Instrumentation course

Oil and Gas Measuring Instruments

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.

Define
Knowledge Determine
& Skills Elements

Measure Learning
& Correction Objectives

Instruction
Activity

Training Cycle.

1


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.

2


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

3


4


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.

5


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.

6


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).

7


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

8


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.

9


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

10


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

11


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.

12


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

13


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.

14


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.

15


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

16


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

17


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

18


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

19


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.

20


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;

21


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

22


2.5 Role Play

Each Trainee should speak thoroughly about one of the learning objective
elements.
 Analog Transmitters
 Smart Transmitters and HART Protocol.

23


24


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.

25


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

26


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.

27


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

32


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.

33


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

34


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.

35


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.

36


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.

41


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

42


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.

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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

44


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.

45


Positive Negative Accuracy*** Range °C
Type Comments
Material Material Class 2 (extension)

Good at high temperatures,
0.5% 50 to 1820
B Pt, 30%Rh Pt, 6%Rh no reference junction
>800°C (1 to 100)
compensation required.

1% 0 to 2315 Very high temperature use,
C** W, 5%Re W, 26%Re
>425°C (0 to 870) brittle

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

-270 to 1000 General purpose, low and
E Ni, 10%Cr Cu, 45%Ni 0.5% or 1.7°C
(0 to 200) medium temperatures

G** W W, 26%Re

-210 to 1200 High temperature, reducing
J Fe Cu, 45%Ni 0.75% or 2.2°C
(0 to 200) environment

Ni, 2%Al General purpose high
-270 to 1372
K* Ni, 10%Cr 2%Mn 0.75% or 2.2°C temperature, oxidizing
(0 to 80)
1%Si environment

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

Ni, Relatively new type as a
Ni, 14%Cr -270 to 1300
N* 4.5%Si 0.75% or 2.2°C superior replacement for K
1.5%Si (0 to 200)
0.1%Mg Type.

A more stable but
P** Platinel II Platinel II 1.0% 0 to 1395 expensive substitute for K
& N types

-50 to 1768
R Pt, 13%Rh Pt 0.25% or 1.5°C Precision, high temperature
(0 to 50)

-50 to 1768
S Pt, 10%Rh Pt 0.25% or 1.5°C Precision, high temperature
(0 to 50)

Good general purpose, low
-270 to 400
T* Cu Cu, 45%Ni 0.75% or 1.0°C temperature, tolerant to
(-60 to 100)
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

46


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

47


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:

48


8 Gauge 16 Gauge 20 Gauge 24 Gauge 28 Gauge 30 Gauge
Type
4.06mm 1.63mm 0.91mm 0.56mm 0.38mm 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.

49


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.

50


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.

51


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

52


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.

53


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:

54


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.

Temperature
Metal Alpha Comments
Range

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

0.00300 Lower cost alternative to platinum in the
Molybdenum Mo -200°C to 200°C
0.00385 lower temperature ranges

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

Ni-
Nickel - Iron -200°C to 200°C 0.00518 Low cost
Fe

0.00385
Platinum Pt -240°C to 660°C Good precision
0.00392

55


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).t3 )

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:
Alpha R0
Standard Polynomial Coefficients
ohms/ohm/°C ohms

200°C < t < 0°C
a = 3.90830x10-3
b = -5.77500x10-7
IEC751
0.00385055 100 c = -4.18301x10-12
(Pt100)
0°C < t < 850°C
a & b as above, but
c = 0.0

a = 3.97869x10-3
SAMA
0.0039200 98.129 b = -5.86863x10-7
RC-4
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

56


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.

57


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

58


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

Dimensions ellipsoid bead 2.5mm x 4mm

59


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.

60


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,

61


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

62


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

63


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

64


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

65


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.

66


 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.

67


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

68


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.

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P  g (h2  h1 )

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

70


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

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4.7 Role Play

Each Trainee should speak thoroughly about one of the electrical
transducers for
 Pressure.
 Temperature.
 Level Gauging and Vibration Sensing.

72


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.
M

V

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.
1 dVs

Vs dT

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5.2.3 Compressibility

The compressibility of a fluid, , is the fractional decrease in
specific volume caused by unit increase of pressure.
1 dVs
 
Vs dP

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 m 2 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.

74


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
vD
Re D 

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

75

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