level measurement

EI 602 Process Control Instrumentation – I Level Measurement

Vishnu Mohan @ College of Engineering Kidangoor 1

LEVEL MEASUREMENT

In modern manufacturing industries which use many solvents, chemicals, steam and

other liquids and in power plants which use vast amount of water, the accurate measurement

of liquid level is very essential.

A. LIQUID LEVEL MEASUREMENT

Generally, there are two methods used in industries for measuring liquid level.

These are

1. Direct Method

2. Indirect Method

Direct method use the varying level of the liquid as a mean of obtaining the

measurement and the indirect method use a variable that changes with the liquid level to

accurate the measuring mechanism.

1. DIRECT METHOD

This is the simplest method of measuring liquid level. In this method, the level of the

liquid is measured directly by means of the following level indicators

i. Sight Glass / Gauge Glass

ii. Float Type / Float - Operated Level Gauges

iii. Torque Tube Displacer / Float Displacement Type Level Gauges

i. SIGHT GLASS / GAUGE GLASS

Sight glass is used for the continuous indication of liquid level within a tank or vessel.

A sight glass instrument consists of a graduated tube of toughened glass which is connected

to the interior of the tank at the bottom in which the water level is required

Fig 1.i.1 shows a simple sight glass for an open tank in which the liquid level in the

sight glass matches the level of liquid in the tank. As the level of liquid in the tank rises and

falls, the level in the sight glass also rises and falls accordingly. Thus, by measuring the level



in the sight glass, the level of liquid in the tank is measured. In sight glass, it is not necessary

to use the same liquid as in the tank. Any other desired liquid also can be used.

When it is desired to measure a liquid level with the liquid under pressure/vacuum,

the sight glass must be connected at the top as well as at the bottom; otherwise the pressure

difference between the tank and the sight glass would cause false reading. In this case, the

glass tube enclosed in a protective housing and two valves are provided for isolating the

gauge from the tank in case of breakage of sight glass.

Fig 1.i.2 shows a high pressure sight glass in which measurement is made by reading

the position of the liquid level on the calibrated scale. This type of sight glass in high pressure

tanks is used with appropriate safety precautions. The glass tube must have a small inside

diameter and a thick wall.

The standard practice is not to go in for a glass tube of more than 900 mm length. In

case the height of the tank is more than 900 mm, two or more sight glass level gauges are

provided at different levels. This type of gauge is made to withstand pressure of 350 psi of

steam pressure at 252°C (500°F) or 1000 psi liquid pressure.

Advantages

Direct reading is possible

Special designs are available for use up to 316°C and 10000 psi.

Glassless designs are available in numerous materials for corrosion resistance.

Disadvantages

It is read only where the tank is located, which is not always convenient.

Since glasses are located on the outside of the tanks, the liquid in sight glass may

freeze in cold weather even though the liquid inside the tank does not, and thus, it

may cause error in the reading.

Heavy viscous liquids or liquid containing materials which fall out of solution and

clog the tube cannot be measured satisfactorily by a sight glass.

Overlapping gauges are needed for long level spans

Accuracy and readability depend on the cleanliness of glass and fluid.

ii. FLOAT OPERATED LEVEL MEASUREMENT GAUGES

In liquid level measurement by floats, the primary device is a float which by reason of

its buoyancy follows the changing level of the liquid, the movement of the float is transferred

to a pointer through the some suitable mechanism. The various types of floats used are

hollow metal spheres, cylinder shaped ceramic floats and disc-shaped floats of synthetic

materials

In the simplest float operated mechanism (fig 1.ii.1) for continuous direct liquid level

measurement, the float is attached to a cable which is wound around a pulley in which the

indicating pointer is attached. The movement of pointer is read on a scale which is calibrated



in terms of the level of the tank. The advantage of this method is that it is possible to read

from ground level, the level of liquid in tanks which are below the ground level.

Simple modification of this arrangement can be as shown in fig 1.ii.2. It does not

permit a wide range of level measurement, but it does have mechanical advantages that make

it excellent for control and transmitter applications.

In another arrangement shown on fig 1.ii.3, the float is used to move a magnet. As the

magnet moves, it attracts a follower magnet connected by a cable to the indicator, thus

providing a reading of liquid level measurement. A float guide tube is inserted downward

into the vessel and is mounted with flange and gasket to the top of the tank, the lower end of

the tube is closed and the inside of the tube is thus completely sealed off from the tank.

Concentricity of float to tube is maintained by guide surfaces to ensure vertical motion and

proper magnetic relationship between inner and outer magnets. The follower magnet inside

the tube seeks a position corresponding to that of the float, thus moving the cable. The cable

winds on a drum located at the top end of the float guide tube. The cable is kept taut by a

spiral wound spring.

Advantages

It is possible to read the liquid levels in a tank from the ground level even if the tank

is kept below the ground level.

Low cost, reliable design.

Operates over a large temperature range

Choice of corrosion- resistant materials to make these



Disadvantages

Normally limited to moderate pressures

They are tailored to tank geometry

iii. FLOAT DISPLACEMENT TYPE LEVEL MEASUREMENT

These instruments work on the Archimedes principle according to which a body when

placed in a liquid is buoyed up by a force equal to the weight of the displaced liquid, and the

apparent change in weight of the body is directly proportional to the level of liquid in which

it is placed. The body immersed is called a displacer which is usually hollow, cylindrical in

shape, with small amounts of lead shots filled into it. Several types of instruments working on

this principle are available, each differing in the method employed to weigh the float and

convert its weight into liquid-level measurement.

Torque tube is the most commonly used device for this purpose. The displacer is

attached to a torque tube assembly whose rotary motion is used for read out/control. The

torque tube consists of a metal tube with a shaft inside of which is connected with the tabular

floats shown in fig 1.iii.1. The displacer is suspended from the torque tube which restricts its

movements to prevent contact with any part of the vessel in which it is placed.

It is designed so that the weight of the float with liquid in its contact will twist the

tube by a maximum amount in one direction. When liquid level rises and touches the float,

the float will get immersed in liquid and loss weight equal to the weight of the liquid

displaced by it. For maximum level desired, the size of the float and its weight can be

selected suitably, and corresponding to maximum level read the torque tube goes to the other

extreme position. The displacer rod which connects the displacer to the tube is designed to

absorb sideward forces and minimize friction by a knife edge bearing. Backlash is prevented

in the bearing because the displacer always pulls in the same direction on the torque tube. As



the torque tube is tubular, the small rotator shaft which is fastened to the inner end of the tube

can transmit the degree of rotation of the inner end accurately to the outside of the vessel. The

outer end of the tube is casketed and clamped rigidly to the vessel wall. Thus the interior of

the torque tube is at atm. pressure and no packing is required for it.

It is important to note that the apparent change in weight of float is also dependent on

the density of the liquid and the weight of the float. Therefore, it is essential that density of

liquid should not change. Obviously this instrument should not be used in tank containing

slurries, heavy viscous liquids, or liquids which form crystals that will deposit on the float.

Otherwise, this instrument is rugged and simple in construction and reliable in

operation. With selection of suitable material for float, float cage, and torque tube, it’s

possible to use this instrument over a wide range of pressure and for many liquids.

Advantages

High accuracy

Reliable in clean liquids

Can be mounted internally or externally (external mounted unit can be disconnected

for maintenance)

Adaptable to liquid interface measurement

Disadvantages

Limited range, devices exceeding 1.2m in length are bulky and difficult to balance

Cost increases appreciably for externally mounted units as pressure ratings increase

External units may require stilling chambers

2. INDIRECT METHODS

Following are the indirect methods of liquid level measurement generally used in

industries.

i. Hydrostatic pressure type

ii. Electrical methods

iii. Ultrasonic level sensor

i. HYDROSTATIC PRESSURE TYPE

A liquid in tank at rest exert a force on the walls of the tank. This force in a liquid at

rest, is known as hydrostatic pressure, and is proportional to the depth (height) of the liquid in

the tank. Hydrostatic pressure methods used for liquid level measurement are listed below.

a. Pressure gauge method

b. Air purge system

c. Diaphragm box type

d. Torque balance type



a. PRESSURE GAUGE METHOD

This is the simplest method used for liquid level measurement in an open tank.

Hydrostatic pressure of any liquid in an open tank is given by the relation

P - pressure

h - Height of liquid (often called head)

ρ - Density of the liquid

Hydrostatic pressure in a closed tank is given by

Therefore pressure measured at the bottom of the tank containing a liquid of known

density and specific gravity is directly proportional to the height or level of liquid in the tank.

This principle is utilized in pressure gauge instrument for liquid level instrument.

The pressure gauge level indicator consists of a pressure gauge connected at the

lowest level of the tank. The level at which the pressure gauge is fitted is known as the

reference level and the static pressure measured by the gauge is a measure of the height of the

liquid column above the reference level and hence the liquid level. A liquid seal is connected

with the piping on the tank including a shut off valve while measuring a corrosive or highly

viscous liquid. This liquid seal consist of a fluid with which the measuring system is filled.

This filling liquid transmits the pressure head of the measured liquid. The free surface of the

filling liquid is kept in indirect contact with the measured liquid. These two liquids must not

mix or react chemically. Fig 2.i.a.1 shows an open tank pressure indicator for measuring

liquid level.

The location of the pressure gauge should be chosen carefully, since any difference in

elevation above or below the lowest level of the tank will affect the reading.



Disadvantage

The instrument must be mounted at same level as the minimum level in the tank. This

is often inconvenient, as a tank may be located at certain height above the control room. In

this case, the level indicator would show an error equivalent to the height of the tank from

control room.

b. AIR PURGE SYSTEM

Air purge (bubbler tube) is one of the most popular hydrostatic pressure types of

liquid measuring system which is suitable for any liquid as shown in fig 2.i.b.1.

An air purge system consists of a hollow tube inserted in the liquid of the tank. Two

connections are made with the bubbler tube, one to the regulated air supply and the other to a

pressure gauge, calibrated in terms of liquid level. A bubbler is connected in the air supply

line which serves simply as a visual check to the flow of the supply air. A level recorder may

be connected with the pressure gauge to keep the continuous record of liquid level as shown

in fig 2.i.b.1.

When there is no liquid in the tank or the liquid in the tank is below the bottom end of

the bubbler tube and the pressure gauge indicates zero. In other words, if there is no back

pressure because the air escapes to the atmosphere. As the liquid level in the tank increases,

the air flow is restricted by the depth of liquid and the air pressure acting against liquid head

appears as back pressure to the pressure gauge. This back pressure causes the pointer to move

on a scale, calibrated in terms of liquid level. The full range of head pressure can be

registered as level by keeping the air pressure fed to the tube, slightly above the maximum

head in the tank. The range of the device is determined by the length of the tube. Because air

is continuously bubbling from the bottom of the tube, the tank liquid does not enter the

bubbler tube and hence the tube is said to be purging. The common purging fluid is air, but, if

air reacts with the tank fluid or is absorbed, different gases (Ca/N) are chosen depending on

the liquid properties.



Advantages

Pressure gauge can be placed above or below the tank level and can be kept as far

away as 500 ft (12.7m) from the tank with the help of piping.

Well - suited for measuring the corrosive/abrasive liquid.

c. DIAPHRAGM BOX METHOD

The diaphragm box liquid level meter is shown in fig 2.i.c.1 and consist of two

flanges in between which is contained a diaphragm element made of rubber or oil resistant

synthetic composition. The box is immersed in the liquid to be measured and an air filled

capillary extends from it to the instrument. The deflection of the diaphragm produced by the

liquid head causes a compression of the air in the connection capillary. The capillary tubing,

on the other end is connected to a secondary instrument which through the air from the

capillary tube indicates the liquid head exerted by the liquid on the diaphragm. The capillary

connecting tube, which enters the top flange, is extended into the diaphragm box and is bent

to one side, to prevent its being sealed by the diaphragm

An alternative method known as air trap method (fig 2.i.c.2) is used when a

diaphragm box cannot be used. Here a box without diaphragm is inserted into the tank. In

effect the box comprises an iron pipe which is inserted into the tank. The upper end of the

pipe is sealed and connected to the secondary instrument through capillary tubing and the

bottom end is exposed to the liquid in the tank. When the liquid level rises, the air in the

capillary tube is compressed and the instrument responds accordingly. Here it is important

that the liquid should be free from the solids, which otherwise might plug the capillary.



Advantage

Where it is necessary to prevent contact b/w liquid and diaphragm, the box may be

installed in a well outside the tank and the well is communicated to the tank with an impulse

piping. The impulse piping and the well are filled with an inert liquid.

Disadvantage

The main disadvantage is that the head developed is not sufficient to meet up the line

losses as well as for a satisfactory indication. Hence ranges are quite limited.

d. FORCE BALANCE TYPE

Pneumatic force-balance type gauge is shown in fig 2.i.d.1. When the hydrostatic

pressure acts on the bottom side of the diaphragm, the valve port closes sufficiently to

develop a back pressure on the upper side of diaphragm which would be indicated by the

gauge. For hydrostatic pressure due to fixed level in the tank, the diaphragm will come in

equilibrium when pressures on both sides are equal. For any change, the valve port will either

close more or open more and a new balance will be obtained.

The force balance method is well adopted for liquid level control.

ii. ELECTRICAL METHODS OF LEVEL MEASUREMENT

Electrical methods consist in converting liquid levels into electrical signals and

measure by electrical/electronic means. Commonly used electrical methods are

a. Resistance type gauge

b. Inductive level gauge

c. Capacitive type level gauge



a. RESISTANCE TYPE LEVEL GAUGE

The simplest form of the electrical type is the resistive method, a scheme of which is

shown in fig 2.ii.a.1. This is also known as the contact point type. A mercury column is

operated by the liquid column so that the resistance R is measured, decrease with increasing

level. A no. of resistances of suitable values have their one end inserted in the column as

shown in fig. resistances R1, R2,……, Rn are selected so that 1/R is a linear function of the

level. But the level variation gives a stepwise resistance variation.

Application

This method is used if the resistivity of the liquid whose level is to be measured is

very high and the contact sets cannot be mounted in the tank. For a fairly large conductive

liquid the ranges of contact with resistances may be directly mounted in the tank. With

increasing level, more number of probes is shorted by the liquid giving a lower value of

resistance. Such type is used in mineral processing units to distinguish the layers of pulp,

froth, air etc. Also because city supply of water, sewage, or sea water has high conductivity

this method can effectively be used in these cases.



b. INDUCTIVE LEVEL GAUGE

Inductive level gauging is suitable for liquid metals which are good electrical

conductors. The container of the liquid must be non-magnetic or at least feebly magnetic. It

may be a side tube to the actual container as shown in fig 2.ii.b.1, with two coils wound on it

and a measuring scheme. The position of the coils may be altered if required, by sliding.

When the liquid moves up, inductance of one of the coils changes and unbalance in bridge

which is directly related to the required height is measured.

c. CAPACITIVE TYPE LEVEL GAUGE

Capacitance methods are the most extensive ones for liquid level measurement in

recent years. They are the most suitable for non conducting liquids like oils, gasoline or

liquid gases, for corrosive acids and high pressure process liquids. The liquid acts as the

dielectric material of the capacitor.

The electrodes are arranged in the form of a pair of concentric cylinders in the tank.

With the level rising, the capacitance of the capacitor b/w the electrode pair increases as the

gas phase has less dielectric constant than the liquid. The basic scheme of the system is

shown in fig 2.ii.c.1. At the bottom, a hole is provided for the liquid to enter into the

electrode assembly. If the dielectric constant of the vapour phase εv, is very small compared

to that of the liquid phase εl, and the dimensions are as shown, the capacitance is given by the

relation



Taking into account the dielectric constant of the vapour and a vapour phase of length

, the approximate value of the capacitance is

The outer cylinder is generally ground to avoid stray capacitance effects. This means

that for measurement of the liquid level in a tank of cylindrical form, a separate outer

electrode is unnecessary. The tank serves this purpose. A central electrode rod is used to form

a concentric pair. It is further assumed that the liquid is not an ideal dielectric but has a

leakage resistance of a large value. The central electrode may be inserted bare for good

dielectric liquid. For a worst case, it is generally enclosed in a dielectric cylinder and a probe

is formed.

iii. ULTRASONIC LEVEL SENSOR

Ultrasonic level detectors use the principle of reflection of an acoustic wave from a

liquid to vapour plane or vice versa. The schematic arrangement is shown in figures 2.iii.1

and 2.iii.2.

Ultrasonic wave sent by the transmitter T is reflected at the interface and received

back by the receiver crystal R. As level changes from l1 to l2, transit time also changes.

Measurement can be made on the basis of the time required by the echo to reach by the

receiver or on the basis of change of phase of the wave during this time.

For phase measurement, a phase sensitive detector is used. The method of time

measurement is quite common and such a meter is known as the Fathometer and has an

accuracy of about 0.1% of full scale. The liquid phase method is used in storage tank of oil

and chemicals and for air-craft or marine equipment tanks. The vapour phase method is used

in mines, oil-wells, shafts, etc.

Piezoelectric crystals such as quartz or barium titanate are used with a frequency

range from 30-300 KHz. A single crystal may be used both as a transmitter and receiver and

an electronic switch is used for changing the mode. A tuned LC resonance circuit is generally

for employed for avoiding spurious crystal oscillations. The received signal reduces in

amplitude by a factor of 103 to 10

6 and hence must be amplified.



B. SOLID LEVEL DETECTORS

With the increasing applications of continuous processing, not only the fluid handling

industries, but also to those industries which handle large quantities of solid materials in bulk,

the importance of measuring and controlling methods for solid level is growing.

Solid level detectors fall into two principal categories:

i. Continuous, which provides a continuity of measurement from the low to high end of

their scale.

ii. Fixed – point, which provides measurement only at one or several specific levels.

The various types of solid level detectors in common use are

1. Gamma Ray Absorption Method

2. Weighing Device

3. Electrical Capacitance

4. Diaphragm Method

5. Rotating Paddle

6. Balanced Paddle

1. GAMMA RAY ABSORPTION METHOD

The working of this device (fig B.1.1) is based on the principle that the amount of γ -

radiation from a fixed source varies with thickness of solid between source and detector.

This device is most ideally used in solid bins and can operate under condition of high

pressure and temperature. It could be designed to give either continuous indication or

annunciation at some fixed point values. The advantage is that it does not require any

opening or connection through vessel wall. This instrument can also be used to determine

density of fluidized bed with known height of bed.



2. WEIGHING DEVICE

This is probably the simplest possible device whenever feasible and gives result most

accurately. The bin containing the solid is supported from scale levers or other types of

force balancing system, which can be mechanical, pneumatic, hydraulic, or electric. This

device can operate under high temperature and pressure. It is very expensive particularly

for large bins where supports etc, causes lots of problems.

3. ELECTRICAL CAPACITANCE

In a device which measures the solid level continuously based on principle of

electrical capacitance, an insulated vertical rod is permanently installed in the bin. The

electrical capacitance of this rod with the material in its surroundings changes in direct

proportion to the level of material. The change of capacitance is detected by suitable

electrical device which could either give continuous indication or alarms at high and low set

points. This device is suitable for continuous indication, recording and control in both bins

and process vessels. It may be noted that level readings in this method are affected by the

density of material and therefore, to some extent by temperature also.

4. DIAPHRAGM METHOD

This device (fig B.4.1) is best suited for detecting solid level at one particular point.

One unit is required to detect level at one point. Thus two units will be required to detect both

high and low levels. If intermediate levels are also to be detected, then more units may be

used.

The device consists of a flexible diaphragm which is exposed to the solid material in

the bin. As solid level rises, pressure forces diaphragm against counter weighted lever

mechanism. Tripping of lever actuates an electrical switch which can be used to operate

signals or conveyors. This device could be operated under high pressure conditions but not

under high temperatures.



5. ROTATING PADDLE

In this method, a paddle is attached to a shaft driven by synchronous motor. When

rotation is restricted by solid material, motor support rotates in horizontal direction, causing

actuation of electric switches which may be used to operate or shut off conveyor. This

method is suitable only for top – level detection.

6. BALANCED PADDLE

This device is designed to measure level of hot catalyst at temperatures upto 550°C

and has rapid response. The device is capable of giving indication at fixed point only.

In this device a balanced paddle extends into the vessel through a diaphragm seal

supported on horizontal arm. It is balanced by pneumatic balance transmitter. When the solid

come upto the level of paddle and contact it, the balance is disturbed and the arm in

pneumatic balance transmitter is unbalanced, which is indicated by an indicator attached to

this transmitter.



C. LEVEL SWITCHES

The most commonly used level switch is a float type one. The working of float switch

is that they ride or float on the changing liquid level and their motion is used for alarm or on

– off functions.

Float switches utilize a float that follows liquid levels and at some point within its

range of travel, it actuates a switch. Most of the float switches are designed for relatively

short travel. In tanks, both high and low levels are of concern. The low level float switch

usually remains submerged while the high level switch hangs freely. When level falls and

reaches past the central line of float, then the low level switch actuates. Most floats are

hollow and made of brass or stainless steel.

Fig C.1 shows the working of a mercury switch attached with float. A permanent

magnet is attached to a switching arm which holds the mercury switch. As float rises, the

sleeve pull magnet towards itself, which tilts the mercury switch. When level falls, the sleeve

moves down the magnetic field and magnet swings out due to spring action, tilting the

mercury switch to other position. In places where vibrations are expected, dry type micro

switches are preferred in place of mercury switches.

Float level switches may be designed for internal mounting. Internal mounted

switches may be flange mounted or screwed from top, or side. These are cheap but the tank

has to be shut down for the removal for maintenance. External piping also serves as an

effective stilling chamber for tanks having turbulent conditions inside.

level measurement

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