CHAPTER
1.1
1
Introduction To measurements
INTRODUCTION
Man is, in general, curious in nature and this trait, coupled with his imagination, skill and
intuition has seen rapid strides in the field of measurement over the last few decades. During
the early days of human civilisation, barter trade system was in vogue as measurement and it
was used as a means to quantify the exchange of goods. Although very crude and unscientific as
the system was, human beings had to do with this system because of non-availability of scientific
and reliable forms of measurements.
As human civilisation continued to evolve and progress, man tried to improve upon the then
existing systems in order to have more meaningful exchange of trade. Man’s continued stride for
betterment in measurement systems gave rise to industrial revolution in the last century, followed
by electronic revolution a few decades later. These developments, followed by the introduction
of microprocessors in 70s and digital computers, have led to hitherto unseen and unimaginable
measurement techniques which are very accurate, reliable and at the same time cheap.
Measurement has become an all pervading one, be it chemical, aeronautical, automobiles or
manufacturing systems of all sorts. As consumers are demanding higher and higher limits of
accuracy coupled with cost control, measurement techniques are reaching the sky to achieve the
above. The success on the above fronts have been made possible because of the ability and a quest
for betterment in measuring the state, condition and the characteristics of physical systems with
sufficient and dependable degree of accuracy. Techniques and technologies employed nowadays
in manufacturing/process industries have undergone a sea change with the introduction of new
technologies like robotics, continuous condition based monitoring (CBM), remote control, etc.
1.2
elements of a measuring system
Fig. 1.1: Basic elements of a measuring system
1
2
Measurement techniques in industrial instrumentation
The basic elements of a measuring system consists of three stages—I, II and III—shown in
figure 1.1. Stage I consists of sensor/transducer part, stage II the signal processing part and the
last or final stage may be indicator, recorder or it may include controller also.
Stage I gives an output which is a function of the measurand or input parameter. An ideal
transducer should give an output which is proportional to the input parameter variations, but
should be insensitive to every other possible input. Thermocouples, strain gauges, liquid-in-glass
thermometers are examples of primary transducer.
Measurands, which are normally measured are temperature, pressure, flow, level, velocity,
acceleration, pH, humidity, force, torque, etc.
Stage II is the intermediate stage and may include variable conversion element and signal
processing circuitry. For force and displacement measurements, a strain gauge is used which
gives an output in the form of a varying resistance. This is then converted into a voltage change
by employing a bridge circuit. The signal processing circuit increases the amplitude or power of
the signal (transducer’s output) or both; it all depends on the transducer output. This enhances
the sensitivity and resolution of measurement. This stage also has several other functions like
filtering, integration, differentiation and sometimes telemetering also.
Final or stage III provides an indication, record or utilised for control of the process. The
measurand may be presented as an indicator (as in a pressure gauge) or a digital output (as in
pulse counting).
1.3
Application areas of measuring instruments
Varied application areas exist for measuring instruments. These are:
l System parameter informations
l Perform mathematical manipulations
l Design studies
lMonitoring
l Simulation of system conditions
l Verification of physical phenomena/scientific formulae
l Testing of materials, product specifications and adherence to standards
l Quality control
l Automatic control of a process/operation
System Parameter Informations
System parameter informations: By determining various system parameters, healthiness of a
process can be ascertained. In fact, condition-based monitoring leads to a continuous assessment
and subsequent corrective actions can be taken, e.g., the health of a patient.
Perform Mathematical Manipulations
A simple pocket calculator performs the functions of addition, subtraction, multiplication,
division, integration, differentiation, logarithmic calculations, etc. In addition, several instruments
are designed to perform functions like signal sampling, linearisation, averaging, ratio control, etc.
Design Studies
Before launching a product in the market, extensive design studies are made on the drawing
board. Thus, before formally launching a car/aircraft, its prototype is developed taking the help
of design data. It is then tested under working conditions and various operating parameters are
noted. Any deviations from expected parameter values are corrected at the design stage before
marketing the product. Thus, experiments supplement design data in the intermediate stage to
ensure acceptability of the final product by the discerning customers.
3
Monitoring
Weather forecasting is made possible by employing instruments like barometers, anemometers,
thermometers, etc. Again, by measuring flow, temperature, pressure, etc. in a process plant, an
operator would be able to take corrective actions. Temperature of a greenhouse can be taken care
of by a thermometer.
Simulation of System Conditions
A prototype of an aircraft is tested in controlled air streams generated in a wind tunnel that
simulates the flow, turbulence conditions that the aircraft would actually face in its flights. Such
simulation testing helps improve the design parameters of different parts of an aircraft. Again, a
passenger car is tested under simulated road conditions to determine its worthiness/acceptability
by customers.
Verification of Physical Phenomena/Scientific Formulae
Experimental studies are at times undertaken where substantial theoretical backgrounds
are unavailable. Postulates proposed by scientists are verified experimentally to ascertain their
veracity. Thus, Coulomb’s postulate that friction between two dry surfaces is proportional to the
normal reaction and independent of area of contact was experimentally verified and since then
is known as Coulomb’s law.
Testing of Materials, Product Specifications and Adherence to Standards
To improve longevity, reliability of product parts and a product as a whole, standard
organisations specify material and product standards which the manufacturers of such parts/
products must adhere to. Again, as accuracy in measurement goes on improving, the specifications/
standards are also changed from time to time. These ensure less downtime for parts/products.
Quality Control
Each and every part of a machine is extensively put under quality control tests so that the
defective parts are outright rejected. This ensures that the end product is flawless and reliable
over a considerable length of time. The machine parts normally tested are for blow holes, cracks,
etc, apart from some long duration tests, such as creep, fatigue, etc.
Automatic Control of a Process/Operation
The block diagram of a closed loop control system is shown in figure 1.2. This is one of the
most important application areas of a measuring instrument. The deviation (called the error
signal) between the reference input and the measured value is fed to a controller, the output of
which drives a servo valve and an actuator. This closed loop system thus helps in maintaining
the value of the measurand to a value in close proximity to the reference input value.
Fig. 1.2: Block diagram of a closed loop control system (for monitoring and control)
Chapter 1
Introduction To Measurements
4
Measurement techniques in industrial instrumentation
The automatic control system has widespread use in process industries like oil refineries,
fertiliser plants, power plants, textile industries as also in sophisticated systems like radar tracking
system, missile guidance, automatic aircraft landing, etc.
1.4
Classification of instruments
It is based on method of energy conversion, nature of output signal and mode of operation.
Different types of instruments are:
l Manual/automatic
type
l Active/passive type
l Null/deflection type
l Analog/digital type
l Contact/non-contact type
They are discussed as under:
1.4.1 Manual/Automatic Type
An instrument, which requires human intervention and operation for recording/displaying the
value of the input (measurand), is manual in nature. Thermocouples and RTDs employed for
measurement of temperature employ null type potentiometers and are examples of manual type.
Employing an automatic self-balancing feedback device in the above will lead to an automatic
type of instrument.
1.4.2 Active/Passive Type
Active or power operated instruments require an auxiliary power source, such as electrical
supply, compressed air, etc, for their operation. Examples of such instruments are linear variable
differential transformer (LVDT), a float-type water tank level indicator, etc. When an LVDT is
used for displacement measurements, its output changes, but it needs an auxiliary power across
its primary for it to be operational. Similarly, in a water tank level indicator, the position of the
wiper along the potentiometer changes when the water level changes. The output is taken from
the wiper as shown in figure 1.3.
In passive type instruments, the force/energy required for driving their indicator/output are
derived from the measurand. Examples are a
To output
(indicator)
Bourdon gauge, a mercury-in-glass thermometer,
a Pitot tube, etc.
Auxiliary power
For active instruments, resolution can be
Wiper
source
varied considerably while the same for passive
ones can be varied in a limited manner. For
example, in the case of a Bourdon type pressure
Float
Tank
gauge (a passive instrument), resolution can be
increased to some extent by making the pointer
longer such that the pointer tip can move
through a longer arc. But obviously, there is a
practical limit to an increased pointer length. In
Fig. 1.3: An example of an active instrument
5
the case of an active instrument, resolution can be increased by increasing say, the supply voltage.
But in such a case, heat dissipation is a restraining factor.
In general, passive instruments are generally cheaper and simpler in construction than their
active counterparts.
1.4.3 Null/Deflection Type
Null type instruments can be manual or automatic type. A dead weight tester is a null type
instrument where the applied external pressure is balanced by adjusting weights placed on a pan.
Another example of a null type instrument is that the thermo-emf developed in a thermocouple
is balanced (nulled) by a potentiometer.
A common Bourdon type pressure gauge is an example of a deflection type instrument. In
this type of instrument, the physical effect produced by the measurand (pressure, temperature,
etc.) is opposed by the moving parts of the instrument, the displacement of which is thus a
measure of the measurand.
Deflection type instruments are simple in construction and have good dynamic response.
However, they interfere with the measurand and hence accuracy is low. Null type instruments
are slow in response and hence their dynamic response is poor. But their accuracy is much more
and they are normally used for calibration purposes.
1.4.4 Analog/Digital Type
An analog instrument gives an output which is continuous or stepless in nature. Thus, the output
can have an infinite number of values within the range of the instrument’s span. Examples of
analog instruments are a Bourdon type pressure gauge, a mercury-in-glass thermometer, a U-tube
manometer, etc. Although theoretically an analog instrument can assume an infinite number of
values, practically it is dependent on the scale length and its graduations.
In digital instruments, the measurand (physical parameter or variable) is expressed digitally,
i.e., by numbers. Thus, the display is discrete and it varies in steps. The measurand is sensed first
and quantised (i.e., assigned a particular analog value) and then it is digitalised, say by means
of an ADC (Analog to digital converter). It is then electronically processed, calibrated and the
measurand is displayed digitally.
Digital instruments have a number of advantages like ease in processing, noise immunity
and direct compatibility with microprocessors/microcomputers, apart from data coding, error
detection, error correction capability, if needed. Digital instruments are more reliable, requires
less calibration but is a little more costly.
1.4.5 Contact/Non-contact Type
A contact type instrument is kept in the measuring medium. A clinical thermometer is an example
of a contact type instrument.
Non-contact type instruments are placed at a distance from the measuring medium. An
example is an optical pyrometer which measures very high temperature from a considerable
distance. A variable reluctance tachometer, which measures the rpm of a rotating device, is also
an example of a non-contact type of instrument.
Chapter 1
Introduction To Measurements
6
Measurement techniques in industrial instrumentation
1.5
Measurement standards
During the early days of civilisation, very crude methods for units of measurements were
used. For example, human torso like foot or hand were used as standard for measurement of
length. As human civilisation evolved over the years, better and accurate methods for different
standards were tried out and improved upon. In 1960, a standard meter was defined in terms of
1.65076373 × 106 wavelengths of the radiation from Krypton-86 in vacuum. Later on, in 1983,
it was redefined as the length of path travelled by light in an interval of 1/299792458 seconds.
Similar improvements were also effected for other kinds of measurement units.
For different types of applications and functions, different standards are specified.
These are: International Standards, Primary Standards, Secondary Standards and Working
Standards.
1.5.1 International Standards
These are standards conforming to highest possible accuracy achievable using very advanced
measurement technologies. These are maintained by International Bureau of Weights and
Measures at Se’vre’s in France. These standards are maintained under recommended
environment conditions and are not available to the ordinary user. International standards
for Kilogram (M), length (L) and time (T) are prototype kilogram, wavelength of Krypton-86
orange-red lamp in vacuum and cesium clock respectively.
1.5.2 Primary Standards
These are maintained by national laboratories/standard organisations. These are calibrated
independently by absolute measurements and are used to calibrate and check secondary
standards. Primary standards are also not available to the ordinary user for use.
1.5.3 Secondary Standards
These are maintained by industrial units to act as basic reference standards. These are
periodically checked and calibrated against primary standards. Secondary standards are
available to the ordinary user for calibration and checking of their instruments.
1.5.4 Working Standards
Working standards are commercially available in the market after their certification by primary
or secondary standards. As for example, the industry working standard for length is the
precision gauge block made of steel of specified compositions. This has a accuracy tolerance
of 0.25 to 0.5 micron range. As other working standards, standard cells and standard resistors
are available conforming to environmental specifications. These standards are very widely
used for calibration of laboratory/field instruments, for checking products, etc.
Table 1.1 shows definitions of standard units, while table 1.2 gives the details of
fundamental, supplementary and derived quantities, their units and symbols.
Introduction To Measurements
7
Standard Unit
Definition
metre
The length of path travelled by light in an interval of
1/299 792 458 seconds
Mass
kilogram
The mass of a platinum–iridium cylinder kept in the
International Bureau of Weights and Measures, Sévres, Paris
Time
second
9.192 631 770 × 10 9 cycles of radiation from vaporized
caesium-133 (an accuracy of 1 in 10 12 or 1 second in
36000 years)
Temperature
kelvin
The temperature difference between absolute zero and the
triple point of water is defined as 273.16° kelvin
Current
ampere
One ampere is the current flowing through two infinitely long
parallel conductors of negligible cross-section placed 1 metre
apart in vacuum and producing a force of 2 × 10–7 Newtons
per metre length of conductor.
Luminous intensity
candela
One candela is the luminous intensity in a given direction from
a source emitting monochromatic radiation at a frequency of
540 terahertz (Hz × 1012) and with a radiant density in that
direction of 1.4641 mW/steradian. (1 steradian is the solid
angle which, having its vertex at the centre of a sphere, cuts
off an area of the sphere surface equal to that of a square with
sides of length equal to the sphere radius)
Length
Matter
mole
The number of atoms in a 0.012 kg mass of carbon-12
Table 1.2: Fundamental, Supplementary and Derived Units
(a) Fundamental units
Quantity
Standard unit
Symbol
Length
metre
m
Mass
kilogram
kg
Time
second
s
Electric current
ampere
A
Temperature
kelvin
K
Luminous intensity
Matter
candela
mole
cd
mol
(b) Supplementary fundamental units
Quantity
Plane angle
radian
Standard unit
Solid angle
steradian
Symbol
rad
sr
Chapter 1
Table 1.1: Definition of Standard Units
Physical Quantity
8
Measurement techniques in industrial instrumentation
(c) Derived units
Quantity
Area
Volume
Velocity
Acceleration
Angular velocity
Angular acceleration
Density
Specific volume
Mass flow rate
Volume flow rate
Force
Pressure
Torque
Momentum
Moment of inertia
Kinematic viscosity
Dynamic viscosity
Work, energy, heat
Specific energy
Power
Thermal conductivity
Standard unit
square metre
cube metre
metre per second
metre per second squared
radian per second
radian per second squared
kilogram per cubic metre
cubic metre per kilogram
kilogram per second
cubic metre per second
newton
newton per square metre
newton metre
kilogram metre per second
kilogram metre squared
square metre per second
newton second per sq. metre
joule
joule per cubic metre
watt
watt per metre kelvin
Electric charge
Voltage, e.m.f., pot. diff.
Electric field strength
Electric resistance
coulomb
volt
volt per metre
ohm
Electric capacitance
farad
Electric inductance
Electric conductance
Resistivity
Permittivity
Permeability
Current density
Magnetic flux
Magnetic flux density
Magnetic field strength
Frequency
Luminous flux
Luminance
Illumination
Molar volume
Molarity
Molar energy
henry
siemen
ohm metre
farad per metre
henry per metre
ampere per square metre
weber
tesla
ampere per metre
hertz
lumen
candela per square metre
lux
cubic metre per mole
mole per kilogram
joule per mole
Symbol
m2
m3
m/s
m/s2
ras/s
rad/s2
kg/m3
m3/kg
kg/s
m3/s
N
N/m2
Nm
kg m/s
kg m2
m2/s
N s/m2
J
J/m3
W
W/m K
Derivation formula
kg m/s2
Nm
J/s
C
V
V/m
W
As
W/A
V/A
F
A s/V
H
S
Wm
F/m
H/m
A/m2
Wb
T
A/m
Hz
lm
cd/m2
lx
m3/mol
mol/kg
J/mol
V s/A
A/V
Vs
Wb/m2
s–1
cd sr
lm/m2
Introduction To Measurements
9
1. Draw the basic elements of a measuring system and discuss the individual blocks.
2. Mention the different application areas of a measuring instrument. Discuss any two of them.
3. State the basis on which instruments are classified and mention their types. Discuss the active and
passive types of instruments.
4. What are the different standards of measurements? Discuss them.
Fill in the blanks:
1.During the early part of civilisation, human hand was used as a measurement standard for
.................... .
2. The elements of a measuring system, in general, consist of .................... stages.
3. A closed loop automatic system maintains the value of the measurand very close to the ....................
value.
4. Prototype of aircrafts are tested under .................... conditions.
5. A clinical type thermometer is an example of .................... type instrument.
6. LVDT is an example of .................... type instrument.
Tick the correct answer:
1. Signal processing circuitry enhances the sensitivity/resolution/both of measurement.
2. An active instrument requires/does not require an auxiliary power source for their operation.
3. A Bourdon type pressure gauge is an example of a passive/active instrument.
4. In the case of an active instrument, resolution can be increased by increasing/decreasing the supply
voltage.
5. Null type instruments are fast/slow in response.
6. A clinical thermometer is a contact/non-contact type instrument.
7. International standard of measurement is more/less accurate than working standard.
Chapter 1
Review Questions