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Process Engineering and Technology – Basic Instrumentation

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Module 9

Objectives of the Module

Module Contents

Basic Instrumentation

At the end of this module, you will be able to:

a. Recall the basics of instrumentation

b. Identify and recall the functionality of temperature

measurement devices.

c. Identify and recall the functionality of pressure measurement devices.

d. Identify and recall the functionality of level measurement devices.

e. Identify and recall the functionality of flow measurement

devices.

f. Identify and recall the functionality of the following key

measurement devices:

Specific gravity,

Haze

Dissolved oxygen

Carbon dioxide

pH

This module covers the following:

1. The basics

2. Temperature Measurement

3. Pressure Measurement

4. Level Measurement

5. Flow Measurement

6. Specific Gravity Measurement

7. Haze or Turbidity Measurement

8. Dissolved Oxygen Measurement

9. Carbon Dioxide Measurement

10. pH Measurement


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Basics

Methods of Measurement

Instrumentation is defined as „the art and science of measurement and control‟.

The FUNCTION of instrumentation is to measure the physical and

chemical parameters of matter.

Physical parameters are temperature, volume, mass, level,

pressure, density and flow

Chemical parameters are carbon dioxide, dissolved oxygen

and nitrogen

The PURPOSE of instrumentation is to control, measure and

record physical and chemical properties and the ACTION of

instrumentation is to transform physical and chemical properties into either mechanical or electrical signals.

Let‟s look at some common types of instrumentation and their

application in the brewery:

Type Control Action Example A Initiating or terminating a process filling or emptying B Controlling within boundaries temperature C Recording quantitative data for product specs alcohol content D Monitoring for quality control oxygen content E Recording process variables to accumulate data for extending the

knowledge of the brewing process, assessing equipment performance, designing maintenance schedules etc

A and B, are most of interest during production; C and D are

necessary for sales of product and E is required for strategic planning

There are three methods of doing measurement within the

brewery environment:

INLINE - an interacting or sensing element located directly

in the material to be measured e.g. conductivity or

temperature

Advantages:

Time lag is reduced to that imposed by design of sensing

element and associated signal processing equipment

Allows real time control of process

Continuously measures the variable


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Requirements for

Instrumentation

Disadvantages:

The location of the instrument is usually in the material

and this could lead to contamination issues especially if the physical location cannot be cleaned

ONLINE - an interacting or sensing element located in a

side stream connected to the process e.g. alcohol content

Advantages:

For a constant relationship between the side stream and

the main process, the time lag is constant and predictable Continuously measures the variable

Disadvantages:

The instrument relies on the control of the side stream to

main process being maintained in a constant state

OFFLINE - a remote measuring device which takes a

specific sample or measurement at a point in time e.g.

pressure gauge

Advantages:

More sophisticated instrumentation can be applied to

make determinations and a whole suite of measurements

performed

Disadvantages:

The instrument could have a variable time lag

Has a discontinuous measurement of the variable

There are 6 key requirements for any instrumentation installed in a brewery.

1. RELIABILITY - there must be a consistent and reliable signal from the instrument and it must originate from the

variable being measured.

2. ROBUSTNESS - the instrument must resist failure in abnormal conditions i.e. hot and wet environments.

3. REPEATABILITY / ACCURATE - the consistency and precision of the instrument is essential.

4. RESPONSE - the signal received from the instrument must

be proportional to the variable being measured.


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Modes of Measurement

Signals and Outputs

Transducers and Sensors

5. RESPONSE RATE - the response time of the signal must be faster than the process variable.

6. HYGIENE - the instrument must meet all the criteria for

hygienic design and must be able to be cleaned.

The MODE of measurement defines that interaction between the

instrument and the parameter being measured

There are three modes of measuring:

Direct interaction with the parameter being measured e.g.

temperature, flow (magnetic flow) and pressure

Indirect interaction with the parameter being measured

e.g. the level of a tank calculated from the pressure exerted

by the fluid on a pressure sensor at the base of the tank

Inferred measurement e.g. measuring the density of a

fluid to get the alcohol content or measuring the volumetric

flowrate to get the mass

For signal types and output types, please consult the module on Process Control.

A transducer is a device, usually electrical, electronic or electro-

mechanical, that converts on type of energy to another for various

purposes including measurement or information transfer. Broadly speaking, a transducer is a device which converts a signal from

one energy form to another.

A sensor is a type of transducer. There are various types of

sensors and some of the more common are listed below:

a. Inductive sensors are electronic proximity sensors i.e. when a metal object moves into the inductive field of the sensor

(created by a coil), then the disturbance is detected by the sensor and this will then trigger an output.

b. Capacitive sensors are also a type of electronic proximity sensor, but instead of using inductive fields, this sensor senses

a change in the intensity of the capacitance of the sensor and then triggers an output.

c. Optical sensors are photoelectric sensors, where a beam is transmitted from the transmitter and a receiver receives the

beam. Typically, if the beam is broken, then the sensor will trigger an output.


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d. Hall Effect sensor is a transducer that has an output voltage that changes with a change in magnetic field density. These

sensors are typically used for positioning or speed detection.

e. pH sensor is a type of voltmeter. As the concentration of

hydrogen ions increases, a proportionally but small voltage is produced because of the electrical circuit which has been

created. This is then equated to a pH.

f. Strain gauges are devices which are used to measure deformation (strain) of an object and this deformation is

normally measured using a Wheatstone bridge.

A Wheatstone bridge comprises of a set of resistors which are

attached to a divided voltage supply. If there is no strain on the system, then the voltage will be balanced between the

resistance circuits. If there is any resistance, then the bridge

becomes unbalanced and current will flow, giving a voltage as an output. Common examples of strain gauges are Pressure

transmitters, load cells and various transducers.


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Temperature Measurement

Bimetallic Instruments

This is the most widely measured and controlled process variable. The word temperature indicates the hotness or coldness of a body

and is measured using a linear scale of degrees, either Celcius (⁰C), Fahrenheit (⁰F) or Kelvin (K). The most common of the

measures is ⁰C.

Temperature measurements are used for one or more of the

following purposes:

to provide continuous input to an automatic controller or to an

operator

to aid operator monitoring of process variables or equipment

to record information for trend indication or historical records

to make frequent spot checking of temperature during a

process

In industry there are two common groups of temperature measuring instruments:

Bimetallic and fluid filled instruments e.g. temperature gauges

Electrical temperature instruments e.g. resistance temperature

detectors (RTD‟s)

A bimetallic strip is used to convert a temperature change into

mechanical displacement. The strip consists of two strips of different metals which expand at different rates as they are

heated, usually steel and copper, or in some cases brass instead of

copper. The strips are joined together throughout their length either by riveting, brazing or welding. The different expansions

force the flat strip to bend one way if heated, and in the opposite direction if cooled below its normal temperature.

If one of the metals has a low coefficient and the other metal has

a relatively high one, the two metals will expand at different rates

and to different lengths as the temperature rises. This will force the bimetallic strip to bend toward the side with the lower rate of

thermal expansion.

The deflection of the tip is small if the strip is short. If the strip is

wound in a spiral or helix, a longer strip can be contained in a relatively small space. Longer strips respond with greater

deflection, since deflection increases with the square of strip

length. For example, doubling the strip length increases deflection four times.


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Types of Bimetallic

Elements

In some applications the bimetal strip is used in the flat form. In others, it is wrapped into a coil for compactness. The greater

length of the coiled version gives improved sensitivity.

Each of the principle types of bimetallic elements used for this type

of thermometer has an increased strip length, which will allow for greater deflection without sacrificing space.

The SPIRAL or HELIX coils tighten as the temperature increases.

As it coils, the centrepost rotates clockwise. If you attached a

pointer to the post, it will also rotate clockwise and, if properly positioned on a printed dial, can be used to indicate temperature.

Similarly, the MULTIPLE HELIX centreposts of the two helixes will also rotate as temperature changes.

Many helical-wound bimetallic thermometers are designed to fit into protecting wells or tubes. You can insert this type of

thermometer into a duct to measure the temperature of a liquid or gas inside the duct.

A tightly wound helical bimetallic strip is located inside the stem of

the thermometer. The strip is attached to a centrepost that

extends from the stem to the centre of the indicating dial. As the coil winds and unwinds with changes in temperature, the post

rotates. This causes a pointer attached to the centrepost to indicate the measured temperature on the dial. Normally, the stem

is charged with silicone fluid to aid heat transfer between the stem

and the enclosed bimetallic helical coil.


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Glass Thermometers

To make bimetallic elements strong enough to operate the pen of a strip-chart recorder just increase the width and thickness of the

metal strips. This type of system then becomes a self-contained measuring and recording device that is independent of electrical

power requirements (except for the strip-chart drive). As it is

difficult to convert movement to pneumatic or electric signals for remote transmission without reducing accuracy, bimetallic

thermometers should be primarily used for local/remote applications.

The above diagram shows two helical bimetallic thermometers

using different attachments.

Bimetallic thermometers are inexpensive, relatively rugged, and

easy to read. They are also reasonably accurate if you handle them carefully. Rough handling can change their calibration.

The most commonly used thermometer is the liquid-in-glass thermometer. This device consists of a small-bore glass tube with

a thin-wall glass bulb at its lower end. The liquid that fills the bulb and part of the tube is usually mercury or an organic compound.

The operation of a liquid-in-glass thermometer is based on the fact that liquids expand as temperature rises. In this type of

thermometer, the expansion causes the liquid to rise in the tube, indicating the temperature.

Mercury is the usual working fluid inside liquid-in-glass thermometers, however at low temperature (below –39⁰C),

mercury freezes. This means other liquids, most frequently alcohol, are used for measuring very low temperatures. Depending on the

manufacturer‟s choice of construction and working fluid, you can

use some laboratory-grade thermometers to measure temperatures down to –200⁰C.


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Electrical Temperature Instruments

The basics

Resistance Temperature

Devices

For measuring high temperatures, the thermometer‟s glass stem above the mercury can be charged with nitrogen. This helps

prevent the mercury from evaporating or boiling, making accurate readings difficult. Even with the nitrogen, however, liquid-in-glass

thermometers are usually limited to temperatures less than about

600⁰C. Temperatures higher than 600⁰C can affect the glass and

cause permanent changes in the volume the bulb can contain,

thus destroying the instrument.

There are three common electrical temperature measuring

instruments:

Resistance Temperature Detector (RTD)

The Thermistor

The Thermocouple

All of these instruments rely on changes in electrical resistance

due to varying temperature. RTDs incorporate pure metals or

specific alloys that increase resistance as temperature rises. Thermistors are solid-state devices that react to change in

temperature many times faster than RTDS. Unlike RTDS, however, a thermistor resistance decreases in response to rising

temperatures. The third instrument, the thermocouple, produces a voltage corresponding to the temperature difference between a

selected junction and a reference junction.

The resistance of certain metals changes as the temperature

changes. This characteristic is the basis for a temperature-measuring instrument called the resistance-temperature detector

(RTD). RTDs act as electrical transducers, converting temperature

changes to voltage signals by measuring resistance. The metals used in RTD sensors must be pure, of uniform quality, stable

within a given temperature range, and able to give reproducible resistance-temperature readings. Only a few metals have these

properties.

Platinum, copper, and nickel are three metals that can meet RTD

requirements. The best of these is platinum as it has a relatively linear relationship between resistance and temperature, where as

at higher temperatures, nickel and copper have non linear relationships.

Resistance elements are usually long, spring-like wires enclosed in a metal sheath. RTD resistance elements are usually constructed

of platinum, copper, or nickel. A metal suitable for RTD applications must be able to withstand repeated temperature

cycles. It should also have a linear resistance temperature curve

and a high coefficient of resistance.

The coefficient of resistance is defined as the change in resistance that occurs with a change in temperature. It is usually expressed

as percent per degree of temperature.


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The resistant material must be pure and capable of being drawn into fine wires so the elements can be easily constructed.

In the standard construction of an RTD, the metal element (in this

case platinum) is surrounded by a porcelain insulator that prevents

a short circuit developing between the wire and the metal sheath. The sheath is made of Inconel, a nickel-iron-chromium alloy with

good corrosion resistance. Leads are attached to each side of the platinum wire to give the electrical resistance.

When you place the RTD in a liquid or gas medium, the Inconel sheath quickly reaches the temperature of the medium. This

change in temperature causes the platinum inside the sheath to heat or cool, resulting in a proportional change in the wire‟s

resistance. You can then use the new resistance reading to

compute the medium temperature.

The diagram shows an RTD, a protecting well, and a terminal head. The well protects the RTD from a possible damage caused

by the gases or liquids being measured. Protecting wells are usually made of stainless steel, carbon steel, Inconel, or cast iron

and are used for temperatures up to 11000 C. Ceramic wells are

used for furnaces or kilns having temperatures higher than 11000 C.

A heavy metal head, usually of cast iron or aluminium, protects the

connections between the bridge circuit and the RTD. A cap, base, and gasket make up the head. The gasket is both heat proof and

moisture proof. The RTD leads from the protecting well enter the

head and are attached to the bridge circuit leads mounted in the head.

Protecting well

containing RTD

Terminal Head

containing electrical

connections


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Thermistors

Thermocouples

This type of RTD is commonly referred to as a PT 100. The PT refers to Platinum and the 100 refers to 100Ω at 0⁰C.

Whilst RTD‟s are extremely accurate (+0.150C) and have a large range (-259 to 6310C), they are expensive as the metal used is

pure. The response time for an RTD compared to other types of electrical resistance devices is very fast. They are complex pieces

of equipment, as they need a bridge circuit, a power supply and a voltage readout device.

Thermistors are a special type of resistor. They are classified as semi-conductors. They are often used to detect small temperature

changes because they have extremely large temperature-resistance coefficients. This means that a thermistor responds to

temperature changes with a greater change in resistance than

platinum, copper, nickel, or other resistance elements.

Thermistors differ from RTD‟s in that the material used in a thermistor is generally a ceramic or polymer, while RTDs use pure

metals. The temperature response is also different; RTDs are useful over larger temperature ranges, while thermistors typically

achieve a higher precision within a limited temperature range

[usually −90 °C to 130 °C].

Thermistors are less expensive and less complex than RTDs, but their measurements lack RTD accuracy and reproducibility.

Thermocouples produce electric current when subjected to

temperature changes. Thermocouples are made by connecting

two different metals to form a closed circuit. Using a dissimilar metal to complete the circuit creates a circuit in which the two legs

generate different voltages, leaving a small difference in voltage available for measurement.

If one of the two connections (or junctions) is heated, current will

flow through the circuit. The amount of current produced depends

on the difference in temperature between the two junctions and on the characteristics of the two metals.

The diagram below illustrates a simplified thermocouple composed

of wires made from two different kinds of metal alloys. The wires

are joined at the ends, forming two junctions, a measuring junction and a reference junction.


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Heating the measuring junction produces a voltage greater than the voltage across the reference junction. You can read the

difference between the two voltages (in mV) on the voltmeter.

The voltmeter reading is then converted to a corresponding

temperature, by referring to a manufacturer‟s conversion table, which lists the specific temperatures that correspond to a series of

voltage readings.

Thermocouples are inexpensive, rugged, and reasonably accurate.

They are also simpler to use than either metal resistance thermometers or thermistors because they do not require bridge

circuits. In addition, thermocouples have extremely wide

temperature ranges, from near absolute zero (-273⁰C) to about

2800⁰C.

The disadvantages of thermocouples are as follows:

If the leads are not housed in metal conduit, low junction

voltages (in the 5- to 50 mV range) can cause the device to pick up stray electrical signals.

Because of the relatively small change in junction voltages

with temperature, you will find that thermocouples have limited use in temperature spans less than about 33⁰C.

You must hold reference Junction temperatures constant or

compensate for any deviations.


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Pressure Measurement

Principles of Liquid Pressure

Nearly all industries use liquids, gases, or both in their processes. Controlling these processes requires measuring and controlling

liquid and gas pressure. Pressure measurement is one of the most important of all process measurements.

To work effectively with pressure measuring devices, you must understand some basic properties of fluid flow and the principles

of pressure in liquids and gases.

Pressure is commonly measured using one of the following units:

Pascal (Pa) which is actually Newtons per square metre

Bar (bar) which equals 1 x 105 Pa.

There are other units of measure, which you may come across:

PSI – pounds per square inch

Torr – millimeters of mercury

Hg - inches of mercury (commonly used in weather reporting)

Consider an empty tank with a pressure gauge fitted at the bottom

of the tank. As liquid is pumped into the tank, the gauge shows

increasing pressure as the level of the liquid rises. The bottom and walls of the tank exert a force on the liquid to keep the liquid from

flowing out into a flat puddle. The force increases as the liquid rises to higher and higher levels.

Pressure acts equally in all directions in a liquid. Think of a tiny

particle of liquid within a container filled with liquid. The

surrounding liquid exerts pressure on the particle. This pressure acts equally in all directions from above, from below, and from all

sides. The particle also exerts the same pressure on the surrounding liquid. This pressure is the same in all directions –

upward, downward, and toward all sides.

The pressure exerted by a liquid on a particle results from the

weight of the liquid above that particle. If the particle is far below the surface, the pressure on it is high. If it is near the surface, the

pressure is low. The pressure at the surface of a tank open to the atmosphere is equal to the atmospheric pressure. From there, the

pressure increases in proportion to the depth.

In a tank with a sealed top, the pressure at the surface of the

liquid may be high or low. However, the pressure under the surface increases in proportion to depth, just as in an open tank.

Note the following:

Pressure depends on depth, not volume

Pressure is defined as the amount of force applied to a unit of

area. To calculate pressure, divide the force by the area.


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Three important facts about liquid pressure:

1. If the liquid in a tank remains at rest, the pressure at any point beneath the surface depends on only three factors.

Depth (H) - The pressure at a point is proportional to the

depth of the point below the surface.

Density (ρ) - The pressure is proportional to the density

(or specific gravity of the liquid)

Surface Pressure (Patm)- Any pressure acting on the

surface is transmitted throughout the liquid and

contributes to the pressure at any point beneath the surface.

For liquids for an open tank, we can say:

P = ρgh + Patm

2. Nearly every process industry must measure and control liquid pressures. Pressure measurements indicate flow rates, levels

of liquids, and other important aspects of processes.

The conversion between pressure and level requires that three

conditions be met.

The liquid must be at rest – not flowing, swirling, or

moving in any other way.

The liquid‟s specific gravity must be known.

The liquid must be homogeneous. That is, it must have

the same specific gravity throughout its depth.

3. Most liquid pressure gauges use atmospheric pressure (101,3 kPa) as a zero point. That is they indicate a pressure of 0 kPa

at the surface of a liquid, even, though the pressure is actually

101,3 kPa. All pressures beneath the surface are indicated 101,3 kPa too low.

A gauge that indicates zero at atmospheric pressure measures

the difference between actual pressure and atmospheric

pressure. This difference is called gauge pressure, abbreviated kPa g.


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Principles of Gas Pressure

A gas differs from a liquid in two important ways.

1. For the same mass of fluid, a gas occupies a significantly higher volume.

2. A gas can be compressed or expanded to fit almost any

volume, unlike a liquid, which can be slightly compressed or expanded.

In order to understand a gas and gas pressure, let‟s consider the

following

A gas is composed of molecules.

These gas molecules are constantly moving. They fly around

at high speed, in all directions colliding with each other and with any other molecules in their way. Pressure is caused by

these collisions. The harder the collisions, and the more often they occur, the greater the pressure.

Even a small volume of gas contains an enormous number of

molecules.

Most of the volume occupied by a gas is empty space. The

volume occupied by the molecules themselves is very small compared to the enormous amount of empty space between

and around them.

There exists a relationship between pressure, volume, and temperature can be summarized in a simple equation

pV = nRT where p = pressure V = volume

n = number of molecules of gas

R = a constant T = temperature (Kelvin)

This equation says that pressure multiplied by volume is equal to

the number of gas molecules multiplied by a constant multiplied by the absolute temperature.

Certain relationships can easily be determined from this equation:

If the temperature does not change (T is constant) and the

amount of gas does not change (n is constant), doubling the volume cuts the pressure in half. Reducing the volume to one

fourth would multiply the pressure by four.

If the volume of a gas is held constant, changes can occur in

pressure as the temperature changes.


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Pressure Terminology

An important fact about gas and atmospheric pressure:

Atmospheric pressure, like liquid pressure, results from the weight of material above the point where the pressure is measured. At

higher levels, less air is above the measurement point and

therefore the air pressure is less.

Most industrial processes take place on the earth‟s surface where changes in atmospheric pressure are slight, process instruments

must take even these slight changes into account if they control pressure sensitive processes.

Calculating air pressure at various levels in the atmosphere is rather difficult. There are two major reasons for this difficulty:

1 . The air at lower altitudes is compressed by the weight of air

above it. This compression makes the weight per unit volume

higher at low altitudes than at high altitudes.

2 . The air temperature is lower at higher altitudes. The lower temperature compresses more air into the same volume

compared to a higher temperature at the same pressure.

It is important to remember that when you take a pressure

reading, you need to consider the atmospheric pressure. At the coast, the atmospheric pressure is equal to 101.3 kPa.

In the processing world, there are various terms for expressing

pressure. Here are some of the common ones:

a. Hydrostatic head – this is the pressure due to the weight of a

fluid b. Static pressure – this is the pressure exerted by a fluid at rest

c. Differential pressure – this is the pressure drop measured

between two points and can be used to calculate flowrate d. Dynamic pressure – this is the additional pressure associated

with fluid flow e. Absolute pressure – this is the pressure referenced to zero

pressure i.e. empty space! f. Atmospheric pressure – this is the pressure at 1 atmosphere

and equals 101.325 kPa

g. Gauge pressure – this is the pressure measured on a gauge.


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Pressure Sensors and Instrumentation

Wet Pressure Instruments

Pressure measuring instruments perform two basic functions in process control.

In order for an instrument to sense the pressure to be

measured, some part of the instrument must respond

physically to the pressure. Some part of the instrument must

stretch, bend, or change position in some other way.

The instrument must convert this response into a standard

signal that can be used by other elements in the control system. A pressure transducer must produce an electrical or

pneumatic signal in response to the pressure sensor. There are two families of pressure sensing instruments:

WET: Contains a liquid that responds to pressure. A

manometer is the most common example of such an

instrument.

DRY: The pressure instrument uses an elastic element that

responds to pressure by stretching, bending or

changing shape in some other way.

The simplest form of a wet instrument is a manometer and it is a very simple instrument. It usually comprises of a U tube filled with

mercury and measures differential pressure. See the diagram below.


pg 18 of 56

Dry pressure instruments

The Bourdon Tube

There are two basic types of dry pressure instruments:

The Bourdon tube

The Diaphragm

Instruments incorporating Bourdon tubes are the most common

industrial pressure instruments. They have been used for more

than 100 years. Improvements have been made in construction, materials and accuracy, however the basic operating principle has

remained unchanged.

A Bourdon gauge uses a coiled tube (commonly bent in the shape of a C), which, as it expands due to pressure increase causes a

rotation of an arm connected to the tube. The pressure sensing

element is a closed coiled tube connected to the chamber or pipe in which pressure is to be sensed. One end of the tube is sealed

shut and the other end is open to the process pressure. As the gauge pressure increases the tube will tend to uncoil, while a

reduced gauge pressure will cause the tube to coil more tightly.

This motion is transferred through a linkage to a gear train connected to an indicating needle. The needle is presented in front

of a card face inscribed with the pressure indications associated

with particular needle deflections.


pg 19 of 56

The range of pressure which can be measured ranges, from 0 – 100 kPa to 0 – 68,9 mPa.

The range is determined by the material used, the flatness of the

tube, and the cross-sectional area of the tube. The pressure

exerted by the fluid inside the tube makes it partially straighten out. As the pressure increases, the Bourdon tube becomes

straighter. Because the open end is fixed and cannot move and the closed end remains free, the tube will move. The amount of

movement indicates the amount of pressure.

See steps 1 – 4 below.

The movement is caused by the difference in pressure exerted the

inner and outer walls of the C shape. The outer wall experiences a higher pressure and thus the tube is pushed outward, and it

partially straightens. As the pressure increases, the tube

straightens out further.

The Bourdon tube is made of resilient metal. The metal acts like a spring. It bends rather easily, and then springs back to its original

shape when the bending force is removed. The tip of the Bourdon

tube moves in proportion to the pressure.

As the tip of the Bourdon tube moves, it rotates the sector. Gear teeth in the sector turn the pinion attached to the dial pointer.

The pointer indicates the pressure on a scale.

Note that the hair spring is to keep the tension on the pinion and

gear and NOT to pull the Bourdon tube back to its original position.

2. The increase in pressure

cause the tip to move out

1. Pressure increases

3. The gears transfer the movement of

the tip to the pinion

4. The indicator will move to

represent the pressure


pg 20 of 56

Diaphragm Pressure Sensor

Important facts about Bourdon Tubes:

A Bourdon tube is basically a spring that stretches when

pressure is applied. The metal must undergo repeated flexing without fatigue (gradual formation of tiny cracks that

eventually meet and cause the metal to break).

The metal must not creep (change shape permanently) when

exposed to maximum pressure for a long time.

The metal in a Bourdon tube must not be subject to

hysteresis. In other words, the metal must not stretch

differently when the pressure is increasing than it does when the pressure is decreasing. If it does, the gauge will give

different readings when the pressure rises than when the

pressure falls.

Many process fluids cause corrosion when they come into

contact with certain metals. The metal in a Bourdon tube is chosen to avoid any possible corrosion problems. If such a

choice is not possible, the metal must be protected from the

process fluid.

As the fluid will move into the Bourdon tube, they are not

applicable for hygienic applications as the fluid cannot be flushed out of the tube.

A diaphragm is a flexible disk that changes shape when the

process pressure changes. The amount of deflection is repeatable

for known pressures so the pressure can be determined by using calibration. The deformation of a thin diaphragm is dependent on

the difference in pressure between its two faces.

The disk is held firmly all around the outer edge. The process

pressure pushes on one side of the disk. The central portion of the disk moves in or out as the process pressure changes. The natural

spring of the diaphragm pushes back against the process pressure. The deformation can be measured using mechanical, optical or

capacitive techniques.


pg 21 of 56

Level measurement

Diaphragm sensors can measure a wide range of pressures, including pressures below atmospheric pressure and are normally

made of ceramic and metallic materials.

In order to measure the movement of the diaphragm, a pressure

transducer is connected to the diaphragm element, via a wire. This forms then a differential pressure transmitter.

Level measurement is one of the commonest of all measurements

on a processing plant.

Process variables can be measured either directly or indirectly.

Both methods are used to measure liquid level. To make a direct measurement of water level, lower a measuring stick into the

water and read the level of the water directly on the stick. Mount a stick permanently in a tank, then read the level without handling

the stick.

But suppose the tank contains sulphuric acid instead of water,

suppose the acid is hot and under high pressure. Then you can‟t measure the level as easily and will probably need to measure the

level indirectly.

As discussed earlier, there are two methods to measure level and

both of these methods have a group of instruments attached to it.

1. Indirect Level Measurement Instruments

Hydrostatic Pressure

Pressure Head

2. Direct Level Measurement Instruments

Surface Sensing Gauges

Sight glasses

Floats and Level Switches

Conductivity Probes

Capacitance Probes

Ultrasonic Probes

Radar Probes

Radiation Level Detectors

Load Cells


pg 22 of 56

Indirect Level Measurement

Instruments

Hydrostatic Pressure

Liquid levels can be measured either directly or indirectly. The principles of level measurement discussed liquid levels that were

measured directly. That is, the primary element is in direct contact with the liquid.

In this part of the module, we will look at two methods of measuring liquid levels indirectly.

Consider a tank that is open to the atmosphere and filled to a

height, h from the bottom of the tank. A pressure gauge is installed at the base of the tank. The water in the tank is said to

be at rest, because it has little or no movement within the tank.

Since the walls of the tank contain the liquid, it exerts a force on

the walls. This force, which is registered on that gauge, is proportional to the depth of the liquid. In other words, the force at

any point in the tank varies with the elevation (or height) of the

liquid. PRESSURE is the term used to describe this force. HYDROSTATIC PRESSURE is the term used to describe this

force in a liquid at rest.

The pressure at any point in an open tank of water depends only upon the elevation of liquid above the measurement point.

Pressure is proportional to liquid elevation.

As seen before, you can use the following equation to measure the

hydrostatic pressure of any liquid in an open tank.

P = x g x h

where = pressure (Pa)

h = height of liquid above measurement point (m)

= density of liquid

And, in a closed tank, pressure is proportional to the liquid

elevation above the measurement point plus any additional pressure applied to the liquid. For the pressure in a closed vessel,

the expression becomes:

P = x g x h + external pressure on the liquid


pg 23 of 56

Differential Pressure Transmitters

Direct Level

Measurement Instruments

In another method, pressure can be measured using a differential pressure transmitter. A differential pressure transmitter is a type of

pressure head device. It detects the differential between the head (or high) pressure and the static (or low) pressure of a liquid in a

vessel. These transmitters are available in many forms, such as

the flange-mounted pneumatic transmitter with a diaphragm interfacing with the fluid. These devices are used with viscous,

corrosive, or slurry-type liquids. They tend to fail at the point where the process fluid is in contact with the instrument.

One way of measuring the level of a liquid in an open tank is to

use a point gauge. A point gauge is a metal rod with a pointed end as shown in the diagram below. To use it, lower the rod until the

point just touches the surface of the liquid. A scale on the rod allows you to read the distance from the surface to the datum

point.

Next diagram shows a level – measuring device similar to a point

gauge. It is a flexible metal tape with a plumb bob on the end. To use it, unwind the tape until the plumb bob touches the surface of

the liquid. Reading the tape gives the distance from the datum

point to the surface.

Point gauge Flexible tape and plumb bob


pg 24 of 56

Sight Glasses

Both methods are relatively inexpensive and are as accurate as the person reading the level, however they are cumbersome and

require remote reading.

The construction of some tanks makes it impossible to touch the

surface with a rod or a float one way you can measure the level in such a tank is to use a sight glass. A sight glass or water gauge

is a transparent tube through which the operator of a tank can observe the level of liquid contained within and it extends up the

outside of the tank. It is connected to the interior of the tank at the bottom.

The liquid level in the sight glass matches the level of liquid in the tank. As the level in the tank rises and falls, so does the level in

the sight glass.

Note that if the tank is under pressure or vacuum, the sight glass must be connected to the tank at the top as well as at the bottom,

otherwise the pressure difference between the tank and the sight glass would cause a false reading.

With appropriate safety precautions, sight glasses can be used in

high-pressure tanks. The glass tube must have a small inside

diameter and a thick wall, it must be enclosed in a protective housing. Valves must be located in the connecting lines to permit

isolating the gauge from the tank. A check valve in each connecting line snaps shut if the sight glass breaks.

The main disadvantage of a sight glass is that you must read it where the tank is located, which is not always convenient.

Weather can be another disadvantage since sight glasses are

located on the outside of tanks. In cold weather, the liquid in the

sight glass may freeze even though the liquid inside the tank does not.


pg 25 of 56

Level Switches and Floats

Many process control instruments are not designed to provide the operator with a reading of the process variable. Instead, the

instruments throw a switch, adjust a valve, or take some other action to keep the process running smoothly.

Similarly, level-measuring instruments do not always provide a reading of the level. Often all the level instrument does is respond

correctly when the level reaches an upper or lower limit. At the upper limit, the instrument acts to reduce the level. At the lower

limit, it acts to raise the level.

Opening or closing a switch sounds like a simple task, but the

mechanism for doing it may be quite complex. Simple actions often become complex when you try to increase their reliability,

especially under harsh or dangerous operating conditions.

Most switches operate by making an electrical contact i.e. when

the switch is not made, the contact is open and when the level reaches the float, the contact is moved together and the switch is

activated.

The picture above, illustrates an example of a mercury level switch

which would be installed in a tank for controlling liquid level. The

ball would hang in the tank at a predetermined level. As the tank fills ups with water, the float will rise until the mercury (which is

contained within the ball) makes contact with the two terminals and the switch is closed.

In a float switch, a magnet holds the pivoted armature in a position so that the switch is open. As the level rises in the tank,

the float lifts the steel tip until it comes near the magnet.


pg 26 of 56

Conductivity Probes

The steel tip interferes with the magnetic field and weakens the magnet‟s pull on the armature. The armature then pivots, and the

switch close.

When the steel tip drops away from the magnet, the armature

swings back to its original position and the switch opens, turning off the pump.

Another popular level switch is a vibronic level switch, which

comprises of two forks which vibrate at a predetermined frequency. When the liquid level reaches the forks, the vibration is

reduced and thus the frequency is reached, signaling that the level

has been reached.

Note that level switches do not measure the level in a tank, but will indicate a predetermined level in a tank.

If you take a beaker and fill it with a liquid that is slightly conductive and immerse two rods (which are connected by a

circuit) and apply a voltage, a current will flow between the rods and the fluid. This is called conductivity and it can be used to

measure a level in a tank or activate a level switch.

Important facts about Conductivity:

The current in the circuit changes in proportion to the depth of

the liquid, and you use this fact to approximate the liquid‟s

level.

Different liquids have different degrees of conductivity.

Conductivity of a given liquid varies with changes in

temperature and concentration


pg 27 of 56

The conductance probe is one of the simplest point level detectors i.e. when the liquid reaches a level it activates a switch. It is based

on the principle that a liquid – no matter how little current it conducts – will be conductive enough to complete a circuit.

There are various different types. In the diagram below, is a single rod conductivity probe.

In the diagram below, two conductive metal rods are shown of different lengths in a metal tank. Because the two rods are

insulated from each other, there is essentially no conductivity until

the liquid rises to the point where it touches a rod and completes the circuit. This activates a relay which, in turn, starts a motor or

sounds an alarm, etc.

When the liquid level drops below the rod, the relay drops out and its contacts open.

The major advantage of conductance probes is that you can

extend or cut off the rods as needed to match the point where you

want switching action.


pg 28 of 56

Capacitance Probes

There is no limit to how long the rods can be or the number thereof, except that they must be stiff enough to withstand any

turbulence in vats and tanks.

Capacitance and conductivity can be measured in a similar technique, however the difference with capacitance measurement

is the fluid being measured has little or no electrical conductivity.

This fluid is called a dielectric liquid.

When you insert two parallel rods in a dielectric liquid and connect them to an electric source, as shown below, an electric field forms

in the liquid. The field remains as long as the voltage is applied to the rods. Note that this is similar in many ways to the action of a

capacitor.

If you drain most of the liquid from the tank (leaving the electric source connected to the rods, the electric charges become

redistributed. The space between the rods and the voltage applied

is the same as before, however the dielectric has charged the air instead of liquid. In other words, the electrical capacitance has

changed.


pg 29 of 56

Capacitance probes are also known as dielectric sensors. The operation of a capacitance probe in dielectric liquids is very simple.

In the metal tank shown, an insulator holds a metal rod near the

side of the tank. The tank and the rod then act as the two plates of a capacitor. A capacitance field is set up between the two

metals. Note that if the sensor is moved, the magnitude capacitance field will change and thus the readings of the probe

will change.

The dielectric liquid in the tank rises and falls between the rod and

the side of the metal tank.

The actual form of the electrodes in a capacitance probe depends on the probe‟s specific application. Any metallic material can be

used for an electrode. But if the probe is to be used in corrosive liquids, the electrode should have a ceramic coating.

Capacitance probes need not only be installed from the roof of a tank, but can be installed in the side wall of the tank, as long as

the wall is linked into the circuit, as shown below.


pg 30 of 56

Capacitance probe mounted in the side wall of a tank

For applications involving liquids that conduct, the electrode is encased in a insulating material. In the diagram the conducting

liquid has the same potential as the metal tank. In this case, the

liquid acts as the capacitor‟s ground electrode, and the insulated conductor serves as the other.

Capacitance probe used with conducting liquids

Some capacitance exists between the insulated electrode and the tank‟s walls in the air above the liquid. However, it is very small

compared to the capacitance from the inner electrode through the insulating coating to the conducting liquid.

When the level rises, more of the capacitance probe is surrounded

by liquid and the capacitance increases. All you need to do now is

to accurately measure the capacitance, amplify it, and display it as level.


pg 31 of 56

Sonic and Ultrasonic Level Detection

Sonic detectors operate by sending out a sonic pulse, receiving the reflected pulse, and accurately measuring the time interval

between the two. This method is essentially the same as radar, except that the frequency of a sonic pulse is about 30 kHz.

A typical sonic level detector consists of a sender (transmitter) and receiver.

The transmitter emits a sonic pulse and a reflected pulse arrives at

the receiver a short time later.

The time interval between the transmitted pulse and the received

pulse is a direct function of the distance the sonic pulse travels in going from the transmitter to the reflecting surface and back to

the receiver. As the distance between the object and the sender/receiver increases, so the time interval increases.

Steam vapour foam or bubbles at the surface of a liquid reduce

the surface‟s ability to reflect. In such cases, a sonic detector is

sometimes installed at the bottom of the tank where the pulses rebound from the liquid‟s inner surface to the receiver.

An advantage of sonic level detectors is that they can be used in

large or small tanks, in a vacuum, or under high pressure.


pg 32 of 56

Radar Level Detection

Radiation

Radar level detection works in a very similar manner to ultrasonic level detection. However instead of using sound as a medium for

transmission, it uses electromagnetic waves (10 GHz range) or microwaves to detect the surface of the liquid or solid.

Basically, all types operate on the principle of beaming microwaves downward from a sensor located on top of the vessel. The sensor

receives back a portion of the energy that is reflected off the surface of the measured medium. Travel time for the signal (called

the time of flight) is used to determine level.

Radiation level detectors are used in difficult applications where

other kinds of transducers would not survive. The most common reason for using a radiation level detector is that it does not need

to come in contact with the liquid being measured.

Nuclear radiation in the form of gamma rays can penetrate a fairly

thick steel plate, but the energy level afterwards is greatly reduced. The radiation received at the gamma detector is

proportional to the thickness of the steel plate between the radiation source (radio-isotope) and the detector. That is, the

thicker the steel, the less radiation received by the detector.


pg 33 of 56

A radiation level detector set up to measure liquid level in a steel tank is shown in the illustration.

The gamma rays from the source are directed toward the detector in a thin band of radiation. Shaping the gamma rays in this way is

called collimation. It excludes all radiation to the detector except for a very thin beam from the radioisotope.

After the unit is set up, a reference measurement is taken of the

empty tank. The amount of radiation received by the detector is

determined after the gamma rays pass through two, tank walls and the air or vapour in the empty tank.

The radiation is beamed through both sides of the steel tank. The

radioactive beam must be strong, enough to penetrate both tank

walls (one near the source and the other near the detector) and the atmosphere in the empty tank.


pg 34 of 56

Load Cells

When liquid enters the tank the radiation beam must pass through a path in the liquid, as well as the tank walls. The liquid in the tank

reduces the radiation received by the detector. The amount of radiation received is inversely proportional to the amount of liquid

between the radiation source and the detector.

Even though the steel tank walls absorb some radiation energy,

this loss is constant whether the tank is full or empty. The difference in the amount of radiation received by the detector in

the two cases corresponds to the level of liquid in the tank. (i.e. the greater the amount of radiation received by the detector, the

lower the level of liquid in the tank, and vice versa).

Radiation point detectors are especially valuable where the liquid

to be measured is hot, corrosive, or otherwise hard to handle. Radiation gauges easily make point measurements, since they

need no openings in the tank.

Instead, a small shaft of radiation is allowed to penetrate the

vessel and to strike a detector. When the shaft of radiation is intercepted by liquid in the tank the amount of radiation reaching

the detector is substantially reduced. The detector circuitry then trips a relay to indicate that the liquid level has reached a preset

point.

It is a common fact that you can determine the amount of solid

material in a bin by weighing the bin and its contents. All you need to know in addition to the weight of the bin and its contents is the

dimensions of the bin and its mass when empty. Then, if you know

the density of the material, you can calculate the level.

If you install load cells at the base of each leg of the tank and weigh the bin and its contents. The load cells used here are strain

gauge load cells. Electric strain gauges produce electric signals in

response to mechanical compression or expansion. The load cells contain one or more sets of matched strain gauges bonded to a

high-strength steel element sealed inside the load cell. The load cells‟ outputs are electric signals proportional to the weight

applied.


pg 35 of 56

Flow Measurement

Using weight to find levels of bulk solids is important because other measurement techniques are less precise. If you are careful,

the weight of the bin and its contents and, therefore, its level can be very accurately measured. The method‟s main drawbacks are

that the readout must be recalibrated for different bulk solids, and

errors can be introduced if the bulk solids pack down.

In nearly every process industry, materials must flow from one location to another. Liquid raw materials flow to a reaction vessel,

finished product flows to a storage tank. Steam flows through heating coils, cooling water flows through condenser jackets. In all

of these examples, the rate of flow must be controlled. By

controlling flow rate, you can usually control reaction rate. Controlling the flow rate can also regulate other process variables,

such as temperature and pressure.

The flow characteristics of a fluid are governed by the following:

DENSITY - the mass per unit volume of the fluid – measured

in kg/m3

SPECIFIC GRAVITY - the ratio between the measured

density of one fluid and the measured density of water

VISCOSITY - the resistance to flow caused by friction with

the sidewall of the pipe – measured in kg.m-1.s-1 or Pa.s.

Viscosity is highly affected by temperature.

COMPRESSIBILITY - Gases compress (and expand) very

easily. Because gases are compressible and liquids are not,

measuring gas flow and measuring liquid flow require slightly different methods.

Flowrate is measured using a variety of methods and thus a variety of instrumentation. In some cases, you have a primary and

secondary device which in combination will give you the flowrate e.g. an orifice plate, alternatively the device directly measures the

flowrate e.g. turbine flow meter.

Flowrate measurement is divided into the following groups:

Differential Pressure Flowmeters and DP Cells

o Orifice Plate

o Venturi o Nozzle

Direct Force Flowmeters

o Rotameter

o Turbine meter o Mass flow meter (Coriolis)

Frequency Flowmeters

o Vortex flowmeter Electromagnetic (Mag) Flowmeters


pg 36 of 56

Differential Pressure Flowmeters

Orifice Plate

The purpose of primary measuring devices is to create a difference in pressure, or a pressure differential, in a flowing fluid (liquid or

gas) e.g. an orifice plate, venturi meter etc. A secondary device then measures or „reads‟ this differential to determine the flow

velocity or volume flow rate of the fluid e.g. a pressure

transmitter.

Any device that uses static pressure or head to measure fluid flow is a head-type flow instrument. If the flow through the venturi

tube (see diagram) is steady, then the fluid is forced to conform to the tube shape – filling it completely. If the fluid enters the tube

from the left (past point A) with a fixed velocity, it must speed up

to move the same fluid volume through the smaller diameter of the throat (point B). Then it must slow down to its original velocity

as it leaves the tube pas point C, where the tube is again the maximum diameter.

To achieve the needed increase in velocity (or kinetic energy) at the throat, the fluid must decrease its energy content by

decreasing its static pressure. Note that lowering the temperature of the fluid will also reduce its energy content.

A head-type flow device (venturi tube, orifice, etc), restricts the fluid flow to produce a difference in static pressure between two

points. This pressure difference is really an energy loss. When used with the principle of mass balance, this energy loss is an

accurate means for determining the velocity and flow rate of a

fluid.

An orifice plate is a plate with a hole through it, placed in the flow; it constricts the flow, and measuring the pressure differential

across the constriction gives the flow rate. It is basically a crude form of Venturi meter, but with higher energy losses.

Instead of gradually decreasing the steam size by reducing the tube diameter, the orifice in the plate reduces the steam size very

abruptly.


pg 37 of 56

The pressure drop across the orifice plate is measured and using

fluid flow mechanics, the flow rate of the fluid is then calculated.

There are three types of orifice plate (see picture below):

Concentric (middle and the two to the right)

Eccentric (extreme left but one)

Segmental (extreme left)

Note fluids often carry solids in suspension. When these fine

particles settle out of the flow, they can cause errors. As they fall to the bottom of the pipe, its area becomes smaller and it is no

longer perfectly round. An eccentric bore with its bottom even with the inside wall of the pipe, will usually overcome this. The fluid

velocity simply sweeps the solids through the orifice.


pg 38 of 56

Venturi Tube

The venturi tube operates in a similar fashion to the orifice plate, but is a more accurate instrument for measuring differential

pressure.

The principle behind the operation of the Venturi flowmeter is the Bernoulli effect. The Venturi measures a fluid's flowrate by

reducing the cross sectional flow area in the flow path and generating a pressure difference. After the pressure difference is

generated, the fluid is passed through a pressure recovery exit

section where up to 80% of the differential pressure generated at the throat is recovered. The pressure differential follows

Bernoulli's Equation.

By knowing the pressure and cross-sectional area at two locations, one can calculate the velocity of the fluid. With the velocity of the

fluid and its density, one can calculate the flowrate.


pg 39 of 56

Flow Nozzle

Pressure Transmitter

The flow nozzle, has greater accuracy than an orifice, but less than a venturi tube. Flow nozzles are ideal for measuring high pressure

high-temperature steam. Remember that the loss in the flow nozzle is much greater than in a venturi tube because of the

increased turbulence beyond the nozzle exit.

Note that for all the differential pressure flow measuring devices, the positioning of the pressure sampling points is critical.

There are many different methods for measuring the differential pressure across the restriction. These can range from a simple set

of pressure gauges, to a manometer to a complex DP cell. Although pressure gauges and manometers are relatively cheap,

they are not very accurate.

The P transmitter, or DP cell, is available in either an electronic or

a pneumatic model. The advantage of this type of transmitter is

that the receiving recorder / controller can be located at a much greater distance from the orifice than is possible with a

manometer.


pg 40 of 56

Direct Force Flowmeters

Rotameter

In the P transmitter, process fluid is piped to each side of a

capsule containing twin diaphragms as shown in the diagram. An increase in high pressure causes the capsule chamber to move

toward the low-pressure side and, in turn, imparts this motion to a laminated force bar. The upper end of the bar reacts by moving

the flapper toward the nozzle, which is the source of supply air. The nozzle feed line is connected to one side of a diaphragm in the

air relay.

These flow meters are governed by balancing forces within the

system.

A rotameter is a vertically installed tube that increases in diameter

with increasing height. The meter must be installed vertically so that gravity effects are easily incorporated into the governing

equations. Fluid flows in through the bottom of the tube and out through the top. Inside the glass tube there is a float that changes

position with the flow rate. When there is no liquid flow, the float rests in the bottom of the meter.


pg 41 of 56

Turbine Flowmeter

The applied concept for the rotameter is differential area. As the

flow rate of the fluid changes, the position of the float changes and annular area change directly, keeping a constant pressure

drop across the meter. Changes in float position and annular area are approximately linear with changes in flow rate. Upon achieving

a stable flow rate, the vertical forces are balanced and hence the

position of the float remains constant.

Generally, rotameters are inexpensive and simple to use.

The turbine flowmeter is a very simple operating instrument. As the name suggests, the flowmeter contains a turbine within the

body of the instrument, which has a set of vanes attached to the

turbine shaft.

As the fluid flows through the body of the instrument, the vanes

are forced to turn and spin on the shaft. The rotating vanes are picked up by a magnetic sensor above the meter. This rotation

speed is then converted into a linear flow velocity, which is proportional to the flowrate. Since the meter contains vanes etc,

hygiene is difficult to maintain in this unit. The units can normally

be found on incoming municipal water lines.


pg 42 of 56

Mass flow meters

The measurement of mass flowrate is as important as fluid

flowrate and as is independent of temperature and pressure, it makes measurement more accurate.

The measurement of mass flowrate is complex and relies on the

inertia forces created when a mass of liquid is forced to change

direction.

If one considers the forces exerted on the human body when it is forced to change direction, say in a car. Since the body is free to

move within the car, the force is dissipated, however if the body is

restrained the body will contort or twist to compensate for these inertia forces.

In a similar fashion, the pipes restrain the fluid that is forced to

flow through the tubes and thus all inertia forces will need to be

absorbed by the tubes. This is exhibited by a twist on the flowmeter tube.


pg 43 of 56

Coriolis flow meters generate this effect by diverting the fluid flow through a pair of parallel U-tubes undergoing vibration

perpendicular to the flow. This vibration simulates a rotation of the pipe, and the resulting Coriolis “drift” in the fluid will cause the U-

tubes to twist and deviate from their parallel alignment. This

Coriolis force producing this deviation is ultimately proportional to the mass flow rate through the U-tubes.

The fluid resists behind accelerated (exposed to inertia forces) in

the vertical plane and this causes the tube to twist, as shown below.

The tubes will then twist in an up and downward motion, causing the tube to vibrate. Magnetic position sensors measure the time

interval for the complete cycle i.e. up and down and back to position.

For small angles of displacement, mass flow is proportional to time

interval and geometric constants.

The carioles tubes are contained within a protective box. They are

however robust, sealed to the outside environment and are

hygienic in design.

Degree of twist is measured

One cycle of deflections


pg 44 of 56

Frequency Flowmeters

Vortex Flowmeter

Note that mass flowrate is independent of the following:

Vibration frequency

Density

Solids content

Viscosity

Gas bubbles

Whilst the above reasons, make the application of a mass

flowmeter seem very appealing, these units are extremely expensive.

These flow meters use frequency and electronic signals to calculate the flow rate.

Based on a phenomenon called vortex shedding. When flowing

fluid passes an unstreamlined body (called a bluff body), like the

one shown in (the diagram) the flow cannot follow the sharp contours of the obstruction. The fluid tends to separate into layers

and roll around the bluff body. This rolling action creates vortices that form on the sides of the body and move downstream. The

formation of vortices by the bluff body is a naturally occurring

phenomenon caused by the shape of the obstruction in the flow path.

The vortices alternately spin clockwise and then counterclockwise.

This is the natural way vortices form and is the basis for the meter‟s operation. As a vortex forms on one side of the body, a

low-pressure area is created. At the same time, the effect of the spinning fluid behind the obstruction starts a vortex on the

opposite side.

It is alternate shedding of vortices from one side of the bluff body

to the other that forms the basis of the meter‟s operation. Remember that pressure decreases when a vortex is formed.

When the vortex is shed, pressure increases until the next vortex

forms, at which time it again decreases.


pg 45 of 56

On the opposite side of the bluff body, pressure also increases and decreases due to vortex formation and shedding. The net result is

a measurable increase and decrease of pressure across the bluff body. Vortex-shedding meters are therefore designed with sensors

on opposite sides of the bluff body to detect this change in

pressure.

The vortex-shedding frequency (the time between formation of vortices) is a direct function of flow velocity.

The sensors mounted on each side of the bluff body detect the change in pressure that accompanies the shedding of a vortex.

The output of the pressure sensors is a noisy sine wave. Its frequency is identical to the vortex-shedding frequency. Since a

definite relationship exists between vortex-shedding frequency and

flow velocity, the velocity can be determined if the frequency is known.

The signal converter accepts the noisy sine-wave signal from the

sensors in the sides of the bluff body. It converts this signal to a

standard output (normally 4 to 20 mA or a standard square wave whose frequency is proportional to flow rate).

The standard output signal from the signal converter can be

applied to various output devices. These include strip chart

recorders, digital or analog displays, flow totalizers, or the input of a computer.

The vortex-shedding meter has many advantages. It is simple in

design and has no moving parts in the flow stream. Its accuracy and repeatability are good, and it has a wide flow range (20:1 is

common). It does not require calibration since the vortex-shedding

frequency is dependent upon the shape of the bluff body, flow velocity, and fluid properties.


pg 46 of 56

Electromagnetic Flowmeters

(called Magflow Meters)

The most obvious disadvantage of the vortex-shedding meter is its inability to handle dirty or abrasive fluids. These fluids would

quickly erode the bluff body, changing its shape. A change in shape would alter its vortex-shedding characteristics.

The bluff body creates a pressure drop across the meter that could be unacceptable for some applications. Vortex-shedding meters in

sizes above (± 150mm) are extremely expensive.

Magnetic flowmeters are very nearly ideal, as they offer no restriction to fluid flow and can meter fluids that are almost

impossible to handle with other instruments. Important metering

applications include sludge in sewage treatment plants, ore slurries in mining operations, liquid metals (liquid sodium) in various

industrial processes.

Magnetic flowmeters operate on the same principle as the electric

generator. When an electrical conductor moves at right angles to a magnetic field, a voltage is induced in the conductor. This is

illustrated in the diagram. The voltage induced is proportional to the strength of the magnetic field and the velocity of the

conductor. If there is an increase in the intensity of the magnetic field or in the velocity of the conductor, the voltage induced in the

conductor also increases, in accordance with Faraday‟s Law of

Electromagnetic Induction.

A simplified schematic of a magnetic flowmeter is shown in (the illustrated diagram). Notice that the moving conductor is replaced

by fluid flowing through the meter flow tube. If the fluid is

electrically conductive (if an electrical current can travel through the fluid), it acts as one big conductor moving down the meter

flow tube.


pg 47 of 56

Just as in the electric generator, there is a conductor moving at right angles to a magnetic field, and a voltage is induced across

the conductor. The electrodes penetrate the meter flow tube, make contact with the fluid, and are used to measure the induced

voltage across the conductive fluid.

The magnetic flowmeter is similar to the electric generator in another respect. The magnitude of induced voltage is proportional

to the strength of the magnetic field, or flux, and to the conductor

velocity. If the magnetic flux increases, the voltage measured by the electrodes also increases. If the velocity of the fluid moving

down the pipe increases, the voltage measured by the electrodes increases. In fact, the induced voltage is linearly proportional to

flow velocity if flow velocity doubles, electrode voltage doubles.

The output of a magnetic flowmeter is a voltage that varies

linearly with flow velocity. The magnitude of the voltage must be measured and used to indicate flow.

Two magnetic coils fit on opposite sides of the meter body‟s inner

surface. These coils provide the magnetic field for the meter. The

steel meter body acts as the return path for the magnetic field generated by the coils. The coils are potted within an epoxy-based

compound. The compound holds them in a fixed position and protects them from moisture and damage.

An insulating liner covers the interior of the meter, protecting the

coils and body from the fluid passing through the meter. The liner

material used depends upon the temperature of the metered fluid and its corrosive and abrasive properties.

Two electrodes penetrate the meter body and liner at right angles

to the magnetic field coils. Their purpose is to measure the voltage

induced across the fluid. The electrodes are completely insulated from the meter body.


pg 48 of 56

They are flush with the liner and in contact with the fluid. In most meters, the electrodes can be replaced without disassembling the

meter or removing it from the pipeline.

Electrodes are made of various materials, to withstand the

corrosive and abrasive effects of the metered fluid.

Certain fluids coat the inside of the meter with deposits that isolate the fluid from the electrodes. When this type of insulation occurs,

no signal can be measured. To avoid this problem, many magnetic flowmeters are equipped with devices that clean the electrodes

and liner. One of them impresses an ultrasonic frequency upon the

electrodes. It keeps them continuously vibrating, thus loosening deposits. Another technique induces an electrical current in the

meter body that heats both the body and liner and prevents build-up of sludge and grease. Techniques such as these can minimise

shutdowns for liner and electrode cleaning.

A magnetic flowmeter offers absolutely no obstruction to fluid

flow. This is its primary advantage. The pressure drop across the meter is quite small, which is an important consideration in many

applications, since pressure drop must be overcome by increased

pumping power. Other significant advantages are:

Magnetic flowmeters have no moving parts, seals, or

diaphragms to fail. Liner materials are available that make the meter suitable for

use with almost any corrosive or abrasive liquid.

The output of a magnetic flowmeter is a voltage signal that

increases linearly with fluid velocity. Magnetic flowmeters are available for fluids with temperatures

in excess of 15000F.

Operation is virtually unaffected by changes in fluid viscosity,

pressure, temperature, and density.

Cost is a prime disadvantage of a magnetic flowmeter. In small

sizes, it is probably the most expensive of all the flowmeters.


pg 49 of 56

Specific Gravity Measurement

Hydrometer

Other disadvantages include:

It is unsuitable for metering gases

The liquid being metered must be electrically conductive

Meter calibration is difficult and needs to be done on a bench

Accuracy greater than 2%, although possible, should not be

expected The meter is heavy and bulky

Specific gravity is the ratio of the density of a sample to the

density of water. The ratio depends on the temperature and pressure of both sample and water.

These are manual instruments and are calibrated for a particular

range of densities. The hydrometer consists of a bulb containing a

mass of material and a calibrated tube.

The hydrometer is inserted into the fluid (in a beaker) and once the hydrometer has stopped bobbing up and down, the density or

specific gravity can be measured.


pg 50 of 56

Vibrating U Tube

The vibrating or oscillating U-tube is a technique to determine the density of liquids and gases based on an electronic measurement

of the frequency of oscillation, from which the density value is calculated. This measuring principle is based on the Mass-Spring

Model.

The sample is filled into a container containing a hollow, U-shaped

glass tube (oscillating U-tube) which is electronically excited into

undamped oscillation (at the lowest possible amplitude). The two branches of the U-shaped oscillator function as its spring elements.

The direction of oscillation is normal to the level of the two branches.

The oscillator‟s eigenfrequency is only influenced by the part of the

sample that is actually involved in the oscillation. The volume

involved in the oscillation is limited by the stationary oscillation knots at the bearing points of the oscillator. If the oscillator is at

least filled up to its bearing points, the same precisely defined volume always participates in the oscillation, thus the sample‟s

mass can be considered proportional to its density.

Overfilling the oscillator beyond the bearing points is irrelevant to

the measurement. For this reason the oscillator can also be employed to measure the density of sample media that flows

through the tube and can thus be used inline.


pg 51 of 56

Refractometers

Haze or Turbidity

Measurement

Refractometers measure the refractivity of a solution using a light source and then calculate the density from the degree of

refraction. You can get bench scale refractometers, as well as inline process refractometers.

Inline process refractometers are a type of refractometer designed for the continuous measurement of a fluid flowing through a pipe

or inside a tank. These refractometers typically consist of a sensor, placed inline with the fluid flow, coupled to a control box.

A digital inline process refractometer sensor measures the refractive index and the temperature of the fluid. The

measurement is based on the refraction of light in the process

medium, i.e. the critical angle of refraction using a light source. The measured refractive index (scatter pattern) and temperature

of the process medium are sent to the control box. It calculates the concentration of the process liquid based on the refractive

index and temperature, taking pre-defined process conditions into account. The output is proportional to process solution

concentration, liquid density, Brix or other scale that has been

selected for the instrument.

Turbidity is the cloudiness or haziness of a fluid caused by individual particles (suspended solids) that are generally invisible

to the naked eye, similar to smoke in air. The measurement of

turbidity is a key test of beer quality.

Fluids can contain suspended solid matter consisting of particles of many different sizes. While some suspended material will be large

enough and heavy enough to settle rapidly to the bottom of the

container if a liquid sample is left to stand (the settable solids), very small particles will settle only very slowly or not at all if the

sample is regularly agitated or the particles are colloidal. These small solid particles cause the liquid to appear turbid.


pg 52 of 56

A haze meter works on a similar principle to that of a refractometer, in that it uses light as a medium for measure. A

haze meter measures the degree of the scatter, after the light waves from the source have collided with the particles in the

solution. They specifically look 90⁰, 25⁰ and 11⁰ degree scatter.

The measured value of scattered light depends on:

particle concentration

particle size and particle size distribution

measuring angle

Note that changes in intensity of light at an angle of 90° does not strictly depend on the size of the particles, however at an angle of

11° the particle size influence the intensity of the scattered light.

The drawing shows clearly, that the scattered light of 25° is a mixture of 90° and 11°.

Forward scattered light under 11° shows a high sensitivity for particles like Kieselguhr and yeast and is not affected by small

particles as colloids.

Scattered light at 25° shows a certain sensitivity for particles like

Kieselguhr, yeast and colloids. Changes in the amount of colloids or Kieselguhr will affect the measured value.

Scattered light under 90° is not affected by the particle size, as it

is normally molecular in nature. It depends on the numbers of

particles only.

Both 11° and 25° hazes can be removed using a filter, however 90° haze is very difficult to remove, so normally you would blend

out the high turbidity!

A standard haze meter has a light source (1) and two sets of

measuring detectors. One at 90⁰ to the light source (9) and one

set opposite the light source (6,7).


pg 53 of 56

The picture below shows an inline haze meter, showing the light

source and the detectors.


pg 54 of 56

Dissolved Oxygen Measurement

Measuring the dissolved oxygen (DO) in a beer is critical to the quality of the beer, as high levels of DO in a beer will result in pre-

mature staling of the beer.

The instrument used to measure DO is an Orbisphere. The

Orbisphere is an electrochemical oxygen sensor and it uses the chemistry of oxygen reactions to determine the levels of DO in a

fluid. Note that this instrument can also be used to measure CO2, but primarily it is used for DO.

The sensor of an Orbisphere is shown below:

It has a gas permeable membrane through which the fluid and

dissolved gas diffuses. Within the sensor are two electrodes and an electrolyte fluid (normally potassium hydroxide or potassium

chloride). The cathode is made of gold, whereas the anode is

made of silver.

The sensor is contained within a probe.


pg 55 of 56

Carbon Dioxide

Measurement

In the presence of oxygen the following reactions occur:

At the cathode:

2 H2O + O2 + 4 e- 4 OH-

At the anode:

4 Ag 4 Ag+ + 4 e-

In the aqueous solution:

2 H2O + 4 Ag + O2 4 Ag+ + 4 OH-

In the electrolyte:

Ag+ + Cl- AgClppt

Using normal electrochemistry, an electrical current will be established between the electrodes. The size of the current will

directly correlate to the concentration of oxygen in the liquid and thus give you a measure of the dissolved oxygen in the fluid.

The level of carbon dioxide in a fluid is measured using a Gehaltemeter. The instrument can be both bench and inline.

The functionality of the Gehaltemeter is based on Henry‟s and

Dalton‟s law. According to these two laws, the CO2 concentration can be calculated from the pressure and temperature, by

measuring the partial pressure of CO2 in the fluid.

From pressure and temperature, the digital manometer

automatically calculates the CO2 concentration of the sample.


pg 56 of 56

pH Measurement

The inline measurement of pH is one of the most difficult operations to perform and is normally performed on a bench in the

laboratory or a workbench in the brewery.

The pH meter measures the concentration of hydronium ions in

the solution and via a mathematical equation you get a measure of pH. pH is used to described as the degree of acidity or alkalinity of

a solution and is measured on a scale from 1 – 14. The lower end being the more acidic. A solution is ocnsidered neutral if the pH is

measured as 7.

The pH meter consists of the pH probe and the meter. The probe

is a glass tube with a glass membrane electrode at the base of the tube. A current is applied to the electrode and depending on the

degree of conductivity of the fluid being measured, a specific voltage will be measured. This voltage is proportional to the

concentration of hydronium ions in the solution.

pH probes are best used in the laboratory or a workbench in the brewery as they are not robust , are difficult to clean and require

temperature correction for accurate results.

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