instrumentation course

Oil and Gas Measuring Instruments

1

Course Aim

The aim of this training course is to build up the procedural and

declarative knowledge required to be recognized by projects engineer that

do not have past background of oil and gas measuring instruments. This

will help them to supervise projects dealing with instrumentation in plants

with a strong background.

In this course, the training cycle is divided in five steps that necessitate

the cooperation between the instructor and the trainees. These steps are

shown in figure below, they are summarized as follows:

1. Define the knowledge and skills required to be developed.

2. Define the elements of each knowledge or skill.

3. Formulate a verbal phrase for the learning objective of each

element.

4. Choose an adequate instructional activity to present each element.

5. Set up an indicator to measure the outcomes of the course and

modify the training skills to adapt the vocational needs.

Determine

Elements

Learning

Objectives

Define

Knowledge

& Skills

Instruction

Activity

Measure

& Correction

Determine

Elements

Learning

Objectives

Define

Knowledge

& Skills

Instruction

Activity

Measure

& Correction

Training Cycle.


2

Knowledge and Elements

Introduction to measurements.

Introduce general terms.

Introduce quantities and units.

Distinguish between different gauges and switches.

Introduce how quantity is measured.

Illustrate main components of instrument.

Classify different types of measuring instruments.

Develop knowledge about different transmitters and sensing

elements.

Establish knowledge base about transmitter technology.

Introduce Sensing Element.

Introduce theory of operation.

Introduce some analyzers.

Gas Chromatography.

Moisture Analyzer.

Oxygen Analyzer.


3

Table of Contents

Section I

Chapter 1 Introduction to Measuements 5

Chapter 2 Transmitters 16

Section II

Chapter 3 Mechanical Transducers 25

Chapter 4 Electric Transducers 36

Chapter 5 Flowmeters 73

Section III

Chapter 6 Analyzers 102

Chapter 7 Basic Considerations 109


4


5

Chapter 1

Introduction to Measurement

1.1 Learning objectives

1. Introduce measurements and instruments.

2. Classify instruments and functions.

3. Understand instruments characteristics.

1.2 Measurements

The measurement of a given quantity is an act or the result of

comparison between the quantity and a predefined standard. Since two

quantities are compared, the result is expressed in numerical values. In

fact, the measurement is the process by which one can convert physical

parameters to meaningful numbers. In order that the results are

meaningful, there are two basic requirements:

1. The standard used for comparison purposes must be accurately

defined and should be commonly accepted.

2. The apparatus used and the method adopted must be proved.

1.2.1 Significance of Measurements

The advancement of science and technology is dependent upon a

parallel progress in measurement techniques. There are two major

functions in all branches of engineering:

1. Design of equipment and processes.

2. Proper operation and maintenance of equipment and processes.

Both of these functions require measurements.


6

1.2.2 Methods of Measurements

Direct Method: The unknown quantity is directly compared against

a standard.

Indirect Method: Measurement by direct methods are not always

possible, feasible and practicable. These methods in most of the

cases are inaccurate because of human factors. They are also less

sensitive.

1.2.3 Instruments

In simple cases, an instrument consists of a single unit which gives

an output reading or signal according to the unknown variable applied to

it. In more complex situations, a measuring instrument consists of several

separate elements. These elements may consist of transducer elements

which convert the measurand to an analogous form. The analogous signal

is then processed by some intermediate means and then fed to the end

devices to present the results for the purposes of display and or control.

These elements are:

A detector.

An intermediate transfer device.

An indicator.

The history of development of instruments encompasses three phases:

Mechanical.

Electrical.

Electronic.


7

1.2.4 Classification of Instruments

Absolute instruments: These instruments give the magnitude of the

quantity under measurement in terms of physical constants of the

instrument. Example: Galvanometer.

Secondary Instrument: These instruments are constructed that the

quantity being measured can only be measured by observing the

output indicated by the instrument.

1.2.4.1 Deflection Type

The deflection of the instrument provides a basis for determining

the quantity under measurement as shown in figure (1.1).

Figure 1.1 Deflection Type

1.2.4.2 Null Type

A zero or null indication leads to determination of the magnitude of

measured quantity as shown in figure (1.2).


8

Figure 1.2 Null Type

1.2.4.3 Contact Type

Often when a measured pressure reaches a certain max or min

value, it is desirable to have an alarm sound a warning, a light to

give a signal, or an auxiliary control system to energize or de-energize. A

micro switch is the device commonly used for this purpose.

Figure 1.3 Contact Type


9

1.2.5 Analog and Digital Modes of Operation

Analog Signal: signals that vary in a continuous fashion and take

an infinite number of values in any given range.

Digital signal: signals that vary in discrete steps and thus take only

finite different values in a given range.

1.2.6 Functions of Instruments

Indicating function.

Recording function.

Controlling Function.

1.3 Characteristics of Instruments

1.3.1 Performance

It is to define a set of criteria that gives a meaningful description of

quality of measurement. Performance characteristics are obtained in one

form or another by a process called calibration. The calibration of all

instruments is important since it affords the opportunity to check the

instrument against a known standard.

1.3.2 Errors in Measurement

Measurements always involve errors. No measurement is free from

errors. An understanding and thorough evaluation of the errors is

essential.


10

Figure 1.4 Visual error

1.3.3 True Value

True Value: The true value of quantity to be measured may be

defined as the average of an infinite number of measured values when the

average deviation due to the various contributing factors tends to zero.

1.3.4 Ranges

Scale range: it is defined as the difference between the largest and

the smallest reading of the instrument, i.e. scale range from 200 to

500 degree C.

Scale Span: It is may be confusing with scale range but it is given

to be 300 degree C.

Effective Range: It is defined as the range over which it meets

some specified accuracy requirements.

Rangeability (turndown): If the effective range is from A to B, then

the rangeability is defined by B/A.

1.3.5 Discrimination, Accuracy, Error, Precision and Sensitivity

Discrimination (Resolution): It is used to describe how finely an

instrument can measure. For example, the discrimination of a


11

digital electronic timer reading in milliseconds is a hundred times

as great as that of a stopwatch graduated in tenths of seconds. It is

often wrongly referred as sensitivity.

Accuracy: It is the closeness with which the instrument reading

approaches the true value of the quantity. Thus accuracy means

conformity to truth.

Error: It is defined as the difference between the measured value

and the true value. One kind of error is observational error.

Precision: It is a measure of the degree of agreement within a group

of measurements. High precision means a tight cluster and repeated

results while low precision indicates a broad scattering of results.

Certainty: It is often used as a synonym for accuracy. However,

Uncertainty is the property of a measurement rather than the

instrument used to make the measurement.

Sensitivity: It is a measure of how an instrument is sensitive to the

measured quantity variation. It is the ability to produce detectable

output.

Figure 1.5 Accuracy and Repeatability

1.3.6 Reproducibility, Repeatability and Hysteresis

Reproducibility: It is the closeness of agreement among repeated

measurements of the output for the same value of input mode under


12

the same operating condition over a period of time, approaching

from both directions.

Repeatability: It is the closeness of agreement among a number of

consecutive measurements of the output for the same value of input

under the same operating conditions, approaching from the same

direction.

Figure 1.6 Repeatability

Hysteresis and Dead Band: It is the maximum difference for the

same input between the upscale and downscale output values

during a full range transverse in each direction.

Dead Time: It is defined as the time required by an instrument to

begin to respond to a change in the measurand.

Dead Zone: It is defined as the largest change in which there is no

output from the instrument.


13

Figure 1.7 Hysteresis and Dead band

1.3.7 Drift

Perfect Reproducibility means no drift. No drift means that with a

given input the measured values do not vary with time.

Zero Drift: if the whole calibration gradually shifts.

Span Drift: If there is a proportional change in the indication all

along the upward scale.

Zonal Drift: In case the drift occurs only over a portion of the span.

Figure 1.8 Drift


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1.3.8 Noise

A spurious current or voltage extraneous to the current or voltage

of interest in an electrical or electronic circuit is called noise.

1.3.9 Linearity

It is the closeness to which a curve approximates a straight line. It

is a measure of the extent to which the instrument calibration curve over

its effective range departs from the best fitting straight line.

Figure 1.9 Linearity

1.3.10 Loading Effects

The ideal situation in a measuring system is that when an element

used for any purpose, the original signal should remain undistorted. In

practical conditions, it has been found that any element in the system

extracts energy and thereby distorting the original signal.


15

1.3.11 Other Effects

Temperature Effect

Pressure Effect

Vibration Effect

1.4 Role Play

Each Trainee should speak thoroughly about one of the learning objective

elements.


16

Chapter 2

Transmitters

2.1 Learning Objectives

1. Introduce history of transmitter technology.

2. Understand analog transmitters.

3. Understand smart transmitters with HART protocol.

2.2 Transmitter Technology

Transmitters are instruments that transfer measured output signal to

distance places where it is needed. The technology development through

years is:

1. Pneumatic and Hydraulic.

2. Electrical (Analog – 4-20 mA).

3. Electronic (Analog – 4-20 mA + Digital – HART protocol).

4. Electronic (All digital – Foundation Fieldbus).

Figure 2.1 Pneumatic Transmitter


17

2.3 Analog Transmitters

Analog transmitter uses a variable conversion element to translate

and accommodate the physical non-electrical measurand to electrical

analog signal (4-20 mA).

Figure 2.2 Analog Transmitter

2.3.1 Measurement Converters of Electrical Quantities

Measuring amplifiers: demands on measuring amplifiers, negative

feedback, ideal operational amplifier, basic circuits of measuring

amplifiers using operational amplifiers (OAs)

Measurement of low voltages and currents using OAs, estimating

uncertainty of measurement (including influence of input voltage

offset and input bias).

Rectifiers (converters of the rectified mean value).

2.3.2 Ideal Operational Amplifiers

Figure 2.3 Ideal OP-Amp


18

2.3.3 Inverting amplifier

Figure 2.4 Inverting Amplifier

2.3.4 Current to Voltage Converter

Figure 2.5 Current to Voltage converter

2.3.5 Voltage Controlled Current Source

Figure 2.6 Voltage controlled Current source

2.3.6 Rectifiers

Figure 2.7 Rectifier


19

2.3.7 Adders

Figure 2.8 Adders

2.3.8 Differential Amplifiers

Figure 2.9 Differential Amplidier

2.3.9 Integrators

Figure 2.10 Integrators


20

2.4 HART Protocol

2.4.1 HART Overview

For many years, the field communication standard for process

automation equipment has been a milliamp analog current signal. HART

field communications protocol extends the 4-20 mA standards to enhance

communication with smart field instruments. It was designed for use with

intelligent measurement and control instruments which traditionally

communicate using mA analog signals. HART preserves the 4-20 mA

signals and enables two way digital communications to occur without

affecting the integrity of 4-20 mA signal.

Figure 2.11 Hart Digital Signal

HART, highway addressable remote transducer, makes use of Bell

202 FSK standard to superimpose digital signal at a low level on top of

analog signal; i.e. 1200 Hz for logic 1 and 2200 Hz for logic 0. HART

communicates 1200 bps without interrupting the mA signal and allows a

host application to get two or more digital updates per second from a field

device.


21

Figure 2.12 HART Connection

HART is a master/slave protocol which means that a field device

(slave) only speaks when spoken to by a master. HART provides for up to

two masters, primary and secondary, as shown in figure (2.12).

Figure 2.13 Master/Slave

The most commonly employed communication mode is the

master/slave, figure (2.13). The optional burst communication mode

where a slave device can continuously broadcast a HART reply message,

figure (2.14).

Figure 2.14 Burst

2.4.2 HART Benefits

2.4.2.1 35-40 data items Standard in every HART device

Device Status & Diagnostic Alerts;

Process Variables & Units;

Loop Current & % Range;

Basic Configuration Parameters;

Manufacturer & Device Tag;


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2.4.2.2 Increases control system integrity

Get early warning of device problems;

Use capability of multi-variable devices;

Automatically track and detect changes (mismatch) in Range

or Engineering Units;

Validate PV and Loop Current values at control system

against those from device;

2.4.2.3 HART is Safe, Secure, and Available

Tested and Accepted global standard;

Supported by all major instrumentation manufacturers;

2.4.2.4 Saves Time and Money

Install and commission devices in fraction of the time;

Enhanced communications and diagnostics reduce

maintenance & downtime;

Low or no additional cost by many suppliers;

2.4.2.5 Improves Plant Operation and Product Quality

Additional process variables and performance indicators

Continuous device status for early detection of warnings and

errors

Digital capability ensures easy integration with plant

networks

2.4.2.6 Protects Your Asset Investments

Compatible with existing instrumentation systems,

equipment and people

Allows benefits to be achieved incrementally

No need to replace entire system


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2.5 Role Play


elements.

Analog Transmitters

Smart Transmitters and HART Protocol.


24


25

Chapter 3

Mechanical Transducers


1. Understand the theory of operation of different sensing elements.

3.2 Springs

Most mechanical input instruments employ mechanical springs of

one form or another. Various common types of springs are shown in

figure (3.1). These range from cantilever, helical and spiral springs.

Figure 3.1 Springs

3.3 Pressure Sensing Elements

Most pressure devices use elastic elements for sensing pressure at

the primary stage. A link and gear mechanism are used to convert the

movement to rotational motion to be connected the scale and pointer.


26

3.3.1 Bourdon Tubes

The bourdon tubes are made out of an elliptical flattened bent tube.

One end is sealed and the other is open for fluid to enter. The pressure of

the fluid tends to straighten out the tube. This motion is transferred to the

pointer.

3.3.1.1 C-Type

It is the most used for local indication.

Figure 3.2 Bourdon Type


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3.3.1.2 Spiral Type

Increasing the number of turns will increase the displacement of the free

tip without changing the wall thickness.

Figure 3.3 Spiral type

3.3.1.3 Helical Type

The displacement of the tip of the helical type is larger than that of the

spiral one.

Figure 3.4 Helical type

3.3.2 Bellows

A metallic bellows is a series of circular parts, resembling the folds

in an accordion. The parts are designed in such a way that there are

expanded and contracted.


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Figure 3.5 Bellows Type

3.3.3 Diaphragms

The operating principle of diaphragm elements is similar to that of

the bellows. The pressure applied causes it to deflect where the deflection

is proportional to the applied pressure.

Figure 3.6 Diaphragm Type


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13.4 Temperature Sensing Elements

3.4.1 Bimetallic Thermometer

They are used for local temperature measurements. It is constructed

by bonding two different metals such that they cannot move relative to

each other. All metals try to change their physical dimensions at different

rates when subjected to same change in temperature. The differential

change in expansion of two metals results in bending or flattening the

structure, which in turn moves the pointer via the intermediate element.

3.4.1.1 Strip

Figure 3.7 Strip Type

3.4.1.2 Spiral

Figure 3.8 Spiral type


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3.4.1.3 Helical

Figure 3.9 Helical Type

3.4.2 Distance Reading

There are three basic types of distant reading thermometers.

Liquid filled

Gas filled

Combination liquid-vapor filled

The thermometers are filled with fluid at some temperature and sealed.

Almost the entire volume of the fluid is in the sensing bulb.

Figure 3.10 Distance Reading Type


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3.5 Level Sensing Elements

Figure 3.11 Installation

3.5.1 Transparent Glass

Sight Glasses for Level Gauges grant the best chemical and

physical properties, holding a very precise place as for chemical

composition within the very large group of "Borosilicate Glass" which is

suitable for many applications.

Figure 3.12 Level Glass

3.5.2 Circular Sight Ports

These are used to allow observation within sealed vessels.

Figure 3.13 Dight Port


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3.5.3 Reflex Type

Reflex level gauges working principle is based on the light

refraction and reflection laws. Reflex level gauges use glasses having the

face fitted towards the chamber shaped to have prismatic grooves with

section angle of 90°. When in operation, the chamber is filled with liquid

in the lower zone and gases or vapors in the upper zone; the liquid level is

distinguished by different brightness of the glass in the liquid and in the

gas/vapor zone. The reflex level gauges do not need a specific

illumination: the day environmental light is enough. Only during the

night an artificial light must be provided.

Figure 3.14 Reflex Type

3.5.4 Bicolor Type

An illuminator with special red and a green filters is fitted on the

gauge at the opposite side with respect to the observer. This special

illuminator conveys light through the filters obliquely to the back glasses

of the level gauge. Said filters allow crossing only to red and green rays.

Such colored rays reach, through the back glass, the media inside level

body. When the gauge contains steam, green rays are considerably

deviated and prevented from emerging by the observer side; then only red

light, whose rays are smoothly deviated by steam, passes through the

whole internal hole, reaching the observer. Conversely when rays find


33

water, red rays are considerably deviated and lost inside the internal part

of level gauge, green rays can reach the front glass and seen by the

observer.

Figure 3.15 Bicolor Type

3.5.5 Magnetic Type

Operation of BONT Magnetic Level Gauge is based on some

elementary physical principles:

The principle whereby liquid in communicating vessels is always

at same level;

Archimedes's principle according to which a body immersed in a

liquid receives a buoyancy equal to the weight of displaced liquid;

The principle of attraction between North and South poles of two

permanent magnets and that of repulsion between like poles.

o This principle has two applications in the BONT magnetic

level gauge:

first between the magnet in the chamber float and

every single magnet of the indicating scale:

Second between the magnets of the indicating scale.


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Figure 3.16 Magnetic Type

3.5.6 Gamma Level Switching

The transmission of gamma radiation through a container is

affected by the level contents. The intensity of the transmitted radiation is

measured and used to activate switches when pre-set intensity levels are

reached.

Figure 3.17 Gamma Rays Type

3.6 Seismic Transducer (Vibration)

A schematic diagram is shown in figure (3.18). The mass is

connected through a spring and damper arrangement to a housing frame.

The housing frame is connected to the source of vibrations to be


35

measured. The mass has the tendency to remain fixed in its spatial

position so that the vibration motion is registered as a relative

displacement between mass and housing frame. The seismic transducer

may be used in two different modes. A large mass and a soft spring are

suited for displacement mode, while a relatively small mass and a stiff

spring are used for acceleration mode.

Figure 3.18 Seismic Type

3.7 Role Play

Each Trainee should speak thoroughly about:

Pressure Sensing

Level Sensing

Temperature Sensing

Vibration Switches.


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Chapter 4

Electrical Transducers


1. Introduce electrical transducers.

2. Understand the theory of operation of different transducers.

4.2 Introduction

In order to measure non-electrical quantities, a detector is used

usually to convert the physical quantity into a displacement. In electrical

transducers the output is different, it is in electrical form. The output

gives the magnitude of the measurand. The electric signal may be current,

voltage or frequency and production of these signals is based upon

electrical effects which may be resistance, capacitance, induction, etc.

A transducer may be defined as a device, which converts energy

from one form to another. In electrical instrumentation, a transducer may

be defined as a device which converts a physical quantity into electrical

signal. Another name of a transducer is pick up.

4.2.1 Advantages of Electrical Transducers

Amplification and attenuation may be done easily.

The mass-inertia effects are minimized.

The effects of friction are minimized.

Low power level.

Use of telemetry.


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4.2.2 Classification of Transducers

The transducer consists of two closely related parts:

Detector Element: It is the part that responds to physical

phenomenon.

Transduction Element: It transforms the output of the sensing

element to an electrical output.

Classification of transducers is as follows:

Based on Transduction: like piezoelectric, thermoelectric, etc.

Primary and Secondary: Example, a primary part that transforms

pressure into displacement and secondary part that transforms

displacement into electrical form.

Passive and Active: Depends on whether the transducer will derive

power from or to the circuit.

Analog and Digital: Analog continuous form like voltage or digital

form like pulses.

Transducers and Inverse Transducers: It depends whether the

transducer convert physical quantity to electrical signal or vice

versa.

4.3 Pressure Sensing Elements

4.3.1 Strain Gauges

If a metal conductor is stretched or compressed, its resistance

changes on account of the fact that both length and diameter are changed.

This property is called piezoresistivity.

Figure 4.1 Strain Gauge


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4.3.2 Inductive Type

Figure (4.2) shows an arrangement which uses coils to form the

two arms of an AC bridge. The pressure acts on the diaphragm and

disturbs the reluctance of the paths of magnetic flux for both coils.

Figure 4.2 Inductive Type

4.3.3 Capacitive Type

They convert pressure into displacement which changes the

capacitance value by changing the distance between the two parallel

plates of a capacitor.

Figure 4.3 Capacitive Type


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4.3.4 Linear Variable differential Transformer

The LVDT is used as secondary transducer for measurement of

pressure. The pressure is converted into displacement which is sensed by

LVDT and converted into a voltage.

Figure 4.4 LVDT

4.3.5 Photoelectric Type

As shown in figure (4.5) the light path is affected by the applied

pressure which in turn affects the quantity of light received by the

photoelectric transducer.

Figure 4.5 Photoelectric Type


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4.3.6 Piezoelectric Type

A piezoelectric material is one in which an electric potential

appears across certain surfaces if the dimensions of the crystal are

changed by the application of mechanical force. The potential is produced

by the displacement of charges. The effect is reversible and is known as

the piezoelectric effect.

Figure 4.6 Piezoelectric Type

4.4 Temperature Sensing Elements


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4.4.1 Thermocouple

The thermocouple is one of the simplest of all sensors. It consists

of two wires of dissimilar metals joined near the measurement point. The

output is a small voltage measured between the two wires.

Figure 4.7 The thermocouple

While appealingly simple in concept, the theory behind the thermocouple

is subtle, the basics of which need to be understood for the most effective

use of the sensor.

4.4.1.1 Thermocouple theory

A thermocouple circuit has at least two junctions: the measurement

junction and a reference junction. Typically, the reference junction is

created where the two wires connect to the measuring device. This second

junction it is really two junctions: one for each of the two wires, but

because they are assumed to be at the same temperature (isothermal) they

are considered as one (thermal) junction. It is the point where the metals

change - from the thermocouple metals to what ever metals are used in

the measuring device - typically copper.

The output voltage is related to the temperature difference between

the measurement and the reference junctions. This is phenomena is

known as the Seebeck effect. In practice the Seebeck voltage is made up

of two components: the Peltier voltage generated at the junctions, plus the

Thomson voltage generated in the wires by the temperature gradient.


42

Figure 4.8 Signal generated by temperature gradient

The Peltier voltage is proportional to the temperature of each

junction while the Thomson voltage is proportional to the square of the

temperature difference between the two junctions. It is the Thomson

voltage that accounts for most of the observed voltage and non-linearity

in thermocouple response.

Each thermocouple type has its characteristic Seebeck voltage

curve. The curve is dependent on the metals, their purity, their

homogeneity and their crystal structure. In the case of alloys, the ratio of

constituents and their distribution in the wire is also important. These

potential inhomogeneous characteristics of metal are why thick wire

thermocouples can be more accurate in high temperature applications,

when the thermocouple metals and their impurities become more mobile

by diffusion.

4.4.1.2 The practical considerations of thermocouples

The above theory of thermocouple operation has important

practical implications that are well worth understanding:

1. A third metal may be introduced into a thermocouple circuit and have

no impact, provided that both ends are at the same temperature. This

means that the thermocouple measurement junction may be soldered,

brazed or welded without affecting the thermocouple's calibration, as long

as there is no net temperature gradient along the third metal.

Further, if the measuring circuit metal (usually copper) is different to that

of the thermocouple, then provided the temperature of the two connecting


43

terminals is the same and known, the reading will not be affected by the

presence of copper.

2. The thermocouple's output is generated by the temperature gradient

along the wires and not at the junctions as is commonly believed.

Therefore it is important that the quality of the wire be maintained where

temperature gradients exists. Wire quality can be compromised by

contamination from its operating environment and the insulating material.

For temperatures below 400°C, contamination of insulated wires is

generally not a problem. At temperatures above 1000°C, the choice of

insulation and sheath materials, as well as the wire thickness, become

critical to the calibration stability of the thermocouple.

The fact that a thermocouple's output is not generated at the junction

should redirect attention to other potential problem areas.

3. The voltage generated by a thermocouple is a function of the

temperature difference between the measurement and reference junctions.

Traditionally the reference junction was held at 0°C by an ice bath:

Figure 4.9 Traditional Thermocouple Measurement

The ice bath is now considered impractical and is replaced by a reference

junction compensation arrangement. This can be accomplished by

measuring the reference junction temperature with an alternate

temperature sensor (typically an RTD or thermistor) and applying a

correcting voltage to the measured thermocouple voltage before scaling to

temperature.


44

Figure 4.10 Modern Thermocouple Measurement

The correction can be done electrically in hardware or mathematically in

software. The software method is preferred as it is universal to all

thermocouple types (provided the characteristics are known) and it allows

for the correction of the small non-linearity over the reference

temperature range.

4. The low-level output from thermocouples (typically 50mV full scale)

requires that care be taken to avoid electrical interference from motors,

power cable and transformers. Twisting the thermocouple wire pair (say 1

twist per 10 cm) can greatly reduce magnetic field pickup. Using shielded

cable or running wires in metal conduit can reduce electric field pickup.

The measuring device should provide signal filtering, either in hardware

or by software, with strong rejection of the line frequency (50/60 Hz) and

its harmonics.

5. The operating environment of the thermocouple needs to be

considered. Exposure to oxidizing or reducing atmospheres at high

temperature can significantly degrade some thermocouples.

Thermocouples containing rhodium (B, R and S types) are not suitable

under neutron radiation.

4.4.1.3 The advantages and disadvantages of thermocouples

Because of their physical characteristics, thermocouples are the

preferred method of temperature measurement in many applications.

They can be very rugged, are immune to shock and vibration, are useful


45

over a wide temperature range, are simple to manufactured, require no

excitation power, there is no self heating and they can be made very

small. No other temperature sensor provides this degree of versatility.

Thermocouples are wonderful sensors to experiment with because of their

robustness, wide temperature range and unique properties.

On the down side, the thermocouple produces a relative low output signal

that is non-linear. These characteristics require a sensitive and stable

measuring device that is able provide reference junction compensation

and linearization. Also the low signal level demands that a higher level of

care be taken when installing to minimize potential noise sources.

The measuring hardware requires good noise rejection capability. Ground

loops can be a problem with non-isolated systems, unless the common

mode range and rejection is adequate.

4.4.1.4 Types of thermocouple

About 13 'standard' thermocouple types are commonly used. Eight

have been given an internationally recognized type designator. Some of

the non-recognized thermocouples may excel in particular niche

applications and have gained a degree of acceptance for this reason, as

well as due to effective marketing by the alloy manufacturer.

Each thermocouple type has characteristics that can be matched to

applications. Industry generally prefers K and N types because of their

suitability to high temperatures, while others often prefer the T type due

to its sensitivity, low cost and ease of use.

A table of standard thermocouple types is presented below. The table also

shows the temperature range for extension grade wire in brackets.


46

Type Positive

Material

Negative

Material

Accuracy***

Class 2

Range °C

(extension) Comments

B Pt, 30%Rh Pt, 6%Rh 0.5%

>800°C

50 to 1820

(1 to 100)

Good at high temperatures,

no reference junction

compensation required.

C** W, 5%Re W, 26%Re 1%

>425°C

0 to 2315

(0 to 870)

Very high temperature use,

brittle

D** W, 3%Re W, 25%Re 1%

>425°C

0 to 2315

(0 to 260)


brittle

E Ni, 10%Cr Cu, 45%Ni 0.5% or 1.7°C -270 to 1000

(0 to 200)

General purpose, low and

medium temperatures

G** W W, 26%Re 1%

>425°C

0 to 2315

(0 to 260)


brittle

J Fe Cu, 45%Ni 0.75% or 2.2°C -210 to 1200

(0 to 200)

High temperature, reducing

environment

K* Ni, 10%Cr

Ni, 2%Al

2%Mn

1%Si

0.75% or 2.2°C -270 to 1372

(0 to 80)

General purpose high

temperature, oxidizing

environment

M** Ni Ni, 18%Mo 0.75% or 2.2°C -50 to 1410 .

N* Ni, 14%Cr

1.5%Si

Ni,

4.5%Si

0.1%Mg

0.75% or 2.2°C -270 to 1300

(0 to 200)

Relatively new type as a

superior replacement for K

Type.

P** Platinel II Platinel II 1.0% 0 to 1395

A more stable but

expensive substitute for K

& N types

R Pt, 13%Rh Pt 0.25% or 1.5°C -50 to 1768

(0 to 50) Precision, high temperature

S Pt, 10%Rh Pt 0.25% or 1.5°C -50 to 1768

(0 to 50) Precision, high temperature

T* Cu Cu, 45%Ni 0.75% or 1.0°C -270 to 400

(-60 to 100)

Good general purpose, low

temperature, tolerant to

moisture.

* Most commonly used thermocouple types, ** Not ANSI recognized types. *** See IEC 584-2 for more details.

Materials codes:- Al = Aluminum, Cr = Chromium, Cu = Copper, Mg = Magnesium, Mo = Molybdenum, Ni =

Nickel, Pt = Platinum, Re = Rhenium, Rh = Rhodium, Si = Silicon, W = Tungsten


47

4.4.1.5 Accuracy of thermocouples

Thermocouples will function over a wide temperature range - from

near absolute zero to their melting point, however they are normally only

characterized over their stable range. Thermocouple accuracy is a

difficult subject due to a range of factors. In principal and in practice a

thermocouple can achieve excellent results (that is, significantly better

than the above table indicates) if calibrated, used well below its nominal

upper temperature limit and if protected from harsh atmospheres. At

higher temperatures it is often better to use a heavier gauge of wire in

order to maintain stability.

As mentioned previously, the temperature and voltage scales were

redefined in 1990. The eight main thermocouple types - B, E, J, K, N, R,

S and T - were re-characterized in 1993 to reflect the scale changes. (See:

NIST Monograph 175 for details). The remaining types: C, D, G, M and

P appear to have been informally re-characterized.

4.4.1.6 Thermocouple wire grades

There are different grades of thermocouple wire. The principal

divisions are between measurement grades and extension grades. The

measurement grade has the highest purity and should be used where the

temperature gradient is significant. The standard measurement grade

(Class 2) is most commonly used. Special measurement grades (Class 1)

are available with accuracy about twice the standard measurement grades.

The extension thermocouple wire grades are designed for connecting the

thermocouple to the measuring device. The extension wire may be of

different metals to the measurement grade, but are chosen to have a


48

matching response over a much reduced temperature range - typically -

40°C to 120°C. The reason for using extension wire is reduced cost - they

can be 20% to 30% of the cost of equivalent measurement grades. Further

cost savings are possible by using thinner gauge extension wire and a

lower temperature rated insulation.

Note: When temperatures within the extension wire's rating are being

measured, it is OK to use the extension wire for the entire circuit. This is

frequently done with T type extension wire, which is accurate over the -

60 to 100°C range.

4.4.1.7 Thermocouple wire gauge

At high temperatures, thermocouple wire can under go irreversible

changes in the form of modified crystal structure, selective migration of

alloy components and chemical changes originating from the surface

metal reacting to the surrounding environment. With some types,

mechanical stress and cycling can also induce changes.

Increasing the diameter of the wire where it is exposed to the high

temperatures can reduce the impact of these effects.

The following table can be used as a very approximate guide to wire

gauge:


49

Type 8 Gauge

4.06mm

16 Gauge

1.63mm

20 Gauge

0.91mm

24 Gauge

0.56mm

28 Gauge

0.38mm

30 Gauge

0.32mm

B 1820 - - 1700 1700 -

C 2315 2315 2315 2315 2315 -

D 2315 2315 2315 2315 2000 -

E 870 620 540 430 400 370

G 2315 2315 2315 2315 2315 -

J 760 560 480 370 370 320

K 1260* 1000* 980 870 820 760

M 1260* 1200* - - - -

N 1260* 1000* 980 870 820 760

P 1395 - 1250 1250 1250 -

R 1760 - - 1480 1480 -

S 1760 - - 1480 1480 -

T 400 370 260 200 200 150

* Upper temperature limits only apply in a protective sheath

At these higher temperatures, the thermocouple wire should be

protected as much as possible from hostile gases. Reducing or oxidizing

gases can corrode some thermocouple wire very quickly. Remember, the

purity of the thermocouple wire is most important where the temperature

gradients are greatest. It is with this part of the thermocouple wiring

where the most care must be taken.

Other sources of wire contamination include the mineral packing

material and the protective metal sheath. Metallic vapor diffusion can be

significant problem at high temperatures. Platinum wires should only be

used inside a nonmetallic sheath, such as high-purity alumna.


50

High temperature measurement is very difficult in some situations. In

preference, use non-contact methods. However this is not always

possible, as the site of temperature measurement is not always visible to

these types of sensors.

4.4.1.8 Color coding of thermocouple wire

The color coding of thermocouple wire is something of a

nightmare! There are at least seven different standards. There are some

inconsistencies between standards, which seem to have been designed to

confuse. For example the color red in the USA standard is always used

for the negative lead, while in German and Japanese standards it is always

the positive lead. The British, French and International standards avoid

the use of red entirely!

4.4.1.9 Thermocouple mounting

There are four common ways in which thermocouples are mounted

with in a stainless steel or Inconel sheath and electrically insulated with

mineral oxides. Each of the methods has its advantages and

disadvantages.


51

Figure 4.11 Thermocouple Sheath Options

Sealed and Isolated from Sheath: Good relatively trouble-free

arrangement. The principal reason for not using this arrangement

for all applications is its sluggish response time - the typical time

constant is 75 seconds

Sealed and Grounded to Sheath: Can cause ground loops and

other noise injection, but provides a reasonable time constant (40

seconds) and a sealed enclosure.

Exposed Bead: Faster response time constant (typically 15

seconds), but lacks mechanical and chemical protection, and

electrical isolation from material being measured. The porous

insulating mineral oxides must be sealed

Exposed Fast Response: Fastest response time constant (typically

2 seconds), depending on the gauge of junction wire. In addition to

problems of the exposed bead type, the protruding and light

construction makes the thermocouple more prone to physical

damage.


52

4.4.1.10 Conversion Table

ITS-90 Table for type J thermocouple

Thermoelectric Voltage in mV

°C 0 1 2 3 4 5 6 7 8 9 10

0 0.000 0.050 0.101 0.151 0.202 0.253 0.303 0.354 0.405 0.456 0.507

10 0.507 0.558 0.609 0.660 0.711 0.762 0.814 0.865 0.916 0.968 1.019

20 1.019 1.071 1.122 1.174 1.226 1.277 1.329 1.381 1.433 1.485 1.537

30 1.537 1.589 1.641 1.693 1.745 1.797 1.849 1.902 1.954 2.006 2.059

40 2.059 2.111 2.164 2.216 2.269 2.322 2.374 2.427 2.480 2.532 2.585

50 2.585 2.638 2.691 2.744 2.797 2.850 2.903 2.956 3.009 3.062 3.116

60 3.116 3.169 3.222 3.275 3.329 3.382 3.436 3.489 3.543 3.596 3.650

70 3.650 3.703 3.757 3.810 3.864 3.918 3.971 4.025 4.079 4.133 4.187

80 4.187 4.240 4.294 4.348 4.402 4.456 4.510 4.564 4.618 4.672 4.726

90 4.726 4.781 4.835 4.889 4.943 4.997 5.052 5.106 5.160 5.215 5.269

100 5.269 5.323 5.378 5.432 5.487 5.541 5.595 5.650 5.705 5.759 5.814

110 5.814 5.868 5.923 5.977 6.032 6.087 6.141 6.196 6.251 6.306 6.360

120 6.360 6.415 6.470 6.525 6.579 6.634 6.689 6.744 6.799 6.854 6.909

130 6.909 6.964 7.019 7.074 7.129 7.184 7.239 7.294 7.349 7.404 7.459

140 7.459 7.514 7.569 7.624 7.679 7.734 7.789 7.844 7.900 7.955 8.010

4.4.2 RTD

Resistance Temperature Detectors (RTDs) rely on the predictable

and repeatable phenomena of the electrical resistance of metals changing

with temperature.

The temperature coefficient for all pure metals is of the same order

- 0.003 to 0.007 ohms/ohm/°C. The most common metals used for

temperature sensing are platinum, nickel, copper and molybdenum. While

the resistance - temperature characteristics of certain semiconductor and


53

ceramic materials are used for temperature sensing, such sensors are

generally not classified as RTDs.

4.4.2.1 How are RTD constructed?

RTDs are manufactured in two ways: using wire or film. Wire

RTDs are a stretched coil of fine wire placed in a ceramic tube that

supports and protects the wire. The wire may be bonded to the ceramic

using a glaze. The wire types are generally the more accurate, due to the

tighter control over metal purity and less strain related errors. They are

also more expensive.

Figure 4.12 RTD

Film RTDs consist of a thin metal film that is silk-screened or

vacuum spluttered onto a ceramic or glassy substrate. A laser trimmer

then trims the RTD to its correct resistance value.

Film sensors are less accurate than wire types, but they are

relatively inexpensive, they are available in small sizes and they are more

robust. Film RTDs can also function as a strain gauge - so don't strain

them! The alumina element should be supported by grease or a light

elastomer, but never embedded in epoxy or mechanically clamped

between hard surfaces.


54

Figure 4.13 Typical Sheath Mounted RTD Probe

RTDs cannot generally be used in their basic sensing element form,

as they are too delicate. They are usually built into some type of

assembly, which will enable them to withstand the various environmental

conditions to which they will be exposed when used. Most commonly this

is a stainless steel tube with a heat conducting grease (that also dampens

vibration). Standard tube diameters include 3, 4.5, 6, 8, 10, 12 and 15 mm

and standard tube lengths include 250, 300, 500, 750 and 1000 mm.

4.4.2.2 Characteristics of RTDs

Metal RTDs have a response defined by a polynomial:

R(t) = R0 ( 1 + a.t + b.t 2 + c.t

3 )

Where R0 is the resistance at 0°C, "t" in the temperature in Celsius, and

"a", "b" and "c" are constants dependent on the characteristics of the

metal. In practice this equation is a close but not perfect fit for most

RTDs, so slight modifications are often be made.

Commonly, the temperature characteristics of an RTD are specified

as a single number (the "alpha"), representing the average temperature

coefficient over the 0 to 100°C temperature range as calculated by:


55

alpha = ( R100 - R0 ) / 100 . R0 in ohms/ohm/°C

Note: RTDs cover a sufficient temperature range that their response needs

to be calibrated in terms of the latest temperature scale ITS90.

It is also of interest to note that the temperature coefficient of an

alloy is frequently very different from that of the constituent metals.

Small traces of impurities can greatly change the temperature

coefficients. Sometimes trace "impurities" are deliberately added so as to

swamp the effects of undesired impurities which are uneconomic to

remove. Other alloys can be tailored for particular temperature

characteristics. For example, an alloy of 84% copper, 12% Manganese

and 4% Nickel has the property of having an almost zero response to

temperature. The alloy is used for the manufacture of precision resistors.

4.4.2.3 Types RTDs

While almost any metal may be used for RTD manufacture, in

practice the number used is limited.

Metal Temperature

Range Alpha Comments

Copper Pt -200°C to 260°C 0.00427 Low cost

Molybdenum Mo -200°C to 200°C 0.00300

0.00385

Lower cost alternative to platinum in the

lower temperature ranges

Nickel Ni -80°C to 260°C 0.00672 Low cost, limited temperature range

Nickel - Iron Ni-

Fe -200°C to 200°C 0.00518 Low cost

Platinum Pt -240°C to 660°C 0.00385

0.00392 Good precision


56

4.4.2.4 Platinum RTDs

Platinum is by far the most common RTD material, primarily

because of its long-term stability in air. There are two standard Platinum

sensor types, each with a different doping level of 'impurities'. To a large

extent there has been a convergence in platinum RTD standards, with

most national standards bodies adopting the international IEC751-1983,

with amendment 1 in 1986 and amendment 2 in 1995. The USA

continues to maintain its own standard.

All the platinum standards use a modified polynomial known as the

Callendar - Van Dusen equation:

R(t) = R0 ( 1 + a.t + b.t2 + c.(t - 100).t

3 )

Platinum RTDs are available with two temperature coefficients or alphas

- the choice is largely based on the national preference in you country, as

indicated in the following table:

Standard Alpha

ohms/ohm/°C

R0

ohms Polynomial Coefficients

IEC751

(Pt100) 0.00385055 100

200°C < t < 0°C

a = 3.90830x10-3

b = -5.77500x10-7

c = -4.18301x10-12

0°C < t < 850°C

a & b as above, but

c = 0.0

SAMA

RC-4 0.0039200 98.129

a = 3.97869x10-3

b = -5.86863x10-7

c = -4.16696x10-12

The international IEC 751 standard specifies tolerance classes as

indicated in the following table. While only Classes A and B are defined

in IEC 751, it has become common practice to extended the Classes to C


57

and D, which roughly double the previous error tolerance. The tolerance

classes are often applied to other RTD types.

Tolerance Class Tolerance Equation (°C)

Class A ± ( 0.15 + 0.002.| t | )

Class B ± ( 0.30 + 0.005. | t | )

Class C ± ( 0.40 + 0.009. | t | )

Class D ± ( 0.60 + 0.0018. | t | )

Where | t | indicated the magnitude of the temperature in Celsius (that is

sign is dropped). Some manufacturers further subdivide their RTD

Tolerance Classes into Tolerance Bands for greater choice in price

performance ratios.

4.4.2.6 Characteristics of Platinum RTDs

The IEC751 specifies a number of other characteristics - insulation

resistance, environmental protection, maximum thermoelectric effect,

vibration tolerance, lead marking and sensor marking. Some of these are

discussed below:

Thermoelectric Effect: Platinum RTD generally employs two metals -

the platinum sensing element and copper lead wires, making it a good

candidate for a thermocouple. If a temperature gradient is allows to

develop along the sensing element, a thermoelectric voltage with a

magnitude of about 7 µV /°C will be generated. This is only likely to be a

problem with very high-precision measurements operating at low

excitation currents.

Wiring Configurations and Lead Marking: There are three wiring

configurations that can be used for measuring resistance - 2, 3 and 4 wire

connections.


58

Figure 4.14 Wiring configurations

IEC751 requires that wires connected to the same end of the resistor be

the same colour - either red or white, and that the wires at each end be

different.

4.4.3 Thermistor

Thermistor temperature sensors are constructed from sintered metal

oxide in a ceramic matrix that changes electrical resistance with

temperature. They are sensitive but highly non-linear. Their sensitivity,

reliability, ruggedness and ease of use, has made them popular in research

application, but they are less commonly applied to industrial applications,

probably due to a lack on interchangeability between manufactures.

Thermistors are available in large range of sizes and base resistance

values (resistance at 25°C). Interchangeability is possible to ±0.05°C

although ±1°C is more common.

4.4.3.1 Thermistor construction

The most common form of the thermistor is a bead with two wires

attached. The bead diameter can range from about 0.5mm (0.02") to 5mm

(0.2'').

Figure 4.15Themistor


59

Mechanically the thermistor is simple and strong, providing the

basis for a high reliability sensor. The most likely failure mode is for the

lead to separate from the body of the thermistor - an unlikely event if the

sensor is mounted securely and with regard to likely vibration. The

sintered metal oxide material is prone to damage by moisture, so is

passivated by glass or epoxy encapsulation. If the encapsulation is

compromised and moisture penetrates, silver migration under the dc bias

can eventually cause shorting between the electrodes.

Like other temperature sensors, thermistors are often mounted in

stainless steel tubes, to protect them from the environment in which they

are to operate. Grease is typically used to improve the thermal contact

between the sensor and the tube.

4.4.3.2 Thermistor characteristics

The following are typical characteristic for the popular 44004

thermistor from YSI:

Parameter Specification

Resistance at 25°C 2252 ohms (100 to 1M available)

Measurement range -80 to +120°C typical (250°C max.)

Interchangeability (tolerance) ±0.1 or ±0.2°C

Stability over 12 months < 0.02°C at 25°C, < 0.25°C at 100°C

Time constant < 1.0 seconds in oil, < 60 seconds in still air

self-heating 0.13 °C/mW in oil, 1.0 °C/mW in air

Coefficients

(see Linearization below) a = 1.4733 x 10-3, b = 2.372 x 10-3, c = 1.074 x 10-7

Dimensions ellipsoid bead 2.5mm x 4mm


60

4.4.4 Semiconductor

The semiconductor (or IC for integrated circuit) temperature sensor

is an electronic device fabricated in a similar way to other modern

electronic semiconductor components such as microprocessors. Typically

hundreds or thousands of devices are formed on thin silicon wafers.

Before the wafer is scribed and cut into individual chips, they are usually

laser trimmed. Semiconductor temperature sensors are available from a

number of manufacturers. There are no generic types as with

thermocouple and RTDs, although a number of devices are made by more

than one manufacturer. The AD590 and the LM35 have traditionally been

the most popular devices, but over the last few years better alternatives

have become available.

These sensors share a number of characteristics - linear outputs,

relatively small size, limited temperature range (-40 to +120°C typical),

low cost, good accuracy if calibrated but also poor interchangeability.

Often the semiconductor temperature sensors are not well designed

thermally, with the semiconductor chip not always in good thermal

contact with an outside surface. Some devices are inclined to oscillate

unless precautions are taken. Provided the limitations of the

semiconductor temperature sensors are understood, they can be used

effectively in many applications. The most popular semiconductor

temperature sensors are based on the fundamental temperature and

current characteristics of the transistor. If two identical transistors are

operated at different but constant collector current densities, then the

difference in their base-emitter voltages is proportional to the absolute

temperature of the transistors. This voltage difference is then converted to

a single ended voltage or a current. An offset may be applied to convert

the signal from absolute temperature to Celsius or Fahrenheit.


61

In general, the semiconductor temperature sensor is best suited for

embedded applications - that is, for use within equipment. This is because

they tend to be electrically and mechanically more delicate than most

other temperature sensor types. However they do have legitimate

application in many areas, hence their inclusion.

4.5 Level Sensing Elements

4.5.1 Radar Tank Gauging

Figure 4.16 RTG

FMCW radar principle and FFT signal analysis, (FMCW =

frequency-modulated continuous wave). A radar signal is emitted from an

antenna, reflected from the target (in this case, the product surface) and

received back after a delay interval t. The distance of the reflecting

product surface is measured by way of the transit time t of the microwave

signal: for every meter from a target the waves travel a distance of 2 m,


62

for which they require a time of approx. 6.7 ns. In general, the measured

distance is a = c x t / 2; where c = the speed of light.

The FMCW radar system uses a linear frequency-modulated high-

frequency signal; transmission frequency increases linearly within a time

interval (frequency sweep). Since the transmission frequency changes due

to the time delay during signal propagation, a low-frequency signal

(typically, up to a few kHz), the frequency f of which is proportional to

the reflector distance a, is obtained from the difference between the

current transmission frequency and the received frequency. The product

level is then computed from the difference between tank height and

distance.

Figure 4.17 RTG Signalling


63

4.5.2 Vibrating Fork

A piezoelectric crystal operated Vibrating Fork type level switch

for detection of level of powders / granules / solids in the hoppers, bins

and silos, etc.

Figure 4.18 Vibrating fork

4.5.3 LVDT

The letters LVDT are an acronym for Linear Variable

Differential Transformer, a common type of electromechanical

transducer that can convert the rectilinear motion of an object to which it

is coupled mechanically into a corresponding electrical signal. LVDT

linear position sensors are readily available that can measure movements

as small as a few millionths of an inch up to several inches, but are also

capable of measuring positions up to ±20 inches (±0.5 m).

Figure 4.19 LVDT Core


64

The figure (4.19) shows the components of a typical LVDT. The

transformer's internal structure consists of a primary winding centered

between a pair of identically wound secondary windings, symmetrically

spaced about the primary. The coils are wound on a one-piece hollow

form of thermally stable glass reinforced polymer, encapsulated against

moisture, wrapped in a high permeability magnetic shield, and then

secured in cylindrical stainless steel housing. This coil assembly is

usually the stationary element of the position sensor.

The moving element of an LVDT is a separate tubular armature of

magnetically permeable material called the core, which is free to move

axially within the coil's hollow bore, and mechanically coupled to the

object whose position is being measured. This bore is typically large

enough to provide substantial radial clearance between the core and bore,

with no physical contact between it and the coil.

In operation, the LVDT's primary winding is energized by alternating

current of appropriate amplitude and frequency, known as the primary

excitation. The LVDT's electrical output signal is the differential AC

voltage between the two secondary windings, which varies with the axial

position of the core within the LVDT coil. Usually this AC output voltage

is converted by suitable electronic circuitry to high level DC voltage or

current that is more convenient to use.

4.5.3.1 Advantages

LVDTs have certain significant features and benefits, most of

which derive from its fundamental physical principles of operation or

from the materials and techniques used in its construction.

Friction-Free Operation


65

One of the most important features of an LVDT is its friction-free

operation. In normal use, there is no mechanical contact between the

LVDT's core and coil assembly, so there is no rubbing, dragging or other

source of friction. This feature is particularly useful in materials testing,

vibration displacement measurements, and high resolution dimensional

gauging systems.

Infinite Resolution

Since an LVDT operates on electromagnetic coupling principles in a

friction-free structure, it can measure infinitesimally small changes in

core position. This infinite resolution capability is limited only by the

noise in an LVDT signal conditioner and the output display's resolution.

These same factors also give an LVDT its outstanding repeatability.

Unlimited Mechanical Life

Because there is normally no contact between the LVDT's core and coil

structure, no parts can rub together or wear out. This means that an LVDT

features unlimited mechanical life. This factor is especially important in

high reliability applications such as aircraft, satellites and space vehicles,

and nuclear installations. It is also highly desirable in many industrial

process control and factory automation systems.

Over travel Damage Resistant

The internal bore of most LVDTs is open at both ends. In the event of

unanticipated over travel, the core is able to pass completely through the

sensor coil assembly without causing damage. This invulnerability to

position input overload makes an LVDT the ideal sensor for applications

like extensometers that are attached to tensile test samples in destructive

materials testing apparatus.

Single Axis Sensitivity

An LVDT responds to motion of the core along the coil's axis, but is

generally insensitive to cross-axis motion of the core or to its radial


66

position. Thus, an LVDT can usually function without adverse effect in

applications involving misaligned or floating moving members, and in

cases where the core doesn't travel in a precisely straight line.

Separable Coil And Core

Because the only interaction between an LVDT's core and coil is

magnetic coupling, the coil assembly can be isolated from the core by

inserting a non-magnetic tube between the core and the bore. By doing

so, a pressurized fluid can be contained within the tube, in which the core

is free to move, while the coil assembly is depressurized. This feature is

often utilized in LVDTs used for spool position feedback in hydraulic

proportional and/or servo valves.

Environmentally Robust

The materials and construction techniques used in assembling an LVDT

result in a rugged, durable sensor that is robust to a variety of

environmental conditions. Bonding of the windings is followed by epoxy

encapsulation into the case, resulting in superior moisture and humidity

resistance, as well as the capability to take substantial shock loads and

high vibration levels in all axes. And the internal high-permeability

magnetic shield minimizes the effects of external AC fields.

Both the case and core are made of corrosion resistant metals, with the

case also acting as a supplemental magnetic shield. And for those

applications where the sensor must withstand exposure to flammable or

corrosive vapors and liquids, or operate in pressurized fluid, the case and

coil assembly can be hermetically sealed using a variety of welding

processes.

Ordinary LVDTs can operate over a very wide temperature range, but, if

required, they can be produced to operate down to cryogenic

temperatures, or, using special materials, operate at the elevated

temperatures and radiation levels found in many nuclear reactors.


67

Null Point Repeatability

The location of an LVDT's intrinsic null point is extremely stable and

repeatable, even over its very wide operating temperature range. This

makes an LVDT perform well as a null position sensor in closed-loop

control systems and high-performance servo balance instruments.

Fast Dynamic Response

The absence of friction during ordinary operation permits an LVDT to

respond very fast to changes in core position. The dynamic response of an

LVDT sensor itself is limited only by the inertial effects of the core's

slight mass. More often, the response of an LVDT sensing system is

determined by characteristics of the signal conditioner.

Absolute Output

An LVDT is an absolute output device, as opposed to an incremental

output device. This means that in the event of loss of power, the position

data being sent from the LVDT will not be lost. When the measuring

system is restarted, the LVDT's output value will be the same as it was

before the power failure occurred.

4.5.3.2 Theory of Operation

This figure illustrates what happens when the LVDT's core is in

different axial positions. The LVDT's primary winding, P, is energized by

a constant amplitude AC source. The magnetic flux thus developed is

coupled by the core to the adjacent secondary windings, S1 and S2 . If the

core is located midway between S1 and S2 , equal flux is coupled to each

secondary so the voltages, E1 and E2 , induced in windings S1 and S2

respectively, are equal. At this reference midway core position, known as

the null point, the differential voltage output, (E1 - E2), is essentially

zero.


68

Figure 4.20 LVDT Signalling

If the core is moved closer to S1 than to S2 , more flux is coupled to S1

and less to S2 , so the induced voltage E1 is increased while E2 is

decreased, resulting in the differential voltage (E1 - E2). Conversely, if

the core is moved closer to S2 , more flux is coupled to S2 and less to S1 ,

so E2 is increased as E1 is decreased, resulting in the differential voltage

(E2 - E1 ). The top graph shows how the magnitude of the differential

output voltage, EOUT, varies with core position. The value of EOUT at

maximum core displacement from null depends upon the amplitude of the

primary excitation voltage and the sensitivity factor of the particular

LVDT, but is typically several volts RMS. The phase angle of this AC

output voltage, EOUT, referenced to the primary excitation voltage, stays

constant until the center of the core passes the null point, where the phase

angle changes abruptly by 180 degrees, as shown in the middle graph.

This 180 degree phase shift can be used to determine the direction of the

core from the null point by means of appropriate circuitry. This is shown

in the bottom graph, where the polarity of the output signal represents the

core's positional relationship to the null point. The figure shows also that


69

the output of an LVDT is very linear over its specified range of core

motion, but that the sensor can be used over an extended range with some

reduction in output linearity. The output characteristics of an LVDT vary

with different positions of the core. Full range output is a large signal,

typically a volt or more, and often requires no amplification. Note that an

LVDT continues to operate beyond 100% of full range, but with degraded

linearity.

4.5.4 Servo Motor

A micro-controller based multi-function instrument for precision

level measurement of liquids stored in Cone Roof, Floating Roof tanks,

pressurized Spheres, Mounded Vessels, Bullets and Cryogenic storage

tanks.

Figure 4.21 Servo-motor Type

4.5.5 Pressure Sensing Type

In this type of level gauging, the pressure or differential pressure is

measured converted to level by the following equation.


70

)( 12 hhgP

If the tank is open to atmosphere the pressure at the bottom is indication

of level. In closed tanks, differential pressure is the measurand that

indicates the level. The linkage may be direct, liquid filled or sealed

liquid filled.

Figure 4.22 Pressure sensing Type

4.6 Vibration Sensing

4.6.1 Inductive Sensor (Eddy Current)

Inductive sensors use currents induced by magnetic fields to detect

nearby metal objects. The inductive sensor uses a coil (an inductor) to

generate a high frequency magnetic field as shown in Figure 4.23. If there

is a metal object near the changing magnetic field, current will flow in the

object. This resulting current flow sets up a new magnetic field that

opposes the original magnetic field. The net effect is that it changes the


71

inductance of the coil in the inductive sensor. By measuring the

inductance the sensor can determine when a metal have been brought

nearby. These sensors will detect any metals, when detecting multiple

types of metal multiple sensors are often used.

Figure 4.23 Inductive Sensor

The sensors can detect objects a few centimeters away from the

end. But, the direction to the object can be arbitrary as shown in Figure

4.24. The magnetic field of the unshielded sensor covers a larger volume

around the head of the coil. By adding a shield (a metal jacket around the

sides of the coil) the magnetic field becomes smaller, but also more

directed. Shields will often be available for inductive sensors to improve

their directionality and accuracy.

Figure 4.24 Shielded and Unshielded


72

4.7 Role Play

Each Trainee should speak thoroughly about one of the electrical

transducers for

Pressure.

Temperature.

Level Gauging and Vibration Sensing.


73

Chapter 5

Flow Measurement

5.1 Learning Objectives

1. Review basic properties of fluid flow.

2. To understand the theory of operation of different flow meters.

3. Select the optimum meter according to the application.

4. To avoid pitfalls in flow metering.

5.2 Basic Principles of Fluid Flow and Measurement

5.2.1 Density and Specific Volume

The density of a fluid is the ratio of its mass to its volume. Its

specific volume is the reciprocal of its density. The density of water is

roughly 1000 times that of air at atmospheric pressure.

V

M

5.2.2 Thermal Expansion Coefficient

The thermal expansion coefficient, , is the fractional increase in

specific volume, Vs, caused by a temperature increase of 1 degree.

dT

dV

V

s

s

1


74

5.2.3 Compressibility

The compressibility of a fluid, , is the fractional decrease in

specific volume caused by unit increase of pressure.

dP

dV

V

s

s

1

5.2.4 Viscosity

The viscosity, , of a fluid is a measure of its resistance to shearing

at a constant rate.

where is the shear stress and is the rate of shear strain. The SI unit of

viscosity is Pascal second, but it is usual to express it in centipoises, cP,

where one cP being 0.001 Pa s. Viscosity is referred to as absolute or

dynamic viscosity to distinguish it from kinematics viscosity, , which is

the ratio of viscosity to density. The Si unit of which is m2 s

-1 and

commonly known by centistokes, cSt, where one cSt being 10-6

m2 s

-1.

5.2.5 Air Solubility of Liquids

Air is soluble in liquids, and its solubility is directly proportional to

the absolute pressure. The solubility decreases markedly as the

temperature of the water increases. It is very much soluble in

hydrocarbons where the solubility is not decreased much with increasing

temperature, until quite high temperatures are reached.


75

5.2.6 Humidity in gases

Gases may be either dry or humid. This is because a gas at a given

temperature is capable of holding a certain maximum amount of water

vapor; this value increases with temperature increase. The relative

humidity is defined as the ratio of the actual partial pressure of the water

vapor to the value of partial pressure that would exist under saturated

conditions at the same temperature.

Sudden changes in humidity may cause errors in gas flow

measurement. In particular, errors easily occur if unsaturated gas is

passed through a wet gas meter, or if a sudden expansion cools a gas

sufficiently to cause precipitation of some of its water vapor.

5.2.7 Reynolds Number

The behavior of fluids flowing through pipes is governed by a

quantity known as Reynolds number which is defined by

vDD Re

where v is the mean velocity and D is the pipe diameter. The numerator is

a measure of the flowing fluid's ability to generate a dynamic forces,

while the denominator is a measure of its ability to generate viscous

forces. This means that Reynolds number indicates which kind of forces

predominate the flowing fluid.

5.2.8 Laminar and Turbulent Flow

Laminar flow occurs at Reynolds numbers below about 2000. This

can be likened to the flow of traffic on a busy motorway, with the traffic


76

in the various lanes traveling on parallel paths at different speeds.

Turbulent flow occurs at Reynolds number above about 2000.

5.2.9 Rotation and Swirl

Bends, flowmeters, valves, etc., produce what is known as rotation

in the flow. The fluid on the outside of the bend has to travel farther than

the fluid in the inside and this distorts the pattern of the flow in highly

complex fashion. On consequence of this is the rotary motion. The flow

returns to steady flow after some distance. For two adjacent bends in

different planes the flow rotates in three dimensions, i.e. swirls. It takes

longer distance for swirl to come back to steady flow.

Figure 5.1 Rotational Flow

5.2.10 Continuity and Bernoulli's Equation

In simple, what goes at one end of the pipe comes out at the other.

This simple fact is the basis of continuity, which holds that the mass flow

rate is the same at all cross-sections of one continuous pipe having no

branches. If the fluid is incompressible, the volumetric flow rate remains

constant also.

The energy possessed by a flowing fluid is the same at every cross-

section along the pipe. Bernoulli's equation expresses this fact in

mathematical terms.

22

1 vP constant at all sections.


77

5.2.11 Velocity Head

The expression v2=2g provides a convenient way of indicating the

amount of kinetic energy possessed by the fluid flowing in the pipe. It has

the dimensions of length and is equal to the head to which the fluid would

rise if it were projected vertically upwards. An important use of this

concept is to express the tendency of pipe fittings to dissipate energy in

terms of velocity heads.

5.2.12 Cavitation

It follows from Bernoulli that when the mean velocity increases the

pressure will decrease. In water, volatile hydrocarbons and liquefied

gases cavitation generally occurs only when the pressure at some point

reaches the vapor pressure of the liquid causing bubbles and vapor

pockets to appear. In viscous oil and non-volatile liquid fuels cavitation

generally takes a different form. It begins at pressures somewhat below

atmospheric, but well above the vapor pressure.

5.2.13 Double Block and Bleed Valve

Flowmeters are frequently installed in complex network of piping

containing a number of shut off valves. To eliminate the bypassed flow a

system of double block and bleed valves are installed to confirm the

operator that the valves are sealing perfectly.

Figure 5.2 Double block and bleed valve.


78

5.2.14 Definitions

The mean pipe velocity is related to volumetric flowrate, QV, and

pipe cross-sectional area, A.

vAQV

Volumetric flowrate, QV, is defined as the rate of change of

volume.

dt

dVQV

Mass flowrate, QM, is the rate of change of mass with time.

The results of calibration may be plotted as a graph of flowmeter

readout against flowrate. The graphs may be linear or non-linear.

A more detailed graph is the performance index, which displays

any small deviations from ideal behavior by the flowmeter.

5.2.15 Factors

Coefficient of discharge, C, is defined by

I

T

Q

QC

where QT denotes true flowrate and QI denotes the flow indicated

by meter.

Figure 5.3 Coefficient of discharge


79

Meter correction factor is defined by

I

IT

V

VV

Meter factor, F, is used in connection with meters for total volume

and is given by

I

T

V

VF

K-Factor is a term used to describe the performance of meters

whose output is in the form of a series of electrical pulses, and

where total pulse count, n, is nominally proportional to the volume

passed, and the pulse frequency, dn/dt, is nominally proportional to

the flowrate.

TV

nK

Figure 5.4 K-factor

5.3 Differential Pressure Meters

5.3.1 Principle of Operation

The meter depends on the fact that when a fluid flows through a

contraction it must accelerate; this causes its kinetic energy to increase,

and consequently its pressure must fall by a corresponding amount. The

volumetric flowrate is given by


80

21

21

12

2 2

1

P

m

ACQV

where is an empirical coefficient, the expansibility factor. This depends

upon the physical properties of the gas being metered, as well as the

geometry of the flowmeter.

Figure 5.5 Differential Pressure measurement

5.3.2 Advantages

Simplicity of construction.

Versatility: used with almost any fluid.

Economy.

Experience.

5.3.3 Disadvantages

Accuracy is not quite enough.

The output signal is not linear to flowrate

5.3.4 Selecting the Meter

It is usually not difficult to decide which type is better for a

particular job. The lengthy expensive venturi meter has a low head loss


81

and its high initial cost is justified in situations where large quantities of

liquids are being pumped, i.e. in main water supply pipelines. Gas plants

where head loss is not important the orifice plates are the decision. A

compromise for intermediate cost and size is the nozzle.

5.3.4.1 Venturi Tubes

The venturi tube is the original form of differential pressure meter.

A typical design is shown in figure (5.6). Because energy losses are low

and flow conditions are not far removed from the ideal, the discharge

coefficient of venturi meters is very near unity.

Figure 5.6 Venturi Tube

5.3.4.2 Orifice Plates

An orifice plate is simply a plate with a hole in it, forming a partial

obstruction to the flow. The flowing fluid follows the same kind of path

as it does in venturi tube. However, the narrowest part of the flow stream

is not in the orifice itself, but some distance downstream; this narrowest

section is known as vena contracta. Between the vena contracta and the

pipe wall, numerous eddies form, which dissipate great deal of kinetic

energy that is responsible for the high head loss.


82

Figure 5.7 Orifice Plate

Concentric orifice plates are made with a circular orifice concentric

with the pipe. In figure 5.7, the tapings are in the adjacent pipes at

distances shown. Another common arrangement is to put the tapings in

the pipe flanges adjacent to the orifice plate. The position of the tapping

affects the discharge coefficient. The orifice plate in figure (5.8) is

described as square-edged because that is the shape of the upstream

although the downstream edge is chamfered. This is used in clean gases

and clean liquids with low viscosity. With viscous liquids it is necessary

to make the edges raduissed or chamfered upstream and square

downstream and they are called quarter-circle or conical-entry.

Concentric orifice plates cannot be used with dirty fluids because

dirt gradually builds up behind the plate until its performance is impaired.

Instead eccentric or chord orifice plates are commonly used but they are

less accurate.

Figure 5.8 Orifice Plate Types


83

5.3.4.3 Nozzles

Nozzles are more costly than orifice plates but they have three

advantages over them: they have a discharge coefficient very much closer

to unity; they can be used to discharge directly into the atmosphere; and

they have no sharp edge to blunted, i.e. they can be used with dirty and

abrasive fluids.

Figure 5.9 Nozzles

5.3.5 Points to watch when Using

Install orifice plates correctly watching the edges back and front.

Installing differential pressure meters, take care the pressure

tapings in acceptable position. These must never be at the bottom

so that would not clog with dirt. With liquids the tapings must not

be positioned at the top so they would fill with bubbles. The best

place is at the side of the pipe.

Stay within the recommended range of flowrates.

Cavitation must not be allowed to occur.


84

Make periodic inspections for meter and pipe work to trace any

film of dirt, corrosion or organic growth.

Inspect sharp edges in the orifice if worn or not.

When used with wet gases, plates are often provided with drain

hole.

5.3.6 Drag Plate

The principle of the drag plate meter is illustrated in figure (5.10)

A circular plate is supported centrally in the pipe by means of hinged

arm. The flowing fluid produces a positive pressure on the upstream side

of the plate and suction on the downstream. This pressure difference

produces forces which tend to move the plate in the direction of flow, but

this force is resisted by a null-balance supporting element at the end of

the support arm. The signal from the null-balance device is proportional

to the force on the plate which is proportional to the square of the

flowrate.

Figure 5.10 Drag plate

5.3.6.1 Advantages

Dirt cannot be built up.

There are no pressure tapings to be blocked.


85

The flowrate range can be adjusted by a simple range switch.

5.3.6.2 Disadvantages

Square root characteristics

For good accuracy, large diameters are used and hence high head

loss.

The force on large drag plate would be too great to be supported

effectively by null-balance system.

5.3.6.3 When to Use

The drag plate is suitable for liquids containing suspended solids.

5.3.7 Rotameters

In the simplest type of rotameter the body is a tapered transparent

tube of glass or plastic with a scale engraved on it. Inside the tube is a

small solid body with a circular cross-section, the float, when there is no

flow the float rests at the bottom. Flow causes it ot be lift off. Its very low

price is its advantage. The high head loss is its disadvantage.

Figure 5.11 Rotameters


86

5.3.8 Spring Loaded Variable Aperture Flowmeters

In the differential pressure flowmeter, the area of constriction is

kept constant and the pressure difference is varying. In variable aperture

flowmeters the reverse effect occurs. In this type two degrees of freedom

are possessed which can be used for meter readout. One degree is for the

pressure difference and the other is for the displacement of the member

controlling the aperture.

Figure 5.12 Spring Loaded Variable Aperture

5.3.8.1 Advantages

Wide range of operation with tolerable accuracy.

Linear output.

Less sensitive for viscosity changes.

Can be installed horizontal, vertical or inclined.


87


Larger in diameter than the pipes.

Expensive.

High head loss.

5.3.9 Laminar Flowmeters

In turbulent flow, pressure drop is proportional to the square of the

velocity. In laminar flow it is linear. The simplest laminar flowmeter

consists of fine capillary tube with highly sensitive differential pressure

micro manometer connected across it.

Figure 5.13 Laminar Flowmeters

5.3.9.1 Advantages

Approximately linear output.

Wide rangeability.

No moving parts.

Used for extremely low flow rates.


Bulky and expensive.

Calibration is upset by dust particles.

Sensitive to changes in viscosity.


88

5.4 Rotating Mechanical Meters

5.4.1 Positive Displacement Meters

In principle, liquids are measured using containers. This technique

is accomplished in continuous process for positive displacement meters.

In gases the mechanism must have very low frictional resistance.

Figure 5.14 Postive displacement meters

5.4.1.1 Advantages

High accuracy.

They are not affected by upstream flow disturbances so they can

be used very close to bends.


Large sizes.

High head loss.

Can be damaged by dirt particles.

If they clutch they will block flow.

5.4.1.3 Points to Watch

One direction flow only. So installation should be supervised.

When used with water check internals for non-corrosive materials.


89

5.4.2 Turbine Meters

It consists of a short length of pipe in the centre of which there are

two bearings supported by spiders. A propeller is mounted so that it can

spin freely on these central bearings. The propeller materials should be

either magnetic or small magnet is inserted in the tip of each blade and a

pick up is installed on the pipe. Meter readout is pulses.

Figure 5.15 Tyrbine Meters

5.4.2.1 Advantages

They are very accurate.

The output is digital.

Moderate head loss.

Compact in size.

If they clutch the flow does not block.


Expensive.

Need periodic calibration to compensate for wear up.

Sensitive to viscosity changes.

Sensitive to flow disturbance and especially swirl.


90

5.4.2.3 Points to Watch

Does the application justify cost?

Examine calibration curves for suitable accuracy.

Is it applicable for use with the fluid, temperature and pressure?

Never blow out the line with compressed air or steam because

over speed would damage it.

For dirty liquids use coarse filters.

Any nearby electrical signal might introduce errors in pick ups.

Avoid cavitation.

5.4.3 Bypass Meters

The flowrate in the bypass is approximately a constant fraction of

the flow in the main pipe. The interesting result is that the relationship

between flowrate and pressure drop in the bypass follows square law and

thus it cancels out the square root effect of the orifice plate itself. The

advantage is economical and the disadvantage is that accuracy and

linearity are inferior to those more expensive meters.

Figure 5.16 Bypass meters

5.4.4 Metering Pumps

A metering pump may be regarded as a combination of pump,

flowmeter and flow regulator. It consists of a piston pump with a variable

stroke, a device counting the number of strokes delivered and a pre-

settable mechanism that will stop the pump when the required number of

strokes has been delivered.


91

5.5 Other Volumetric Flowmeters

5.5.1 Electromagnetic Flowmeters

It utilizes the same basic principle of electrical generator: when a

conductor moves across a magnetic field a voltage is induced in the

conductor, and the magnitude of the voltage is directly proportional to the

speed of the moving conductor. If the conductor is a section of a

conductive liquid flowing in a non-conductive pipe through a magnetic

field, electrodes are mounted in the pipe wall at the positions shown in

figure (5.17). The voltage induced across the electrodes is proportional to

the flowrate.

Figure 5.17 Electromagnetic Flowmeter

5.5.1.1 Advantages

There is no obstruction whatever to the flow, suitable for

measuring flow rates of heavy suspensions like mud, sewage and

wood pulp.

Zero head loss.

Wide range of meter sizes.


92

Not affected with upstream flow.

Not affected by density or viscosity variation.

Linear output.

Bi-directional.


Fluid must be electrical conductive.

Not very accurate.

Not cost effective for small pipe sizes.

5.5.1.3 Things to Watch

Make sure of whole range of duty.

Is a built in electrode cleaning device needed?

How the meter is calibrated?

If installed below ground level make sure it withstands drowning.

Never to alter meter duty.

Never install meter with electrodes in vertical diameter because

they would be affected with air bubbles.

Check zero reading periodically.

If the pipe system is electro galvanic corrosion prevention system,

then bonding straps are used to bypass the currents around the

meter.

5.5.2 Ultrasonic Flowmeters

Ultrasonic flowmeters use sound waves to determine the flowrate.

Pulses from a transducer travel through a moving fluid at the speed of

sound and provides an indication of fluid velocity.


93

The first method uses a transit-time method, in which two opposing

transducers are mounted so that sound waves traveling between them are

at 45 degree angle to the direction of the flow. The speed of sound from

the upstream transducer to the downstream transducer represents the

inherent speed of sound plus a contribution due to fluid velocity. The

opposite direction transducer is used to extract the fluid velocity from

speed of sound. It is essential that the fluid is free of entrained gas or

solids to prevent scattering of sound waves.

Figure 5.18 Ultrasonic meters

The second method uses the Doppler Effect. This type uses two

transducer elements mounted in the same side of the pipe. An ultrasonic

sound wave of constant frequency is transmitted into the fluid by one of

the elements. Solids or bubbles within the fluid reflect the sound back to

the receiver element. The Doppler principle states that there will be a shift

in apparent frequency when there is a relative motion between the

transmitter and receiver. Doppler ultrasonic meters require entrained

gases and suspended solids within the flow.

Ultrasonic meters advantages are freedom of obstruction in the

pipe and negligible cost-sensitivity with respect to pipe diameter. The

disadvantages are that performance is very dependent on flow conditions

and that fair accuracy is attainable when properly applied to appropriate

fluids.


94

5.5.3 Vortex Shedding Meters

The operating principle is based on the phenomenon of vortex

shedding known as the von Karman effect. As a fluid passes a bluff body,

it separates and generates vortices that are shed alternately along and

behind each side of the bluff body. These vortices cause areas of

fluctuating pressure that are detected by a sensor. The frequency of vortex

generation is directly proportional to fluid velocity. Vortex shedding

meters are aimed the section of market as orifice plates. They have the

same moderate accuracy as orifice plates, similar head and the same

sensitivity to upstream flow disturbances. There is no rotating mechanism

so there is no wear. It scores over orifice plates by having linear output.

Figure 5.19 Vortex shedding meters

5.5.4 Thermal Flowmeters

This type of flowmeters is for mass flowrates. The mass flow rate

is given by

12 TTc

HQ

p

M

where H is the power supplied in the form of heat and cp is the specific

heat capacity at constant pressure. The main use for this type is with

gases at relatively low pressure and flowrates.


95

Figure 5.20 Thermal Flowmeter

5.5.5 Coriolis Meters

The Coriolis meter uses an obstruction less U-shaped tube as a

sensor and applies Newton's second law of motion to determine flow rate.

Inside the sensor housing, the sensor tube vibrates at its natural

frequency. The sensor tube is driven by an electromagnetic drive coil

located at the center of the bend in the tube and vibrates similar to that of

a tuning fork.

Figure 5.21 Sensor vibration

The fluid flows into the sensor tube and is forced to take on the

vertical momentum of the vibrating tube. When the tube is moving

upward during half of its vibration cycle, the flowing into the sensor

resists being forced upward pushing down on the tube. The fluid flowing

out of the sensor has an upward momentum from the motion of the tube.

As it travels around the tube bens, the fluid resists changes in its vertical


96

motion by pushing up on the tube. The difference in forces causes the

tubes to twist. When the tube is moving downward during the second half

of its vibrating cycle, it twists in the opposite direction. This twisting

characteristic is called Coriolis effect.

Figure 5.22 Forces on sensor

Due to Newton's second law of motion, the amount of sensor tube

twist is directly proportional to the mass flowrate of the fluid flowing

through the tube. Electromagnetic velocity detectors located on each side

of the flow tube measure the velocity of the vibrating tube. Mass flow is

determined by measuring the time difference exhibited by the velocity

detector signals. During zero flow conditions, no tube twist occurs,

resulting in no time difference between the two velocity signals. With

flow, a twist occurs with a resulting time difference between the two

velocity signals. This time difference is directly proportional to mass

flow.

Digure 5.23 Sensor Twisting


97

5.6 Velocity Measuring

5.6.1 Pilot Tubes

It is the oldest and simplest form of fluid meter. The fluid in the

mouth of the tube has been brought to rest, and its kinetic energy has

been converted to pressure energy, which creates an enhanced pressure

inside the pilot tube.

22

1 vP

Figure 5.24 Pilot Tubes

5.6.2 Hot Resistor Anemometers

The basic principle is an electrically heated element is placed

within the stream flow; the higher velocity the more it tends to cool the

element; the change in temperature causes a change in resistance, which

can be measured by some appropriate circuitry.

5.6.3 Laser Doppler Velocity Meters

The schematic arrangement is shown in figure (25). The laser beam

is first passed into a beam splitting prism, and then the two parallel


98

component beams are passed through a lens which makes them converge

at a point where the flow velocity is to be measured. Whenever a dirt

particle passes through the bright spot where the two beams intersect, it

reflects light in all directions. This reflected light possesses a Doppler

frequency shift. Some of it is picked up by a collecting lens and focused

on a photo detector which reads out the velocity.

Figure 5.25 Laser Doppler velocity meter

5.7 Two Phase Flow

Wherever possible it is better to separate the gas and liquid phases

and meter each one on its own. If it is not practical, there are some

recognized techniques for measuring two phase flow. You will have to

work very hard to obtain accuracy approaching 10%.

Figure 5.26 Two Phase flow behavior


99

5.8 Choosing the Right Flowmeter


100

5.9 Calibrating Flowmeters

Calibration of flowmeters can be done using any of the following

techniques depending on how practical the technique is.

Volumetric Tank

Figure 5.27 Volumetric calibration

Weighing

Figure 5.28 Dynamic Weighing calibration

Master Meter

Figure 5.29 Master meter calibration

5.10 Role Play


elements.


101


102

Chapter 6

Analyzers


1. Understand the theory of operation of oxygen, moisture and gas

chromatography analyzers.

6.2 Oxygen Analyzers

The analyzer uses an electrochemical sensor technology to achieve

the measurement of oxygen. See Figure (6.1). The sensor is a self

contained disposable unit which requires no maintenance. The sensor

utilizes the principle of electrochemical reaction to generate a signal

proportional to the oxygen concentration in the sample.

The sensor consists of a cathode and anode which are in contact via

a suitable electrolyte. The sensor has a gas permeable membrane which

covers the cathode allowing gas to pass into the sensor while preventing

liquid electrolyte from leaking out. As the sample diffuses into the sensor,

any oxygen present will dissolve in the electrolyte solution and migrate to

the surface of the cathode. The oxygen is reduced at the cathode.

Simultaneously, an oxidation reaction is occurring at the anode

generating four electrons. These electrons flow to the cathode to reduce

the oxygen. The representative half cell reactions are:

Cathode:

4e- + 2H2O + O2 → 4OH-

Anode:

4OH- + 2Pb → 2PbO + 2H2O + 4e-


103

The resultant overall cell reaction is:

2Pb + O2 → 2PbO

This flow of electrons constitutes an electric current which is

directly proportional to the concentration of oxygen present in the sample.

In the absence of oxygen, no oxidation / reduction reaction occurs and

therefore no current is generated. This allows the sensor to have an

absolute zero.

Figure 6.1 Oxygen analyzer sensing element

6.3 Gas Chromatography

The word “Chromatography” is at present used as a collective

term for a group of methods that at first sight appear somewhat diverse.

These methods, however, have a number of common features. All

chromatographic separations, for instance, involve the transport of a

sample of a mixture through a column. The mixture may be a liquid or a

vapor. The column contains a substance, the stationary phase, which may

consist of a solid absorbing agent or of a liquid partitioning agent

supported by a solid.

The transport of the constituents of the sample through the column

is affected either by a gas or a liquid, the moving phase. Owing to the


104

selective retention exerted by the stationary phase, the components of the

mixture move through the column at different effective rates. They, thus

tend to segregate into separate bands or zones. The column is designed to

affect this separation at the exit of the column where the individual bands

may be directed to a detector for determination. The separation obtained

with this principle of operation is easily observed by using a piece of

filter paper and putting a drop of oil in the center of the paper. With time,

the light molecules of the oil will travel through the capillaries of the

filter paper faster and farther than the heavy molecules. These light

molecules are small in size and, generally, have fewer side branches or

“arms and legs” to cause restriction to flow; and therefore, tend to move

through the capillaries in an easier way than the larger molecules. It is

easy to see the heavy, large, dark molecules of the oil restricted and

retained near the center of the filter paper where the original drop was

placed. Some oils will even show slight color bands as the separation of

molecules occurs while traveling toward the edge of the filter paper.

Figure 6.2 Filter Paper

Similar separation takes place in a packed column, (stationary

phase), where the sample molecules are injected at the head of the column

and begin to move through the column under the motive forces of the

carrier gas, (moving phase), where the light molecules travel through the

column faster than the heavy ones. Therefore, the time that the light


105

molecules are in the column will be shorter than the time the heavy

molecules stay in the column. It is this difference in column retention for

different molecules that provides the separation. A detector is then

employed to measure the relative concentration of each component while

the elution time sequence can be employed to identify each component.

Figure 6.3 Sample column

6.3.1 Thermal Conductivity Detector (TCD)

The Thermal Conductivity Detector used in the GCX utilizes two

filaments, one for sample gas and one for reference gas flow. The

unbalancing of the bridge due to the dilution of the carrier gas by the

sample, and hence the change in Thermal Conductivity offers excellent

sensitivity for most applications.

6.3.2 Flame Ionization Detector (FID)

When a hydrocarbon sample passes through a hydrogen flame, the

molecular structure is altered so that the bond is broken and the carbon

atom becomes a negatively charged and the hydrogen atoms become

positively charged. When placed in an electric field, the ions may be

collected. In the case of the GCX, a positive potential on the polarizing


106

plate causes all positive ions are collected on the measuring plate or

collector and al negatively charged particles are collected on the

polarization plate. This current may be converted to a voltage for further

processing. The hydrogen/air mixture that supports the flame is converted

to water and exits the burner through the vent. Most of the oxygen is

consumed by the flame with only small amounts of excess hydrogen

remaining. The excess hydrogen (H2) passes through the flame to the

vent without being ionized. The only ionization that occurs is with the

hydrocarbon samples. By placing the burner tip within the effective

electric field, all positive ions will be collected and measured by the

measuring circuit. All negative Ions will be attracted to the positive

polarizing plate. Any extraneous electric fields that exist within the

system will change the performance of the burner. Thus, maintaining a

constant electric field and a clean system is of the utmost importance.

Response of the burner to hydrocarbon components. - In a chromatograph

system, each component to be measured is separated so that there is no

interference between components. Each component is calibrated using a

known concentration to determine response of the system to that

component. The relationship between components does not depend upon

each other but only on the calibration factor.

Figure 6.4 FID


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6.4 Moisture Analyzer

The electrolytic moisture sensor consists of a pair of spirally

wound, parallel, electrode wires, partially embedded within the length of

a glass tube. A thin layer of highly absorbent, phosphorous pentoxide

(P2O5) completely coats the interior of the tube. In operation, the sample

gas stream passes through the tube, giving up its entrained water

molecules to the absorbent coating. A current is applied to the electrode

windings, whereby the water molecules are completely and continuously

electrolyzed into their respective hydrogen and oxygen elements.

The stream's moisture level is derived from the current required for

complete electrolysis of the absorbed water. Interpretation of this value is

based on the application of Faraday's Law of Electrolysis, which

describes the quantitative relationship of electrolyte production with the

application of electric current.

H2O + e- -> H2 + ½ O2

The current required to electrolyze the absorbed water is directly

proportional to the number of moisture molecules present, as electrolyzed

over a given time; that is the 'mass rate' of water entering the sensor. As

the current measurement is completely dependent upon the mass rate, it

becomes crucial that pressure and flow are strictly regulated.

Figure 6.5 Moisture Sensing element


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6.5 Role Play

Each Trainee should speak thoroughly about one of the analyzers.

Oxygen

Moisture

Gas Chromatography


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Chapter 7

Basic Considerations


1. Introduce basic considerations for transmitter selection and installation.

7.2 Corrosion Effects

Corrosion is the gradual destruction of a metal by chemical or

electrochemical means. The most generic form of corrosion is galvanic

corrosion. A combination of a cathode, an anode, and an electrolyte must

be present for this type of corrosion.

Material Selection Guide as for Rosemount

E= Excellent Resistance, Corrosion Rate (CR) < 0.05mm/year.

G= Good Resistance, CR < 0.5mm/year.

F= Fair Resistance, CR < 1.27 mm/year.

P=Poor Resistance, CR > 1.27 mm/year.

-- = Data Not Available.

7.3 Lightning and Static Effects

Lightning is the attraction of a charged cloud to an oppositely

charged earth, another cloud, or another area within the same cloud.

Clouds produce lightning with the help of strong updraft air currents.

These air currents cause rapid freezing of water droplets, which inherit a

charge as they crystallize. Among the many types of lightning, cloud to


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ground strikes are the greatest threat to industrial electronic equipment.

Four factors are important in assessing the threat of lightning damage to a

plant or facility.

Frequency and severity of lightning storms.

Vulnerability of existing and proposed instrumentation.

Exposure of systems wiring to possible lightning discharge.

Potential harmful impact of instrument failure on the process.

Comparing the above factors to the costs of not protecting electronic

equipment will help to decide if protection is beneficial.

Figure 7.1 Globalannual Lightning stroms

Three strategies are effective in minimizing lightning induced

transients on industrial electronics.

Diversion: Grounded metallic structures form a cone of protection for

equipment and cabling.


111

Attenuation: Careful wiring practices, such as metallic raceways, cable

shields, twisted pairs, and extensive grounding and earthing reduce the

magnitude of transients.

Suppression: Add-on devices limit the magnitude of the transient

appearing at the instrument.

7.4 Winterizing Transmitters

Ensuring that electronic pressure transmitters operate under all

weather conditions requires consideration of three important variable:

installation, protective measures, and cost. First, the transmitter must be

located properly with respect to the process pipe. Second, once optimum

installation is determined, consider the degree of temperature protection

required. Third, the degree of weatherization needed should then be

balanced against bottom line cost. Failures can be caused by the freezing

water or of solutions containing significant amounts of water. A volume

of water will increase about ten percent as it changes to ice at atmospheric

pressure. If the expansion is contained, the pressure exerted by the frozen

fluid increases the magnitude of this increase is large in comparison with

each incremental decrease in temperature.

Temperature (F) Pressure (psia)

32 14.7

30 2100

25 7000

18.5 12660

9.5 20056

5.0 23115

0.5 26103


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In any case, proper installation is necessary for good transmitter

performance. In determining the best location, remember the following

guidelines:

Keep corrosive or hot process material out of contact with the

transmitter.

Prevent sediment from depositing in the impulse piping.

Keep the liquid head balanced on both legs of the impulse piping.

Keep the impulse piping as short as possible.

Avoid ambient temperature gradients and fluctuations

7.4.1 Liquid Service

For liquid flow measurement, mount the transmitters below the

process taps with the drain/vent valves facing downward. This allows the

trapped gases to vent into the process line. Make the taps to the side of

the line to avoid sediment deposits.

Figure 7.2 Liquid service connection


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7.4.2 Gas Service

For gas flow measurement, install the transmitter above the process

taps with the drain/vent valves facing upward. This provides automatic

drainage and ensures that no liquid accumulates at the transmitter.

Figure 7.3 Gas service connection.

7.4.3 Protective Measures

Although winterizing a transmitter is relatively easy, protection

should not end there. Impulse lines must also be protected from the point

of measurement to the transmitter this may be accomplished in several

ways. Lines may be protected by tracing, insulation or both.

7.5 Total Probable Error

Total Probable error, TPE, is amore realistic number than would

obtained by simply adding up all the possible errors, since it is unlikely


114

that all errors would go in the same direction from their means. The root

sum square method, RSS, determines TPE by summing the squares of the

individual errors and taking the root square of the total. Below is a

comparison of two transmitters.

7.6 Discussion

An open discussion is to be opened about different consideration for

selection of transmitters.


115

References

1. Hugh Jack, "Automating Manufacturing Systems with PLCs",

([email protected]); version 4.6 December, 2004.

2. A.K. Sawhney, "Electrical and electronics measurements and

instrumentation", J.C. Kapoor for Dhanpat Rai Co, Ltd. Naisarak,

Delhi 1999.

3. Leamington Spa, "Flowmeters", 1979.

4. R.B.Helson, " The HART –protocol- a solution enabling

technology", HART communication foundation, 9390 research

blvd., suite II-250, Austin, Texas 78759.

5. Rosemount Measurement Catalog.

6. AEA Technology, "Level gauging", United Kingdom: 329

Harwell, Didcot, OX11 0QJ.

7. KROHNE, "Level Radar BM700".

8. Integrated Publishing – Engine Mechanics, www.tpub.com.

9. SBEM, www.sbem-india.com

10. Sensor Network, www.sensornet-work.com.

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