Type Float


Float Collar and MagnetLoaded drum

Principle of a magnetic-bond-type ball-float gage.

Magnetic-Type Float


DescriptionThe instrument consists of 3 main parts: the float, the chamber and the indicator

Operating principle On the dry side of the chamber is the Indicator, either a magnetic slider inside a glass tube or a column of magnetic flaps which are red on one side and white on the other side . As the float moves up or down the magnetic field of the permanent magnet pulls the rollers through a rotation of 180° thus changing their color. As the float rises the flaps are truned from white to red, as the float falls, they are changed back to white again. This means that at any given tile the amount of liquid in the tank is constantly reprensented by a red column without any external power supply.

General Assembly

Magnetic-Type Float


Magnetic Gage Magnetic indicator gage

Magnetic-Type Float


Magnetic-Type Float

Schematic of magnetic level indicator installation. Courtesy, Weka Besta Ltd.


Stainless steel profil

316L stainless steel

housing

Bi-colour aluminium

wafers

Silicon seal ‘’Pyrex’’ window

Magnetic-Type Float. Flap Version


INDICATOR OPTIONSINDICATOR OPTIONS

Magnetic-Type Float. Flap Version


A magnetic, coloured slider driven by a float, slides in a borosilicate glass tube. The level indication is directly given using two graduated scales assembly, placed on the primary tube. These graduated scales may be moved in translation on the primary tube, allowing the adjustment of the specific gravity from the reference point situated on the bottom of the scale. This assembly is mounted to ensure a maximum shock protection of the glass tube.

Magnetic-Type Float. Slider Version


Reading systemTwo versions are available: slider or flaps version..

Float chamberSo called the primary tube, it consists of a stainless steel or synthetic tube fitted with flanges (in standard) for external side mounting.

The tube is also fitted with a Bottom flange assembly equipped with a draining plug for draining and cleaning.

Plug + Vent (½ BSP or NPT)For air draining according to customer process or application.

Name plateManufacturer name plate including all main technical data and specifications according to applicable rules and standards.

Alarm contacts

Process Connections

4-20 mA Transmitterfor remote measuring.Available with standard housing or flame-proof version.

Float Equipped with 360°magnet– follow the variations of liquid inside the chamber.

Magnetic-Type Float. Description


H2

H1

Magnetic-Type Float


Advantage and Limitation:

Advantage:

•They are reliable and require very little maintenance and calibration.

Limitation:

•They are limited to liquid-gas interfaces. These types of float devices are usually not accurate enough for foaming liquids.

Visual Level Sensor


Application:

•This type of instrument is particularly useful in corrosive process systems or in processes where the viscosity of the liquid could plug or deteriorate a cable or steel tape arrangement. In such systems, it would be cost prohibitive to create an entire measurement system of materials able to withstand the effects of the corrosive elements. In this arrangement, the only part of the system in contact with the corrosive process material is the float, so only the float requires SOMR- type Of protective coating.

Visual Level Sensor


Application:

• A variation of the magnetic float level measuring device can be designed to actuated switches. A magnet attached to a float is housed inside a non-magnetic tube. The non- magnetic tube is attached to the vessel by small pipes at both the top and bottom of the tube. This arrangement allows the liquid in the tube to be at the same level as the liquid in the vessel. As the float rises with the liquid level, the magnet moves up into position opposite the switch mechanism. The magnetic field produced by the magnet attracts the iron armature on the switch. The switch is held in the open position by a spring until the magnet is raised to a certain point (A). Then, the magnetic pull will overcome the spring tension and the switch will close, as shown in (B). This movement tilts a glass tube containing mercury. The mercury flows to cover the terminals and complete the electrical circuit.

Visual Level Sensor


This electrical circuit might then be used to sound an alarm indicating a high level has been reached, or to activate a control device. When the liquid level falls below the set point, the magnet moves, decreasing the magnetic field and the spring tension returns the switch to the original position. Other magnetic-type float devices use the same principle described in the mercury switch to actuate micro-switches and pneumatic controls.

This type of level measuring arrangement can be used on open or closed processes. In addition, it can be used to provide remote level indications or alarms.

Visual Level Sensor


Magnetic float switches mechanism

Visual Level Sensor


Variable Displacement Devices


When a body is immersed or partly immersed in a liquid, it loses weight equal to the liquid weight displaced. Variable displacement level devices utilize this principle by measuring the weight of the immersed displacer.

Archimedes' Principle:

Archimedes' Principle states that a body immersed in a liquid will be buoyed up by a force equal to the weight of the liquid it displaces. This upward pressure acting on the area of the displacer creates the force called buoyancy.



Theory of Buoyancy:

Archimedes principles states that the resultant pressure of a fluid on a body immersed in it acts vertically upward through the center of gravity of the displaced fluid and is equal to the weight of the fluid displaced.

Relationship between level position and displacer element. (a). Level position at the bottom or below the displacer (b). Level position at the top or above the displacer (c). Level position between the top and bottom of the displacer.

With reference to the figure above, it will be noted, by measuring the difference in weight of a partially submerged element at various degrees of submergence, the level of the liquid in which the displacer element is submerged can be determined, the following equations are useful in determining the forces exerted by the displacer.



Condition 1:Level position at the bottom or below displacer (a)

F = WF = Force or weight to be supportedW = Weight of displacer

Condition 2:

V = volume of displacerG = specific gravity of liquid at a reference temperature (usually 60° F)D = Density of fluid

D

VGWF



Level position in intermediate position

Lin = Level position in intermediate positionLo = Level position at the bottom or below displacerd= length of the displacer

d

LL

D

VGWF oin



With a cylindrical displacer, F varies as the level position around the displacer varies. The value of F is measured by a suitable means, such as a torsion spring, pneumatic force balance, or electronic transducer. Thus, this value becomes a function of the level position above the datum line and is governed by the relationship shown in figure below.

The force F versus level height



Buoyancy Type



Classification


It should be noted, that displacers are only sensing elements. To be a useful measuring device, a displacer must be connected to a measuring mechanism which, when sensing the changes in buoyant force, converts this force into an indication of level. A displacer body can be suspended directly in a tank, or installed in a float chamber on the outside of the vessel. In the first case, the displacer is directly exposed to the process and indicates level as buoyancy changes due to changes in the level of the process liquid.



Displacer suspended in Tank Displacer outside Tank



Direct top mounting External chamber mounting NB. If in an agitated process fluid a protective cage or pipe

should be used

Typical Mounting Arrangements



Two important points are demonstrated here:

• First, when the liquid level is lowered to completely uncover the displacer, the displacer can no longer measure level. Any changes in level below the lower end of the displacer will not be measured.

• Second, the same is true when liquid level rises to the top of the displacer. Then, any changes in liquid level above the top of the displacer will not be detected. However, it is possible to increase measurement span. Longer displacer bodies can be used and, in some installations, several displacer bodies (see at figure at right) may be connected together. Displacer Bodies



Liquid-Liquid Interface Measurement


When a displacer is used to determine the level of an interface between two liquids, it is always completely submerged as shown in figure at right. The displacer should be positioned so that its mid-section is at the liquid-liquid interface. When the interface level changes, the buoyant forces acting on the displacer will change the weight of the displacer. This change in weight is a result of the change in buoyancy caused by the difference in specific gravity between the two liquids. The weight of the displacer is then a function of the interface position and a direct indication of the level of the interface.

Displacer Indicating Liquid-Liquid interface



Auxiliary Devices:

Regardless of how the displacer is installed, it must be connected to a device that can sense changes in displacer weight resulting from changes in buoyancy. This information is then used to indicate level.



Auxiliary Devices:

Torque Tube:

One such torque tube displacer level instrument is shown (a). With this instrument, the displacer is suspended from an arm that is attached to a torque tube or torque rod. A knife-edge bearing supports the movable end of the torque tube. This type of bearing provides an almost frictionless pivot point. The torque tube must be of sufficient strength to support the full weight of the displacer in the absence of buoyancy, or when the level is at minimum. It is a solid or hollow tube that transfers displacer motion to an electronic instrument or a pneumatic instrument that will produce a signal proportional to the changes in the weight of the displacer.



This illustration shows the torque as part of a whole displacement-style level transmitter:



(a) Torque Tube Displacer Gage



A force measuring mechanism that senses changes on the buoyant force and converts this force into an indication of level.



Assembly view of Fisher torque unit. Courtesy, GEC Elliot Control Valves Ltd.



Auxiliary Devices:

Spring Balance Displacer:

Spring balance displacers are devices similar to torque tube displacers. In these devices, the torsional spring of the torque tube is replaced by a conventional range spring. The indicating portion of the instrument is located in a separate housing to isolate it from the process. The motion of the displaced is transferred to the indicator by means of magnetic coupling. As shown in figure (b), the displaced is suspended in the liquid by means of an extension range spring. As the level in the tank rises or falls and the weight of the displacer changes, the spring expands or contracts. A magnet attached to the displacer rod rises or falls in response to the displacer movement. Another magnet in the indicator housing follows the displacer magnet and transmits this movement to either a rotating earn on pneumatic units, or to a slide wire on electronic units.



Spring balance displacer



The photograph shows a Fisher “Level-Trol” model pneumatic transmitter measuring condensate level in a knockout drum for natural gas service. The instrument itself appears on the right-hand side of the photo, topped by a grey-colored “head” with two pneumatic pressure gauges visible. The displacer “cage” is the vertical pipe immediately behind and below the head unit. Note that a sight glass level gauge appears on the left-hand side of the knockout chamber (or condensate boot) for visual indication of condensate level inside the process vessel:



Two photos of a disassembled Level-Trol displacer instrument appear here, showing how the displacer fits inside the cage pipe:

The cage pipe is coupled to the process vessel through two block valves, allowing isolation from the process. A drain valve allows the cage to be emptied of process liquid for instrument service and calibration.



Relative Advantage:

An advantage of variable displacers is that they are capable of detecting liquid-liquid interfaces as well as liquid-gas interfaces.

Application:

This application can be used in liquid separators. It must be remembered, however, that the level indication only represents the position of the liquid-liquid interface and does not indicate the overall level in the tank.



Applications:

1. Pneumatic and Electronic applications: Displacers can be used to generate a transmitted signal, which controls process level at the field location, and provides measurement indications at a remote location.

2. Chemical and Petroleum Industries: Control of the interface position is an important consideration. Field-mounted level controllers are often used for such liquid-liquid interface applications. For example, if the water-petroleum distillate interface level in a separator is controlled at some point around the midlevel value, the water can be drawn off through a valve at the bottom of the tank. The valve must be closed when the water level is low to prevent the loss of product, and open when the water level is high to maintain the interface level between the water and the distillate. Field- mounted liquid level controllers open or close the valve to maintain the interface.



Applications:

3. Measurement and control: When it is desirable to measure as well as control a liquid level, devices such as duplex-type controller- transmitters are available. This type of device operates from a single displacer and torque tube assembly to generate two independent signals. One is from the control segment that positions a control element for control purposes; the other operates for the remote level indication or records the level value.



Electrical Level Sensors


A number of widely used level-sensing devices operate by detecting differences in electrical properties created by the interface of process materials. These properties are:

• Capacitance

• Conductivity

• Resistance

Electrical Level Sensor



A common arrangement is shown in (a). Two electrodes are positioned in a tank. One extends to the minimum level; the other is positioned so that its lower edge is at the maximum level. The tank is grounded and functions as the common, or third electrode. Usually, a stilling well is provided to ensure that the interface is not disturbed and to prevent false measurement.

Conductivity


The terminals of the electrodes are connected to relays, which transmit signals to a display or control device. If the process level contacts any portion of the electrode that extends to the minimum level, a conductive path is established through the grounded tank to the electrode. If the level falls below this electrode, the path is interrupted given that the gas or vapor is nonconductive. The level condition may then activate a device, which sounds an alarm or energizes a control device to operate a pump, feed controller or other processing equipment that automatically adjusts the system level.

Conductivity


(a) Conductivity Switch

Electrode or probe-type level system.

Conductivity


If the level rises to the point that both electrodes are in contact with the conductive material, current will flow through both electrodes and the tank, or common.

A relay connected to the electrode positioned at the maximum level will detect the high level and transmit a signal to an alarm or a control device to correct the condition.

Conductivity


Conductivity


ConductivityLevel Sensors

Conductivity


Conductivity


Multiple-conductivity probe sensing pulp and froth layers over set increments. Courtesy, Mineral Control Instrumentation.

Conductivity


Limitations:

•The process substance must be conductive.

• Only point detection measurements can be obtained.

• The possibility of sparking also makes this method prohibitive for explosive or flammable process substances.


Advantages:

• Low cost and simple design.

• No moving parts in contact with the process material. These advantages make this type of system an effective method of detecting and indicating level for many water-based materials.

Conductivity


Advantages

Inexpensive to purchase and install Reliable Low maintenance costs Relay is only moving part Activation from wide range of conductivities

Disadvantages

Limited to conductive fluid Possibility of malfunction if conductive

buildup occurs on electrode holder

Conductivity


Application:

A material's ability to conduct electric current can also be used to detect level. This method is typically used for point measurement of liquid interfaces of relatively high conductivity. These liquids include water-based materials such as brine solutions, acids, caustic solutions, and certain types of beverages. Conductivity applications are usually limited to alarm devices and on/off control systems.

Conductivity


Hydrastep Steam Drum level control

Conductivity


A vertical row of electrodes is installed in the water level column attached to boiler to measure water level. The resistance measurement is made between the insulated tip of each electrode and the wall of the column.

The “cell constant” defining the actual resistance measured is determined by the length and diameter of the electrode tip and the column bore.

In practice, the cell constant is chosen so that the resistance in water is less than 100k ohms and the steam resistance is greater than 10M ohms, since the resistivities of water and steam are substantially different.

Principle of Operation

Conductivity


Amplitude - proportional to resistance Steam - high resistance, large amplitude Water - low resistance, small amplitude Fault - open or short circuit, very low amplitude

Drive Resistor5.0V Drive

< 1.5V = Water

< 0.1V = Fault

> 3.0V = Steam Amplifier

Principle of Operation

Conductivity



Conductivity


Sight Glass

Density Error

Conductivity


Correcting Density Error

Conductivity


Hydrastep – Typical set up

Conductivity


Conductivity Level Sensors

Conductivity



Conductivity



Capacitance is the property of an electrical device that permits it to store energy. Capacitors can be used to store electricity. Figure below illustrates a capacitor that consists of two plates separated from each other by an insulating material called dielectric. By connecting these plates to a power supply, electrons are attracted from one plate and on to the other. The result is that the plates have opposite charges.

Capacitor formed by two plates

Capacitance


Capacitors are capable of storing and holding the charge until they are discharged. The amount of charge that can be stored is determined by three factors: the area of the plates; the distance between them; and the type of dielectric used. The relationship is expressed by the following equation:

D

KAC

where: C = capacitance K = dielectric constant A = area of the plates D = distance between the plates The unit used to indicate the value of capacitance is the farad (F).

Capacitance


B

Capacitance


The change in capacitance is a direct function of the dielectric constant. As the level of the measured material increases, it replaces the air or dielectric between the electrodes or plates. The dielectric constant of air and most gases is 1. The Table (a) indicates dielectric constants for some solids. Dielectric constants for granular material are listed in Table (b) and for liquids in Table (c).

Capacitance


Table (a) Dielectric constant for Solid

MaterialDielectric constant Material Dielectric Constant

Acetic Acid (36oF)Aluminum phosphateAsbestosAsphaltBakeliteBarium sulfateCalcium carbonateCelluloseCerealsFerrous oxideGlassLead oxideLead sulfateMagnesium oxideMicaNapthaleneNylon

4.16.14.82.75.011.49.13.93-5.014.23.725.914.39.77.02.545.0

PaperPhenolPolyethlylenePolypropyllenePorcelainPotassium Carbonate (60oF)QuartRiceRubber (hard)Sand (silicone dioxide)SulphurSugarUreaTeflonZinc sulfide

45.02.04-5.01.55-7.05.64.34.33.53.03-5.03.43.03.52.08.2

Capacitance


Table (b)

MaterialDielectric constant

LooseDielectric constant

Packed

Ash (fly)Coke

Gerber (Oat meal)Linde 5A molecular

1.Sieve dry2.20% moisture

PolyethylenePolyethylene powder

Sand-reclaimed foundryCheer

Fab (10.9% moisture)Tide

VEL (0.8% moisture)

1.765.31.471.810.42.21.254.81.71.3+1.551.25

2.070.0Not testedNot TestedNot testedNot TestedNot Tested4.8Not Tested1.3+1.25

Capacitance


Table (c) Dielectric constant for Liquids

MaterialTemp. oF

DielectricConstant

Material Temp. oF

DielectricConstant

AcetoneAmmoniaAmmoniaAnilineAnilineBenzeneBromineButaneCarbon DioxideCarbon TetraflorideCastor oil ChlorineChlorocyclohexaneChloroformCumeneCyclohexaneDibromobenzeneDidromohexaneDowthermEthanolEthyl AcetateEthylene ChlorideEthyl EtherEthyl etherFormic AcidFreon-12GlycerineGlycolHeptane

71-306832686868303268603276326868687670776868-40686070686868

21.422.015.57.87.32.33.11.41.62.24.72.07.65.52.42.08.85.03.3624.36.410.55.74.358.52.447.041.21.9

HexaneHydrogen ChlorideHydrogen sulfideIsobutyl AlcoholKeoseneMethyl AlcoholMethyl alcoholMethyl etherNapthaleneOctaneOil, transformerPentanePhenolPhenolPhosphorousPropaneStyrene (phenylethene)SulphurSuphuric AcidTetrachloroethyleneTolueneTrichloroethyleneUreaVinyl etherWaterWaterWaterXylene

688248687032687868686868118104933277752687068617168326821268

1.94.65.818.71.837.533.15.02.51.962.21.89.915.04.11.62.43.484.02.52.43.43.53.988.080.048.02.4

Capacitance


Capacitive Level Probes

Capacitive level probes come in two basic varieties: one for conductive liquids and one for nonconductive liquids. If the liquid in the vessel is conductive, it cannot be used as the dielectric (insulating) medium of a capacitor. Consequently, capacitive level probes designed for conductive liquids are coated with plastics or some other dielectric substance, so that the metal probe forms one plate of the capacitor and conductive liquid forms the other: In this style of capacitive level probes, the variables are permittivity (ε) and distance (d), since rising liquid level displaces low-permittivity gas and essentially acts to bring the vessel wall electrically closer to the probe. This means total capacitance will greatest when the vessel is full (ε is greatest and effective distance d is at minimum), and the least when the vessel is empty (ε of the gas is in effect, and over a much greater distance).

Capacitance. Conductive Liquids


Capacitive level probes for conductive liquids

Capacitance


If the liquid is non-conductive, it may be used as the dielectric itself, with the metal wall of the storage vessel forming the second capacitor plate.

In this style of capacitive probe, the variable is permittivity (ε), provided the liquid has a substantially greater permittivity than the vapor space above the liquid. This means total capacitance will be greater when the vessel is full (average permittivity ε is at a maximum), and least when the vessel is empty

Capacitance. Non-conductive Liquids


Capacitive level probes for nonconductive liquids.

Capacitance


Capacitance probe without a shield. (Robertshaw Controls.)

Capacitance probe with a shield. (Robertshaw Controls.)

CapacitanceLevel Sensors

Capacitance


Cutaway view of capacitance level sensor. Courtesy, Kent Industrial Measurements LTD.

Capacitance


Capacitor Probe Capacitance installation. (Robertshaw Controls.)

Capacitance


Capacitance


Electrode formed from insulated steel cable for measurements in tall vessels or

bins [up to 125 ft (38 m)]. Example of solid detection (Robertshaw Controls.)

Cage-mounted capacitance level probe. (Robertshaw Controls.)

Capacitance


On–Off Capacitance level instrument with remote –

mounted detector and probe – mounted detector

Remote–mounted continuous capacitance level monitoring system (Robertshaw Controls)

Capacitance


Capacitance Level Sensors

Capacitance


Capacitance. Hydrostatic Level


Hydrostatic level



Capacitive Plates(gold plated)

Hybrid, surface mount electronicswith temperature correction

Atmospheric vent hole. Air space evacuated and hole sealed for absolute pressure

Force (pressure)

As the applied pressure changes, the ceramic wafer flexes. Thus varying the distance between the capacitance plates (deflection at full range is 0.025mm) causing a change in capacitance.

Increase ceramic thickness to increase range. (high pressures are smaller diameter)

Sensor’s electronics convert deflection in to output of 1 to 4V, 4-20mA is produced from main PCB which can be integral or remote.

Operating principle- Ceramic capacitive sensing cell



Positive seating(metal to metal)

Double CableGland

Strain reliefgland

Vented cable

‘O’ Ring

Ceramic Sensor

Sensor Head Construction



Pole mounted

Cablesuspended

Flanged

Mounting Options

Clamped,Cablesuspended



Applications examples for hydrostatic level transmitters

Indicator

Cable or pole mounted



Advantages and Limitations:

Advantages:

•The primary sensing element is very simple and rugged and has no moving parts.

• Capability for temperature, pressure, and corrosion resistance is easily obtained.

• The sensing elements are easily cleaned, and sanitary standards are readily met.

• Intrinsically safe elements with explosion-proof instruments are readily available.

• The cost of most capacitance systems is competitive with that of the simple mechanical or pneumatic units; however, more costly special purpose capacitance systems are available for special difficult-to- measure applications.

Capacitance


Limitations:

•If the dielectric constant of the measured medium changes with the temperature, a measurement error will result unless a dielectric compensated detector is used.

• Viscous conducting liquids, which coat the sensing element can cause erroneous or false readings unless a detector which compensates for coatings is used.

• Air bubbles in the liquid or foam on top of the liquid can give erroneous readings.

• Sensing the interface level between two conducting liquids is difficult, depending on the magnitude of the conductivity.

Capacitance


Capacitance Type - Continuous Measurement Advantages

Can be used for some applications where other types are not feasible

Moderate cost Fair accuracy Can be used for high temperature and high pressure

applications Can be used in polymer and slurry services

Capacitance


Capacitance Type - Continuous Measurement Disadvantages

Special calibration required in many instances Affected by density variations of measures

materials Limited applications data Erroneous readings occur when coatings form on

the probe. (Note: at least one manufacturer surmounts this problem by precoating the probe and impressing a radio frequency signal on the probe.)

Capacitance


Capacitance Type – Point Detection Advantages

Reasonable cost Easy to install Useful on applications such as powders, pellets and

other solids-containing materials, as well as slurries and corrosive materials where many other level devices will not work

Simple in design No moving parts

Capacitance


Capacitance Type – Point Detection Disadvantages

Accuracy affected by material characteristics Coating of probes troublesome on some designs Lack of data on dielectric constants of some

materials

Capacitance


Application:

In applications involving capacitance measuring devices, one side of the process container acts as one plate and an immersion electrode is used as the other. The dielectric is either air or the material in the vessel. This configuration is illustrated here. The area of the plates and the distance between them are both fixed values. The dielectric varies with the level in the vessel. This variation produces a change in capacitance that is proportional to level. Thus, level values are inferred from the measurement of changes in capacitance, which result from changes in the level.

Capacitor Probe

Capacitance


Application:

1. Non-conductive Tank Walls: A single probe which contains two electrodes can be used. This type of probe is used for point measurement and is installed horizontally. The terminals of the electrodes are connected to a measuring device. When the interface passes between the electrodes, the capacitance changes because of the differences in the dielectric constants of the process materials. A second two-electrode probe can also be installed for high and low level measurement.

2. Conductive Tank Walls: For conductive materials, an insulated probe, such as a teflon-coated probe, is used.

Capacitance


Application:

3. Continuous Level Measurement: In applications requiring continuous measurement, the probe is positioned vertically. Its length corresponds to the measurement span. As level changes occur, the total capacitance will change reflecting different dielectric constants of the process liquid. In some applications in which probes are used, the process material adheres to the probe. As the level in the tank falls, a layer of liquid can remain on the probe. This layer can create the effect of a change in plate area, or it may affect the distance between the plates of the capacitor. In either instance, the amount of capacitance stored will be affected as well as measurement accuracy.

Capacitance



Resistance type level detectors use the electrical relationship between resistance and current flow to accurately measure level. The most common design uses a probe consisting of two conductive strips. One strip has a gold-plated steel base; the other is an elongated wire resistor.

The strips are connected at the bottom to form a complete electrical circuit. The upper ends of the strips are connected to a low voltage power supply. The probe is enclosed in a flexible plastic sheath, which isolates the strips from the process material

Resistance type level detector

Resistance


When the probe is installed in an empty tank or a tank containing vapor, the strips are only in contact at the lower end. When the circuit is energized, current passes through the probe and a base current reading can be taken. As the level of the process material rises, the hydrostatic pressure forces the resistance strips together up to the interface. This action shorts the circuit below the interface level, and total resistance is reduced proportionately. Above the surface, the strips remain separated and un-shorted.

The resistance element is wound to have two to four contact points per inch. Consequently, as the resistance element contacts the base conducting strip, specific amounts of resistance are removed from the circuit. The uniform separation on the resistance windings and the known resistance per unit length make it possible to determine the height of the interface. This is accomplished by measuring changes in current flow resulting from changes in circuit resistance.

Resistance


Relative Advantage:

As with the other electrical level sensors discussed, resistance-type level detectors require relatively little maintenance.

Application:

When resistance type level measuring devices are used in closed-process applications, the inside of the protective sheath is vented to the atmosphere if the tank is at atmospheric pressure. Because it is pressure that causes the resistance element to short out a part of the probe's resistance, a special venting system is required to equalize pressure inside and outside the protective sheathing. The venting system prevents pressurized vapor above the interface from shorting the circuit. It is also necessary to keep the air inside the sheath clean and dry.

Resistance sensing devices can be used for liquid-gas interfaces and for slurries or solids.

Resistance


Level Switches

CapacitanceDiaphragmTilt SwitchRotating PaddleNuclear

For Continuous Level Monitoring

Weight (Load Cells)Sonic/UltrasonicRadar/Microwave

Solid Level Measurement


Solid Level Measurement


There are several parallels between the level sensors for bulk solids and those used for liquids. In fact, considerable adaptation of devices from one technology to the other has occurred during the last few decades. Progress has been accelerated by solid-state circuitry and digital methods. Several detection principles are applied in both solids- and liquid-level measurements, including the employment of electronic field effects, sonic and ultrasonic principles, photoelectric transmission, radio- and microwave absorption, nuclear methods, and diaphragms and tilt switches. Diaphragm sensors may utilize capacitance, reluctance, and strain gage transduction, among other principles.

Bulk Solids – Level Sensors


Importance of Proper Mounting Location

A characteristic of bulk solids, referred to, as the angle of response, is a very important consideration. See Table (A). If this factor is not properly evaluated in location determination, level control devices will not respond properly. The angle of repose may be defined as the maximum angle with the horizontal at which an object on an inclined plane will retain its position without tending to slide. The tangent of the angle of repose equals the coefficient of static friction. The term is used in a closely related way to describe the maximum angle with the horizontal at which loose materials, such as grain, sand, coal, and stone, will retain a position without tending to slide. The moisture content and distribution of fine and coarse particles have a marked effect on the value of this angle.



Table (A)



Other factors which enter into consideration in designing instrumentation systems for bulk solids include arching and flushing, both related to the angle of response. Many materials, such as lampblack, activated carbon, zinc oxide, titanium oxide, fine soda ash, and hydrated lime, tend to arch when placed in containing vessels such as hoppers. Arching is best overcome by keeping the individual solid particles continually in motion. Electric vibrators are commonly used for this purpose. Very few materials are crushed or pulverized so well that all particles are of equal size. In the absence of hopper vibration, the smaller particles and fines tend to segregate and roughly collect in a center cone, while the larger particles roll to the side. Consequently, when a hopper is discharged, the fines drop out first, while the larger particles discharge later, the end result of which is formation of an arch. Flushing is a condition caused by the sudden breaking of an arch or otherwise clogged state. Vibrators also help to eliminate flushing.




Other conditions which affect selection and location of a level sensor include moisture content of the material and physical characteristics such as stickiness, temperature, and pressure.



This type of device makes use of a pressure-sensitive diaphragm which when depressed by the presence of material produces a corresponding switch actuation. A technique that has been used for about a half-century, it is considered quite reliable. Diaphragm controls are available in a wide variety of materials, making It possible to use them over a wide range of product densities. The controls are usually externally mounted and offer no restriction to normal product flow. Other than the switching circuitry, the devices require no external power for their operation. They can be used interchangeably for high- and low-level detection. Advantages include simple installation and maintenance and comparatively low cost. The devices are not applicable to positive pressure environments such as those encountered in pneumatic conveying systems.

The added pressure results in a false indication. Special attention should be given to mounting diaphragm controls, as they can be position-sensitive, especially when used on hopper underslopes or when mounted horizontally.

Diaphragm Controls


Diaphragm Controls


Very commonly used, these controls make use of a motor which slowly rotates a paddle in the absence of material, and rotates itself to actuate a switch when the paddle is prevented from turning by the presence of material. These controls are modestly priced, are versatile, and are available with a variety of paddle options corresponding to various product densities. They may be used for both high and low-level point control. Paddle wheel controls are not position-sensitive, and their operation is not adversely affected by positive-pressure environments, but checking with the specific manufacturer on this point is suggested. The electrical components are contained in either weatherproof or explosion proof enclosures and are easily serviced in the field, requiring no special skills. For installation,

RotatingPaddle

Paddle Wheel Controls


Typical application - Lime SilosTypical application - Lime Silos

Industry: Water and Waste Water Treatment

Application: Lime Silo Measurement, Hi level and re order levels

Problems:Material can hang up Solution:Paddle Switches

RotatingPaddle

RotatingPaddle



This is probably the simplest device used for point level detection of solid materials. These controls make use of a free-hanging sensor which produces a switch contact when the bulk material tilts the unit off its vertical plane. Other than switching circuitry, tilt switches require no external power for their operation. They are usually quite rugged and reliable. Because of mounting requirements, tilt switches are not quite as versatile as most level devices and are usually limited to high-level control.

Tilt Switch


– 120/240 V at 13 amps– Mechanical or Mercury– 20 ft cord length Standard– 32° to 160° F

Tilt Switch


This type of control features a tuning fork as a sensing probe. A piezoelectric crystal vibrates the probe when there is no material surrounding it. When material contacts the probe, the circuitry detects its presence and operates a contact closure. Vibratory controls can be used for both high- and low-level detection. Solid-state circuitry adds to their cost, but they are considered very reliable. A skilled technician is required at the time of a failure. Vibration controls work well with free-flowing materials and can tolerate both temperature and pressure extremes. Because vibration may cause cavitation and subsequent false signals in connection with moist materials, pre-purchase experimentation may be indicated.

VibratingProbe

Vibration Sensors


Vibration Sensors


Typical application - Spent GrainTypical application - Spent Grain

Industry: Brewing and Distilling Application: Spent Grain

Measurement Problems:Damp, Sticky Solution: Ultrasonic or capacitance

Ultrasonic

VibratingProbe

CapacitanceProbe

or

Vibration Sensors


Sometimes called "yo-yo" controls, these devices are useful for generating inventory control information. In operation, a weight attached to a cable is lowered into a storage vessel. When the plumb bob strikes the material, the slackened cable triggers retraction of the cable and weight. The amount a cable is paid out or withdrawn is measured and indicated by an appropriate readout. The devices are easy to install, simple to operate, and accurate to ± 0.1 ft (3 cm). They have the advantage of being able to operate in vessels well over 100 ft (30.5 m) deep. They are not affected by dust, moisture, or most ambient conditions, and the system operates on demand (manual or sequentially timed). They are not to be regarded as continuous measurement devices. For installation, see (a).

Plumb Bob System


Mounting of plumb bob and rotating-paddle types of solids-level detectors. (Bindicator)

Installation

Plumb Bob System


These systems are among the fastest growing methods of solids-level control. A variable capacitor is formed by a probe mounted in the vessel and in the vessel wall. Variations in this measured capacitance can be correlated with material level. Capacitance controls make use of solid – state electronics and have no moving parts within the vessel. They are not position – sensitive and work well in the presence of relatively high temperatures and pressures. They are available for both point control and measurement of solids is usually avoided. The principal limitation is materials which tend to adhere to and build up on the probe. The principle of capacitance measurement are described in more detail earlier in this article in connection with liquid – level applications.

CapacitanceProbe

Capacitance-Type Sensors


Typical application - Spent GrainTypical application - Spent Grain

Industry: Brewing and Distilling Application: Spent Grain

Measurement Problems:Damp, Sticky Solution: U/sonic or capacitance


Ultrasonic

VibratingProbe

CapacitanceProbe

or


Installation of Capacitance Level Probes



In these systems, a source transmits a beam of radiation across a bin to a detector on the other side. The nuclear radiation is capable of penetrating the bin wall, and no holes have to be cut to mount the transmitter and receiver. The nuclear beam is partially absorbed by the material in the bin, and the sensitivity of the receiver can be set to detect when material occupies the gap between the transmitter and receiver. Nuclear devices provide a comprehensive approach to level control, incorporating and servicing. The principles of nuclear level detecting systems are described in more detail earlier in this article in connection with liquid – level application.

Nuclear Devices


Head Pressure Measurements


Pressure HeadInstruments

Hydrostatic pressure - the pressure of liquid at rest.



Theory of Hydrostatics:

Hydrostatic head may be defined as the weight of the liquid existing above the reference or datum line and can be expressed in various units , such as pounds per square inch, or grams per square centimeter. The head is a real force, due to liquid weight, and is exerted equally in all direction. It is independent on the volume of liquid involved or the shape of the containing vessel. Measurement above the datum line may be expressed by the following relationship.



G

PM

GD

P

D

PH

W

H = Height of the liquid above the datum lineP = Pressure due to the liquid head.D = Density of liquid at operating temperatureDW = Density of water at reference temperature G = Specific gravity of liquid at operating temperatureM = multiplying factor , depending on the units of measurement used. For example, if H is in inch, P is in psi, G is in 62.4 lb/in3, and the reference temperature is 60 ºF, M = 27.70




From this relationship , it is seen that the measurement of pressure P at the datum or reference point in a vessel provides a measure of the height of the liquid above that point, provided the density or specific gravity of the liquid is known. Also, this relationship shows that changes in the specific gravity of the liquid will affect liquid level measurement by this method, unless correction are made for such changes.

Basic Element of Hydrostatic Head





During the past relatively few years, compensation environment changes which affect measurement accuracy has been automated in some system through the use of microprocessor and sensor that continuously or intermittently detect changes in such factors as liquid temperature and or/ density.

When a pressure greater than atmospheric is imposed on the surface of a liquid in a closed vessel, this pressure adds to the pressure due to the hydrostatic head and must be compensated for by a pressure measuring device which records liquid level in terms of pressure.




The application of the hydrostatic principle to this apparatus is fundamental. If a pressure gage is installed in the wall of an open vessel, its reading will provide an indication of the height of the interface above the measuring point.

In (a) illustrates an application where the level value is inferred from a pressure measurement. A pressure gage placed at a point on the tank that is level with the surface of the liquid would indicate zero. The pressure at the surface of an open vessel is always at atmospheric pressure and, thus, will always indicate zero. The gage will only provide other than a zero indication when the pressure applied to it exceeds atmospheric pressure. When the level in the tank is raised, the pressure created by the hydrostatic head of the liquid is applied to the gage. The gage indication will be used to infer a level measurement. If the indicated pressure is 1 psi, then the level would be 2.31 feet or 27.7 inches.



If level is to be determined and indicated by measuring pressure, the specific gravity of the liquid must be known. The specific gravity of water is 1.00. If the liquid has a lower specific gravity, the pressure exerted by the column of liquid will be less than that exerted by a column of water of the same height. For liquids with a specific gravity greater than 1.00, the pressure exerted by the column of liquid will be greater.



To compensate for the difference in specific gravity, the following equation is used:

G

ftph

.)31.2(

h = height in feetp = pressureG = specific gravity

(a) Hydrostatic level measurement – Open tank




Hydrostatic pressure - the pressure of liquid at rest.Density - mass (or weight) per unit volumeRelative density or specific gravity - the ratio of a liquid's density to water density




The pressure of liquid in an open tank depends on two factors:•The elevation of the liquid above the measurement point•The specific gravity (relative density) of the liquid

∆P = Level x SG


Direct Connection of Gage to Open Vessel

Pressure gage for open vessels




Another open-tank level measuring instrument which uses the hydrostatic head principle is the diaphragm box. The diaphragm box is submerged in the process liquid and connected to a pressure gage by a gage line. The hydrostatic head produced by the level of the liquid in the tank exerts pressure on the bottom of the diaphragm causing it to flex upward. This action compresses the gas in the box and the gage line. The pressure is applied to a gage or other pressure element that is part of an indicator assembly calibrated to indicate liquid level units. As the level in the vessel rises, the pressure exerted by the hydrostatic head on the diaphragm increases in direct proportion. The diaphragm will continue to flex until the pressure of the gas in the box and the gage line is equal to the pressure exerted by the level of the liquid on the bottom of the diaphragm.

Diaphragm Box


Diaphragm Box

Diaphragm Box


Diaphragm-box system used for liquid-level measurement in open vessels. (a) Open-type diaphragm box submerged in the liquid, used for measurement of noncorrosive fluids under condition that permit installation of the box in the vessel approximately the minimum level. The box must be at least 2 or 3 in (5 to 7.5 cm) above any sediment in the vessel bottom to avoid clogging of openings on the box. (b) Closed-type diaphragm box located outside the tank, used where the liquid is noncorrosive but conditions do not permit installation within the vessel. (c) Closed-type diaphragm box for use with corrosive liquids.

Diaphragm Box


Force-balance diaphragm system for open vessels

Diaphragm Box


It is essential to the accurate operation of the diaphragm box that the box and connecting gage line be free of leaks. If the gas inside the tube leaked, there would be less increase in gas pressure with an increase in liquid level, and the indications would be inaccurate.

To place the diaphragm box in operation, the liquid level is lowered to zero or the same level as the bottom of the diaphragm. Then, the gas in the diaphragm box, the gage line, and the pressure element are all at atmospheric pressure.

Diaphragm Box


Disadvantages:

• The diaphragm is in contact with the process liquid. This may preclude its use in corrosive process applications.

• A torn diaphragm can admit gas into a process liquid. In some instances, this can be detrimental to the process.

• The system is also sensitive to changes in volume and pressure caused by changes in temperature.

Diaphragm Box


Hydrostatic level



Capacitive Plates(gold plated)

Hybrid, surface mount electronicswith temperature correction

Atmospheric vent hole. Air space evacuated and hole sealed for absolute pressure

Force (pressure)

As the applied pressure changes, the ceramic wafer flexes. Thus varying the distance between the capacitance plates (deflection at full range is 0.025mm) causing a change in capacitance.

Increase ceramic thickness to increase range. (high pressures are smaller diameter)

Sensor’s electronics convert deflection in to output of 1 to 4V, 4-20mA is produced from main PCB which can be integral or remote.

Operating principle- Ceramic capacitive sensing cell



Positive seating(metal to metal)

Double CableGland

Strain reliefgland

Vented cable

‘O’ Ring

Ceramic Sensor

Sensor Head Construction



Pole mounted

Cablesuspended

Flanged

Mounting Options

Clamped,Cablesuspended



Applications examples for hydrostatic level transmitters

Indicator

Cable or pole mounted




A variation of the diaphragm box system uses an air- trap sensor, or inverted bell, instead of a diaphragm box. As the liquid level rises, the hydrostatic head forces liquid up into the bell. As the level of the liquid rises, it compresses the air trapped in the bell and the gage line until an equilibrium between the air pressure and the pressure exerted by the hydrostatic head is reached. The pressure of the compressed air can be used to determine level.

Air trapped method

Air-Trap Sensors


Advantages:

•This design is useful where extreme operating temperatures or corrosive fluid applications might damage a diaphragm.

• Although air trapped in the bell and the gage line is more prevalent at high temperatures, it does affect the accuracy of the system and in many cases precludes its use.

Advantage and Disadvantage:

Disadvantage:

• One distinct disadvantage to this system is the loss of air trapped in the bell and the gage line due to air absorption by the process liquid.

Air-Trap Sensors


Air Bubble or Surge Tube



If we lengthen the straw and measure pressure at all points throughout its length, it will be the same as the pressure at the submerged tip of the straw (assuming negligible friction between the moving air molecules and the straw’s interior walls:



This is an alternative to variations of the diaphragm system. It is known by various names, including an air bubble, a surge tube, an air purge and a dip tube. This type of system uses a continuous air supply that is connected to a tube that extends into the tank to a point that represents the minimum level line.

An air regulator controls airflow. It increases airflow to the tube until all liquid is forced from the tube. At this pressure and flow rate, the air begins to bubble out of the bottom of the tube. This indicates that the air pressure forcing the liquid out of the tube is equal to the hydrostatic head produced by the height of the process liquid being forced into the tube. The air pressure acting against the hydrostatic head provides the pressure indication to the gage.



To minimize the need to visually inspect the tank for the presence of bubbles, a sight device is usually installed in the air line leading to the tank to monitor air flow. It can be assumed that air is being forced through the tube if there are bubbles in the sight glass. When the liquid head pressure and the air supply pressure are equalized, as can be determined by constant bubbling in the sight device, the pressure on the airline can be used to provide a level measurement.

Constant air flow at the correct pressure must be maintained to produce bubbling. If there is an air flow restriction in the tube, it can create a back pressure on the gage which results in a false measurement reading.



Bubbler System (Open Tank)

Gas or air purge system for measurement of the liquid level in

open vessels.



Hydrostatic - Bubbler




A key detail of a bubble tube system is to provide a means of limiting gas flow through the tube so the purge gas backpressure properly reflects hydrostatic pressure at the end of the tube with no additional pressure due to frictional losses of purge flow through the length of the tube. Most bubble systems, therefore, are provided with some means of monitoring purge gas flow, typically with a rotameter or with sightfeed bubbler:


Bubbler System (Open Tank)



BubblerSystem

(Closed Tank)

Closed-Tank Applications

This can be a differential pressure transmitter

Purge gas (Nitrogen)


As with all purged systems, certain criteria must be met for successful operation. Listed here are some of them:

•The purge gas supply must be reliable. If the flow stops for any reason, the level measurement will cease to be accurate, and the dip tube may even plug with debris.

•The purge gas supply pressure must exceed the hydrostatic pressure at all times, or else the level measurement range will fall below the actual liquid level.

•The purge gas flow must be maintained at a low rate, to avoid pressure drop errors (i.e. excess pressure measured fue to friction at the purge gas through the tube).

•The purge gas must not adversely react with the process.

•The purge gas must not contaminate the process.

•The purge gas must be reasonable in cost, since it will be continuously consumed over time.



In open tanks, measurements are referenced to atmospheric pressure. At atmospheric pressure, the pressure on the surface of the liquid is equal to the pressure on the reference side of the pressure element in the measuring instrument. When atmospheric pressure changes, the change is equal on both the surface of the liquid and the reference side of the measuring element. The result is that no change occurs in the measurement value.

In closed processes, pressure will exert an additional force on the surface of the liquid. Pressure applied to the level surface reacts according to Boyle's Law and has an undiminished effect on the pressure exerted on all surfaces in contact with the liquid. Thus, changes in pressure affect the measurement regardless of where the measurement is taken.



As with all purged systems, certain criteria must be met for successful operation. Listed here are some of them:

•The purge gas supply must be reliable. If the flow stops for any reason, the level measurement will cease to be accurate, and the dip tube may even plug with debris.

•The purge gas supply pressure must exceed the hydrostatic pressure at all times, or else the level measurement range will fall below the actual liquid level.

•The purge gas flow must be maintained at a low rate, to avoid pressure drop errors (i.e. excess pressure measured fue to friction at the purge gas through the tube).

•The purge gas must not adversely react with the process.

•The purge gas must not contaminate the process.

•The purge gas must be reasonable in cost, since it will be continuously consumed over time.



Limitation:

However, as with other hydrostatic pressure systems, the major limitation of these systems is that they are generally limited to open-tank applications.


Advantage:

An important advantage to the bubble system is that the measuring device can be mounted at any location and elevation with respect to the tank. This is most useful for applications such as underground tanks and water wells.



Optional


Compensation on the effect of pressure variation in close tanks: To compensate for the effects on level measurement caused by such pressure variations in closed-tank applications, a differential pressure (d/p) cell is often used to measure and indicate level. A typical application is shown in (a). In this application, the d/p cell only responds to differences in pressure applied to two measuring taps. Pressure taps are connected to opposite sides of the d/p cell. One pressure tap is the measuring point on the tank, which is usually below ft minimum level point for the liquid. The other tap is usually located near the top of the tank. The tap in the liquid region of the tank is referred to as the high-side; the other tap, located above the level of the liquid, is referred to as the low- side. System pressure is sensed by both the high and low sides. In addition to system pressure, the high side also senses the pressure exerted by the height of the liquid. Since both sides are exposed to the same system pressure, the effects of system pressure are cancelled and the differential pressure cell only indicates liquid level.



Factor affecting accuracy: One potential problem with the differential pressure method of measuring level is that vapor may condense in the space above the surface of the liquid and fill the low pressure tap of the d/p cell with liquid (wet leg problem). This can affect accuracy because the condensed liquid produces its own static head, which causes the d/p instrument reading to be inaccurate. For example, it would read below zero when there is no liquid in the tank.



Solutions to the Wet Leg Problem:

•The instrument can be calibrated to compensate for the additional static pressure created by the condensed liquid. This compensation or adjustment is called zero elevation.

• The low pressure leg is deliberately filled with liquid. This is referred to as a wet leg installation.

• The use of a device called a pressure repeater or one-to-one relay. The repeater is installed at the top of the tank and linked by pipe to an air relay. The pressure in the tank actuates the air relay, which is connected to an air supply. When the pressure in the tank increases, the relay increases the air pressure on the low-pressure leg. The relay regulates the air pressure so that it is equal to that of the tank pressure. When the pressure in the tank decreases, the relay vents air from the low pressure leg to maintain the equilibrium.



For corrosive, viscous, or slurry-type process liquids, d/p cells are available with diaphragm capsules that protect the cell from corrosion and plugging. The diaphragm is made of corrosion-resistant material. The high side of the d/p cell is flanged directly to the process vessel. The diaphragm actually serves a dual purpose since it also prevents solid material in the bottom of the tank from clogging the differential pressure cell.

In some applications, it may become necessary to locate the differential pressure measuring device at a level below the reference point on the tank. The head pressure caused by the liquid in the connecting tap from the zero point to the high side pressure tap will cause a measurement indication on the instrument when there is no liquid in the tank. To account for this, the zero must be suppressed. This operation, known as zero suppression, is the correction adjustment required to compensate for error caused by the mounting position of the instrument with respect to the level measurement reference.



Pressurized Fluids(Non-condensing Vapor)

(a) Differential Pressure Cell in

Close tank



Piping arrangement sometimes used for the differential-pressure method of measuring the liquid level in a closed tank with a noncondensable atmosphere.



Pressurized Fluids(Condensing Vapor)



Piping arrangement sometimes used for the differential-pressure method of measuring the liquid level in a closed tank with a condensable atmosphere.



Ultra Sonic and Sonic Detector


Ultrasonic sensors (also known as tranceivers when they both send and receive) work on a principle similar to radar or sonar which evaluate attributes of a target by interpreting the echoes from radio or sound waves respectively. Ultrasonic sensors generate high frequency sound waves and evaluate the echo which is received back by the sensor. Sensors calculate the time interval between sending the signal and receiving the echo to determine the distance to an object.

Systems typically use a transducer which generates sound waves in the ultrasonic range, above 20,000 hertz, by turning electrical energy into sound, then upon receiving the echo turn the sound waves into electrical energy which can be measured and displayed.

The technology is limited by the shapes of surfaces and the density or consistency of the material. For example foam on the surface of a fluid in a tank could distort a reading.


This technology can be used for measuring: wind speed and direction (anemometer), fullness of a tank and speed through air or water. For measuring speed or direction a device uses multiple detectors and calculates the speed from the relative distances to particulates in the air or water. To measure the amount of liquid in a tank, the sensor measures the distance to the surface of the fluid. Further applications include: humidifiers, sonar, medical ultrasonography, burglar alarms and non-destructive testing.


Introduction

In level measuring applications where it is not acceptable for the measuring instrument to contact the process material, it may be feasible to use a sonic or ultrasonic device. These devices measure the distance from one point in the vessel, which is usually a reference point, to the level interface.

Ultrasonic Level Measurement


Introduction

Sonic and ultrasonic devices operate on the echo principle. When sound waves contact solid or liquid surfaces, only a small proportion of the sound energy in the wave penetrates the surface. Most of the sound is reflected. The reflected sound wave is an echo. A sound wave is usually generated at a frequency of about 1 to 20 kHz.

Ultrasonic devices differ from sonic devices according to their operating frequency range. For ultrasonic instruments that range is around 20 kHz; for sonic instruments, the operating range is approximately 10 kHz or below. These instruments can be used to detect continuous and point measurement. Continuous level detector designs are categorized as under-liquid sensors and above-liquid sensors. Point detectors can be used to measure gas-liquid, liquid-liquid, liquid-foam and solid-gas interfaces.



What is Ultrasonic?What is Ultrasonic?

Humans: 20Hz to 20KHz Pigeons: down to 0.1 Hz Dogs: up to 40,000 Hz Cats: 100 to 60,000 Hz Bats: 1,000 to 100,000 Hz Dolphins: up to 150,000 Hz



Operating Principle:

Sonic conductivity can be precisely defined for any particular substance under a given set of conditions. In large measure, sonic conductivity is dependent upon the density of the substance and the characteristics of its surface. Sound waves tend to pass through most gases. The waves lose their kinetic energy through friction and are gradually absorbed. Liquid surfaces are highly reflective. Most liquids reflect more sound than gases, but absorb more than solids. Solid surfaces are also highly reflective. Substances that tend to reflect sound waves are referred to as live media. Substances that do not reflect sound waves are called dead media. Sonic level measuring instruments are dependent on the sound wave striking a live medium and reflecting the wave. Dead media absorb most of the sound energy. Consequently, the reflection or echo is too slight to produce reliable results.




The sound absorption coefficient of a material is defined as:

D = Sound energy absorbed / Sound Energy incident on surface

The value is strongly dependent upon the frequency of the sound wave. It is also influenced by certain properties of the medium such as surface porosity, material thickness and rigidity.




The importance of the absorption coefficient becomes apparent when it is understood that the interruption or detection of the generated sound waves is the basis for point detection. For continuous level measurement applications, measurements are based on the time that elapses from the generation of the sound wave to the detection of the reflected wave. Most particularly for continuous measurements, the operation of the echo method depends on the transmitted sound wave being reflected at the interface of the media.




Continuous ultrasonic level measurement systems are designed to measure the time of flight of a reflected sound wave. In other words, it measures the time required for an ultrasonic sound wave to travel to the process surface and be reflected back to the receiver. The generic name for such devices is sonar.

Classification. Continuous Measurement


Ultrasonic (20-50 Khz)Sonic (9500 Hz)

Ultrasonic Level Measurement Principles


Variations of Sound Speed:

The speed of sound, traveling through various substances, changes with the media and with the physical parameters of a medium. For example, if the medium is air, sound travels at a velocity of 1129 ft./sec., when the air is at 20ºC. If the medium were alcohol, the speed at which sound travels at the same temperature is considerably higher: 3890 ft/sec. A change in the physical parameters will also affect the speed of sound. This is especially true with temperature. Increasing the temperature of the air from 20ºC to 100ºC, increases the velocity at which sound travels to 1266 ft/sec. Provided that information on the temperature and composition of process materials is known, level measurements can be determined by measuring the time of flight.



Physics of soundPhysics of sound

Piezo-electric crystal

Frequency 50kHz

Beam angle:+/- 6 degrees Half-Power Point



The TechnologyThe Technology

Plastic Cup Moulding

Piezo-Electric Crystal

Cable to PCB

Matching Layer

Potting

Cork disc



The transducer is based on a Piezoelectric crystal being ‘pinged’ by a sharp voltage pulse of size up to +1000V, causing a vibrational pulse to be transmitted. The vibration is transmitted into the atmosphere. A ‘window’ is used between the crystal and the atmosphere for Reflection and Transmission coefficients.

The aim is to reduce the change step changes in acoustic impedance between the source and the medium you are using.

– The more gradual the change, the lower the energy reflected and the greater the energy transmitted. (crystal to air)

– The sharper the change, the greater the energy reflected, and the lower the energy transmitted. (air to liquid)

Transmission of Ultrasonic Signal



Typical Echo Trace

Time

Sig

na

l

Ringing

Dead Zone

Round trip time

Multiple echoTrue echo from liquid surface



The instrument itself consists of an electronics module containing all the power, computation, and signal processing circuits; plus an ultrasonic transducer to send and receive the sound waves. This transducer is typically piezoelectric in nature, being the equivalent of a very high-frequency audio speakers. The following photograph show a typical electronics module (left) and sonic transducer (right).

Classification. Point Measurement


Physics of sound - Temperature EffectPhysics of sound - Temperature Effect

1 1 °C change of temperature means 0.6 m/s change in Speed of Sound °C change of temperature means 0.6 m/s change in Speed of Sound which is equal to 0.18% change in measured distancewhich is equal to 0.18% change in measured distance

Example (at 20 Example (at 20 °C)°C)::

Speed of Sound = 343 meter / secondRound trip time = 20 milliseconds

Distance to surface = 343 x (0.020 / 2)Distance to surface = 3.43 meter

Example (at 22 Example (at 22 °C)°C) : :

Speed of Sound = 344.2 meter / secondRound trip time = 20 milliseconds

Distance to surface = 344.2 x (0.020 / 2)Distance to surface = 3.442 meter

3.43 – 3.442 = 0.012 meter = 1.2 cm / ~0.5”



Principle of operation - 1Principle of operation - 1

Distance to surface

= Speed x (Round Trip Time / 2)

ExampleExample:

Speed of Sound = 343 meter/second

Round trip time = 20 milliseconds

Distance to surface = 343 x (0.020 / 2)

Distance to surface = 3.43 meter


Bottom reference

Reference Height

Level = Ref.Height – Distance

Non-contacting Level Measurement;Non-contacting Level Measurement;

Distance to surface

Principle of operation - 2Principle of operation - 2

Reference: Transmitter Face



Principle of operation - 3Principle of operation - 3Dead Band :No measurements are possible in this area.

Blanking Distance



Sensor Mounting:

In continuous measurement applications, the sensors must be mounted and aimed to provide the most direct path. The transmitter and receiver may be enclosed together in one device, which can be mounted at the bottom or top of the process vessel. For solid level applications, the sensors are mounted above the medium to be measured. The measurement range can vary from 6 inches to 100 feet, depending on the application.

Continuous level detector designs are categorized as under-liquid sensors and above-liquid sensors.



Continuous sonic-type level measuring units. (a) Liquid phase, (b) vapor phase



General Mounting TipsGeneral Mounting TipsAvoid near wall effectsAvoid turbulence & splashing Align vertically Do not mount to the centre

of a domed tankProtect from excessive heatUse plastic flange or thread



Nozzle length

Nozzle diameter

size

Distance from

tank wall



Using either type of system, point measurement is based on the detection or non-detection of a transmitted sound wave. Point measurement devices, which are commonly used to measure liquid-gas or liquid-liquid interfaces, are typically categorized as single-sensor element or two-sensor systems. Point detectors can also be used to measure liquid-foam and solid-gas interfaces.




Common type of single-element sonic system made up of a transmitter and a receiver. An air gap separates the transmitter from the receiver. The transmitter generates sound waves in the ultrasonic frequency range and the receiver detects these waves. The transmitter and receiver are contained in a single probe.

When the gap between the transmitter and the receiver is filled with air or vapor, the sound waves or beams will be transmitted to the receiver, although the dead media may absorb some of the energy. The strength of the ultrasonic signal increases when the process level rises and the gap is filled with liquid or live media. The signal can then be used to energize a relay to operate an alarm or control device.


Single Element Sonic System



Alternative configuration is where the transmitter and receiver are positioned on opposite sides of the tank.


Single Element Sonic System


Single-sensor sonic-type liquid-level indicator using side-mounted sonic probe. (Delavan Industrial Controls.)



Single probe level detector



Problem areas for contact transmitters

Changing specific gravity of liquid

Corrosive properties of liquid

Hygienic considerations

Sludge build-up/ deposits

Problem areas for non-contact ultrasonic transmitters

Vacuum/High Pressure Vapour Levels CO2 & Other Dense Gases Temperature Swings Excessive Foam or Turbulence

Liquid

HydrostaticTransmitter

Ultrasonic transmitter

Remote Programmer for MSP systems

1 2 3 PAR

4 5 6 RCL

7 8 9 OP

C 0 . #

No contact with medium : Corrosion effects minimised Product build-up effects

eliminated

No moving parts Low maintenance

Generally easy to install



Advantages and Disadvantages

Advantages:

• Capability for continuous measurement without contacting the process material.

• Minimal maintenance requirements.

• Freedom from concern regarding the impact of process variables on measurement accuracy. The temperature and the consistency of the process material are generally the only factors that must be considered.



Advantages

Essentially no moving parts Utilizes solid-state circuitry requiring little

maintenance Accuracy good, where application is suitable Applicable to some difficult-to-measure streams

such as powders, solids, solids-containing fluids and slurries

Easy to install



Disadvantages:

• More expensive and sophisticated than the more conventional measuring systems.

• Ultrasonic techniques are generally reserved for those applications in which the use of conventional systems would present serious difficulties and yield less successful results.



Disadvantages

Insufficient application data Tendency to bridge for some sensor types

and for some materials Relatively high cost Difficulty in fixing distance between

transmitting and receiving units in two-element systems – this objection is overcome by fixed distances in some models



Wet-well installationsWet-well installations

Applications


Open Channel FlowOpen Channel Flow

Applications


“V” Notch weir installations

Applications


Flume installations

Applications


Typical inlet to a flume

Applications


Radiowave Type Level Sensors


WHAT IS RADAR?

Radar, an acronym for the phrase radio detection and ranging as applied to military use in the 1930s to detect the presence of aircraft.

Useful Applications

- the detection of storms to help pilots and weather forecasters avoidand predict bad weather,

- the use by law enforcement to measure the speed of an automobile, and

- the use of radar on small boats as a safety and navigation aid.

- microwave oven. Radar devices and microwave ovens both produce electromagnetic waves. However, that is where the similarity stops.Whereas a microwave oven uses an electromagnetic wave to heat foods, radar typically uses an electromagnetic wave to determine distance and speed.

Radar Level Measurement


What is micro wave ?

26 GHz

10 GHz

6 GHz1

10100

HzHzHz

110

100

kHzkHzkHz

5,8 / 6,3 GHz

10 GHz

24 GHz

110

100

MHzMHzMHz

- (10 Hz)15

- (10 Hz)12

- (10 Hz)18

- (10 Hz)21

- (10 Hz)24

110

1001

10100

110

1001

10100

TV- (10 Hz)8

110

100

GHzGHzGHz

110

100

THzTHzTHz

TelephoneAC current

Radio waves

TV

Microwaves

Heat radiation

Visible light

Ultra violet

X-rays

Gamma wavesCosmic radiation

Kilometer

Meter

DecimeterCentimeterMillimeter

Micrometer

Nanometer

Picometer

Femtometer

Frequency Wave length

110

100

GHzGHzGHz

DecimeterCentimeterMillimeter



Electromagnetic Spectrum.


Radar/Microwave

Uses Time of Flight technology of radar

and microwave signals which is used for level

measurement.



What is the principle ?

c :Speed of micro waved :Distancet :Time

c = d = c • 2dt

t2

d



RADAR SIGNAL CHARACTERISTICS

A basic principle of radar is its capability to reflect off the surface of materials based on the material’s dielectric constant. The dielectric constant of a material is directly proportional to the amount of electromagnetic energy that reflects from it. A vacuum has a dielectric constant of 1.0, which means it does not reflect a radar signal. Any material that has a dielectric constant greater than 1.8, such as water, crude oil, or ammonia, will easily reflect radar signals.

The higher the dielectric constant of the material, the more signal that is reflected and available for level measurement. On the other hand, radar signals tend to pass through materials that have a dielectric constant less than 1.8, such as air, vapor, certain gases, or foam. That is why radar is an excellent technology for measuring the level of a material in a tank; the air, vapor, or foam has minimal effect on level measurements, as compared with other level measurement technologies. In addition, changes in dielectric constants caused by changes in temperature or pressure have a minimal effect on the signal.



Radar gauges determine the level of a product in a tank by measuring the ullage (also called “outage”). The ullage, or vapor space, is the distance from the radar gauge mounting location to the surface of the material. The level measurement is determined by subtracting the ullage from the distance the radar gauge is mounted above the tank bottom (or reference gauge height.)

Radar Measurement



The reference gauge height is the fixed distance from the bottom of the tank (or strike plate) to the face of the radar gauge mounting flange. Thereference gauge height establishes a reference point from which all level and calibration measurements are taken.



This photo shows the Tank Gauge Unit from inside the tank. It is perfectly safe to enter the tank while the Emerson Process Management Marine Solutions equipment is in operation. It is also safe to handle the Gauges while they are in operation, since the transmitter power is so low.



This gauge is still operating and gauging accurately despite the heavy contamination. The parabolic antenna has for several months been exposed to blown bitumen heated to 160 ºC (320 ºF)



BASIC RADAR GAUGE COMPONENTS

The radar electronics is the heart of the radar gauge. It produces an electromagnetic wave by using an oscillator that converts direct-current (dc) power into a microwave frequency or signal. It also receives the return signal

The signal passes from the electronics through a waveguide. The waveguide is the entire path from the electronics to the antenna.

The antenna is typically a cone shaped device made of noncorrosive stainless steel. The antenna controls the signal beam width by helping to keep the signal focused on its target so it doesn’t spread out over the entire tank and give false echoes. The size of the antenna is inversely proportional to the frequency; the higher the frequency the smallerthe antenna



BASIC RADAR GAUGE COMPONENTS

At the top of a typical radar gauge, is the gauge housing. The gauge housing includes specially designed electronics for signal processing.



Antenna


Antenna. Horn


The transition of microwaves from the low dielectric waveguide into the metallic horn where they are focused towards the product being measured

Antenna. Horn


Antenna. Horn


Antenna. Parabolic


Extended waveguide horn antenna to enable measurement in long nozzles or through a concrete tank or sump roof

Waveguide


Waveguide


Rex gauges are installed using existing nozzles and manways.



WHY FREQUENCY IS IMPORTANT

Radar gauges that operate in the 24 GHz frequency range provide significant advantages over lower frequency gauges. One advantage relates to beam width. The width of a radar beam is determined byusing the formula:

70 x speed of lightbeam angle = ---------------------------------------- frequency x antenna diameter




To illustrate, assume the height of a tank, or Reference Gauge Height, is 20 feet (6.1 m).

Frequency Beam Width Example.




Using the previous formula yields the following results for 5, 10 and 24 GHz frequencies (Table 1).

Table 1 shows that the lower frequencies (A, B) that use a 4-inch antenna transmit a wider beam; at 24 GHz (C), beam width is only four feet. To achieve an equal beam width, the lower frequencies require a much larger diameter antenna (D, E).

The narrower beam width helps to reduce unwanted signals from vessel obstructions such as agitators, heat exchangers, filling pipes, baffles, thermowells, intermittent filling streams, and other obstructions.




It allows greater flexibility in mounting the gauge on existing flanges located close to tank walls, without any problems.

Another advantage of the 24 GHz frequency relates to the use of a smaller, and thus lighter weight, antenna. A smaller and lighter weight unit makes it easier to transport and install on top of the tank. Inaddition, the antenna more easily accommodates existing small flanges.



RADAR GAUGE APPLICATIONS

Radar gauges are a non-contacting method of measuring level. The gauges provide an attractive alternative in processes where a standard insertion device becomes fouled or corroded.

The gauges are insensitive to many problematic liquid characteristics such as changing density, dielectric, or conductivity.



Guided wave radar type level instrument

Free space radar type level instrument (non-contact radar)

Rope radar type level instrument

Radar Type


Radar level instruments send radio waves out through open space to reflect off the process material. The instruments relying on open space for signal propagation are called non-contact radar.

Radar Type. Non-contact


Radar Type. Non-contact


Radar level instruments use waveguide “probes” to guide the radio waves into the process liquid. The instruments using waveguides are called guided-wave radar instruments.

Radar Type. Guided-wave Radar (GWR)


Transition ZonesThese zones are near the extremes of the probe length. Measurements of liquid level or interface level within these zones may not be accurate or even linearly responsive. Thus, it is strongly advised to range the instrument in such a way that the lower- and upper-range values (LRV and URV) lie between the transition zones:

The size of these transition zones depends on both the process substances and the probe type. The instrument manufacturer will provide you with appropriate data for determining transition zone dimensions.

Radar Type. Guided-wave Radar


What is the probe ?

+ =



What is the probe ? Radar waves are always guided along the rope -

giving strong reflection signals



What is the difference ?

Free radiating radar wavescan be deflected

Whereas guided radar waves are always bounced back directly



What is the difference ?

Rod probes: measuring ranges

up to 4 m independent from

turbulent surface ideal mounting in

bypass or stilling well

very low influence by obstacles fixtures and fittings, if the dist. is 300 mm

lateral force:30 Nm

Coaxial probes:

independent from mountingno influences from nozzles- no influences by obstacles, fixtures and fittings in vessel

lateral force: 300 Nm

Rope probes:

measuring ranges> 4 m

at narrow installationconditions, e.g.low ceiling space



WHICH RADAR TO USE: GUIDED WAVE OR NON-CONTACTING?

Although non-contacting radar works well in pipe applications, contacting or guided wave radar (GWR) may be a simpler choice. Non-contacting radar must meet certain installation requirements for optimum results. The guided wave radar has simpler installation requirements and provides better performance than non-contacting radar. GWR can maintain its accuracy and sensitivity independently of the pipe.

Guided Wave Radar is the preferred technology for shorter installations where rigid probes may be used. This makes it a suitable replacement for caged displacers, which are often less than 10 ft. (3 m). The probes are available in a variety of materials to handle corrosive fluids. For taller applications or those with limited headspace for installing rigid probes, non-contacting radar may be advantageous. Non-contacting radar is also the preferred technology for applications with heavy deposition or very sticky and viscous fluids.



ADVANTAGES OF USING BYPASS PIPES AND STILLING WELLS

Pipes offer a calmer, cleaner surface A pipe can increase the reliability and robustness of the level measurement, especially for non-contacting radar. It should be noted that the coaxial probe of a guided wave radar is essentially a probe within a small stilling well. It should be considered as an alternative to stilling wells for clean fluid applications.

Pipes eliminate issues with disturbing obstacles. Pipes completely isolate the transmitter from disturbances such as other pipes, agitation, fluid flow, foam and other objects. The pipes can be located anywhere in the vessel that allows access. For GWR, the microwave signals are guided by the probe, making it resistant to disturbing objects.

Pipes may be more accessible to the area of interest Bypass pipes may be located on a small portion of a tank or column and allow access to the measurement instrument. This may be especially important for interface measurements near the bottom of a taller vessel or for measurements in a distillation column.



Example of a bypass mount (left) and a stilling well mount (right).



Applications

Applications


Bulk-Solid Level

The level measurement of solid materials has been a frustrating experience for many users. The number of available technologies suitable for solids measurements is limited. Radar, ultrasonic, nuclear capacitance, phase shift, weigh cells, and mechanical devices are the most common technologies found in these applications. Of the technologies available, all have varying results and many restrictions.



Why are solids so difficult to measure?

• The surface of solid materials is rarely flat or horizontal. The angle of repose, or surface inclination, will change as the vessel fills and empties.

• The dielectric value of most solids is fairly low. For radar, this is a key indicator of the amount of signal that will be reflected back to the gauge.

• There is often a lot of dust during the fill cycle. While radar can handle this dust fairly well, ultrasonic devices cannot.

• Heavier materials can create a pull force that can break cables. While this is more likely to be an issue in vessels taller than about 50 ft. (15 m), care must be taken to guard against this possibility.

• The weight of the material may push cables towards other structures in the tank, causing false targets.




Radar gauges are a non-contacting method of measuring level. The gauges provide an attractive alternative in processes where a standard insertion device becomes fouled or corroded.

The gauges are insensitive to many problematic liquid characteristics such as changing density, dielectric, or conductivity.




radar gauges effectively overcome the difficulties of level measurement problems, such as:

• agitation• density changes• vessel obstructions• temperature changes• vapors• condensates• changing dielectric

When selecting a level measurement technology, consider the reliability, accuracy, and versatility of radar technology.



Radar technology does offer distinct advantages over other technologies. Radar signals can penetrate vapor spaces containing dust or steam which is problematic for ultrasonic devices. Non-contacting radar eliminates the breaking and pushing issues associated with technologies that use probes or other mechanical structures. In addition, it is not susceptible to mass changes or ambient temperature changes as are load cells. Unlike nuclear technologies, no special licenses or training are needed for radar devices. There are no empty tank requirements during installation for the non-contacting radar.



The main advantages for using radar for tank gauging are:

• Radar waves are extremely robust to any conditions in the tank.

• Radar waves are generally not affected by the atmosphere above the product in the tank

• The only part located inside the tank is the antenna without any moving parts

• High reliability

• High accuracy

• Can easily be serviced and replaced during closed tank conditions



• Radar is an acronym for Radio Detection And Ranging.• Radar is just as safe as the radio, TV, and cellular communication waves that surround us everyday.• The two most common types of radar signals are pulse radar and FMCW.• Radar signals easily reflect from materials that have a dielectric constant greater than 1.8.• Radar gauges determine the level of a product in a tank by measuring the ullage (distance to the product surface).• The basic components of a radar gauge include: gauge housing, electronics, mounting flange, waveguide, and antenna.• Radar gauges that operate in the 24 GHz frequency range are smaller and lighter weight than gauges operating at lower frequencies.• A smaller beam width reduces unwanted signals from vessel obstructions.• A smaller antenna and beam width provides easier installation and increased flexibility for mounting on existing flanges.• Radar, in most situations, is impervious to conditions such as agitation, obstructions, vapors, temperature and density changes, condensates, andchanging dielectric.• Because there are no moving parts, radar gauges minimize maintenance costs.

Basic Radar Principle Summary


Nozzle/socket


Extended Waveguide and Curved Waveguide Horn


Measurements of Liquids – Rod Antenna


Nozzle/socket


Rod Antenna Incorrectly Installed in a Nozzle


General Installation Considerations:

Horn and Rod Antennas in Liquid Applications


Parabolic EffectMounting in Dished Topped Vessels


False Reflections


Avoiding False Echoes


Vessel Installations


Build-up


Orientation of the Radar Transmitter on Liquids (Horn or Rod Antenna)


In Flowing Liquid


Sensor Too Close to the Vessel Wall


Measurement Through Plastic Tank Top and Low Dielectric Windows


Measurement of Solids – Horn Antenna


Radiation Type Level Sensors


The trefoil symbol is used to indicate radioactive material.

Radioactivity


Radioactivity


Radioactive decay is the process by which an unstable atomic nucleus spontaneously loses energy by emitting ionizing particles and radiation. This decay, or loss of energy, results in an atom of one type, called the parent nuclide transforming to an atom of a different type, named the daughter nuclide. For example: a carbon-14 atom (the "parent") emits radiation and transforms to a nitrogen-14 atom (the "daughter"). This is a stochastic process on the atomic level, in that it is impossible to predict when a given atom will decay, but given a large number of similar atoms the decay rate, on average, is predictable.

Radioactivity


The SI unit of activity is the becquerel (Bq). One Bq is defined as one transformation (or decay) per second. Since any reasonably-sized sample of radioactive material contains many atoms, a Bq is a tiny measure of activity; amounts on the order of GBq (gigabecquerel, 1 x 109 decays per second) or TBq (terabecquerel, 1 x 1012 decays per second) are commonly used. Another unit of radioactivity is the curie, Ci, which was originally defined as the amount of radium emanation (radon-222) in equilibrium with of one gram of pure radium, isotope Ra-226. At present it is equal, by definition, to the activity of any radionuclide decaying with a disintegration rate of 3.7 × 1010 Bq. The use of Ci is presently discouraged by the SI.

Radioactivity


Radioactivity


This diagram demonstrates the constitution of different kinds of ionizing radiation and their ability to penetrate matter. Alpha particles are stopped by a sheet of paper whilst beta particles halt to an aluminium plate. Gamma radiation is dampened when it penetrates matter. Gamma rays can be stopped from 4 meters of lead. Tungsten and tungsten alloys can stop Gamma radiation with much less mass than lead

Radioactivity


Radioactivity


Nuclear Type Level Sensors

Cesium 137

Nuclear Level Measurement


One of the most effective methods of shielding against gamma ray radiation is with very dense substances such as lead or concrete. That is why the source boxes holding gamma-emitting radioactive pellets are lined with lead, so the radiation escapes only in the direction intended:

These “sources” may be locked out for testing and maintenance by moving a lever that hinges a lead shutter over the “window” of the box. This lead shutter acts as an on/off switch for the radioactive source. The lever actuating the shutter typically has provisions for lock-out/tag-out so a maintenance person may place a padlock on the lever and prevent anyone else from “turning on” the source during maintenance. For point-level (level switch) applications, the source shutter acts as a simple simulator for either full vessel (in the case of a through-vessel installation) or an empty vessel (in the case of a backscatter installation). A full vessel may be simulated for neutron backscatter instruments by placing a sheet of plastic (or other hydrogen-rich substance) between the source box and the process vessel wall.



Radiation-Type Level Sensors

Radiation Source: Cesium 137Holder: Lead/ Depleted UraniumCollimator: Depleted Uranium



Nuclear gauging system needs mounting only on the outside of existing pipes or containers. Courtesy, Control Instrumentation.



Typical installation of a radiation-type detector



Benefits of Nuclear Level Measurements

Low Maintenance & High Reliability

• Noninvasive to Vessel

No exposure to the corrosive, abrasive, high pressure or high temperature process conditions.

• No moving parts

Detectors employ no moving parts to wear, blind, corrode or fall on process.

• Proven Technology

Nuclear measurements have proven reliable over time in thousands of applications.

• Gauge Installed Without Process Shutdown

No alterations required to the vessel interior for gauge installations in standard application.

• No Intrusion into Vessel Required

With no intrusion necessary in a standard installation, no changes to coded vessels are required.

• High First-time Success Rate

With nuclear level systems, there is a high success rate on the first installation. Time and money are not wasted trying multiple level technologies.



Solves Difficult & Extreme Applications

• High Temperature

Process temperature has no effect on measurement.

• High Pressure or Vacuum

Measures in all process pressures.

• Corrosive

Non-contact technology allows use in the most aggressive services.

• Volatile & Biohazard

Non-invasive nature eliminates process connections and leak paths.

• Agitator, Baffles, Coils & Other Obstacles

Non-contact technology allows use in the most aggressive services.



Universal Solution

• Applicable Across Virtually All Industries, Materials and Processes

Nuclear gauges use radiation as a means to make measurements, but this radiation does not affect the process material that are being measured.

• No Effect On Process Materials

Will not change product being measured, even foods and pharmaceuticals.

• High Accuracy & Repeatibility

When properly installed, accurate to ± 1% or better.


Nuclear Continuous Level and Point Measurements

Nuclear Continuous Level & Point Level Detection/Measurement


Nuclear continuous level measurement works by directing a narrow fan of radiation through the vessel to a detector. As the process level rises, it shields the detector from the from the radiation. The more radiation the detector “sees”, the lower the process level (discernable to 1% of span). The less radiation detected, the higher the process level.

Nuclear Continuous Level Measurement


Ionization is the physical process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. The process works slightly differently depending on whether an ion with a positive or a negative electric charge is being produced. A positively-charged ion is produced when an electron bonded to an atom (or molecule) absorbs enough energy to escape from the electric potential barrier that originally confined it, thus breaking the bond and freeing it to move. The amount of energy required is called the ionization potential. A negatively-charged ion is produced when a free electron collides with an atom and is subsequently caught inside the electric potential barrier, releasing any excess energy.



Ionization chamber Device for detection of ionizing radiation by measuring the electric current generated when radiation ionizes the gas in the chamber and therefore makes it electrically conductive.



Radioactive Source

Radioactive Detector (ionization chamber)



Radioactive Source Holders



Gamma-ray emission-type gage. Radiation source is in float. (Ohmart.)

Typical application of an Ohmart cell for level measurement. (Ohmart.)



Detector Types. Ion Chamber Detectors and Scintillator Detector



Continuous Level Detectors



Level gauges similar to the one shown above, have been in service since the 1970’s in a paper mill located in Midwester United States.



Applications. Continuous Level


Point level detection uses a focused beam of radiation directed at a small detector, that senses the presence or absence of the beam. When the beam is blocked by the process, the detector sees the radiation from the source. Point level detectors can be set as either high or low alarms.

Nuclear Point Level Measurement

Radioactive Source

Radioactive Detector (Geiger-Muller, GM, switch)


Geiger-Muller counter

This instrument contains an easily ionized gas. If a radioactive particle penetrates the tube, across with a high potential difference is applied. This discharge is registered on the counter, and in this way the instrument counts the number of particles which penetrate the tube through 'window‘ in a specific time.



Geiger Counter or Geiger Tube

An instrument for detecting the presence of and measuring ionizing radiation such as alpha particles, beta particles, and gamma rays. A Geiger-Müller counter an count individual particles at rates up to about 10,000 per second and is used widely in medicine and in prospecting for radioactive ores.

The device is named for Hans Geiger who invented it in 1908, and Walther Müller who collaborated with Geiger in developing it further in 1928.



A fine-wire anode runs along the axis of a metal cylinder which has sealed insulating ends, contains a mixture of argon or neon and methane at low pressure, and acts as the cathode, the potential between them being about 1,000 volts. Particles entering through a thin window cause ionization in the gas; electrons build up around the anode and a momentary drop in the inter-electrode potential occurs which appears as a voltage pulse in an associated counting circuit. The methane quenches the ionization, leaving the counter ready to detect further incoming particles.



The detectors for a radiation-based instrument is by far the most complex and expensive component of the system. Many different detectors design exist, the most common being the ionization tubes such as the Geiger-Muller (G-M) tube. In such devices, a thin metal wire centered in a metal cylinder sealed and filled with inert gas is energized with high voltage DC. Any ionizing radiation such as alpha, beta, or gamma radiation entering the tube causes gas molecules to ionize, allowing a pulse electric current to travel between the wire and tube wall. A sensitive electronic circuit detect and counts these pulses, with greater pulse rate corresponding to a greater intensity of detected radiation.



Point–level detector for rapid response, non-contact, and high– or low–level alarm systems, as well as for material flow detection. The design permits easy field installation and adjustment. (Ohmart.)



Geiger-Mueller Technology


This unit has the unique option of employing one to six tubes in the detector. With additional tubes, sensitivities can be greatly increased allowing smaller source sizes, reduced radiation fields and measurement in thick walled vessels or over large distances.


Application Configuration for Point Level


Radiation Type – Continuous Measurement

Advantages Sometimes works, when no other method is available External mounting often possible Easy zero check Motor-driven models available for high-accuracy

applications



Radiation Type – Continuous Measurement

Disadvantages – Continuous Measurement Costly to install Requires licensing by regulatory agency Dangerous to handle unless precautions are followed Original calibration and checkout often difficult and

costly Errors caused by density variations in measured

materials Lack of application data Difficult to obtain linear readout over wide ranges Problems presented by materials that coat walls



Radiation Type – Point Detection

Advantages Sometimes work when other methods fail Mounted outside the process

Disadvantages Costly to purchase and install Requires licensing by regulatory agency Dangerous to handle unless precautions are followed Lack of application data Problems presented by buildup of material on vessel

walls


level measurement

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actual level

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certain level

level span

continuous level detection

highlow level detectors

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