Unit v MEASUREMENT OF POWER, FLOW AND TEMPERATURE RELATED PROPERTIES

KIT – KALAIGNAR KARUNANIDHI INSTITUTE OF TECHNOLOGY,
COIMBATORE / ME6504 / V-SEM / 2015 METROLOGY & MEASUREMENTS
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LESSON NOTES
ME6504 METROLOGY AND MEASUREMENTS
UNIT – V

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UNIT V MEASUREMENT OF POWER, FLOW AND TEMPERATURE
Force, Torque, Power – Mechanical, Pneumatic, Hydraulic Electrical type, Flow Measurement :
Venturimeter, Orificemeter, Rotameter, Pitottube – Temperature: Bimetallic strip, thermocouples,
electrical resistance thermometer Reliability and calibration – Readability and Reliability.
CONTENTS
5.1 MEASUREMENT OF POWER (MEASUREMENT OF FORCE, TORQUE AND POWER)
5.1.1 MEASUREMENT OF FORCE: (Mechanical, Pneumatic, Hydraulic and Electrical types)
5.1.1.1 DIRECT METHODS
(i) Equal Arm Balance
(ii) Analytical Balance
(iii) Unequal Arm Balance
(iv) Multiple Lever System
(v) Pendulum Scale
5.1.1.2 INDIRECT METHODS
(i) Acceleration Method
(ii) Electromagnetic Balance
(iii) Using Elastic Loaded Members
(iv) Using Cantilever Elastic Member.
(v) Load Cell (Electrical Based)
a. Hydraulic Load Cell
b. Strain Gauge Load Cell
c. Pneumatic Load Cell
(vi) Elastic Force Meter
a. Proving Ring – (Mechanical Based)
b. Load Cell Type Elastic Loaded Members
c. Electronic Weighing System.
5.1.2 MEASUREMENT OF TORQUE (Mechanical, Pneumatic, Hydraulic and Electrical types)
5.1.2.0 Basic Principle Of Mechanical Torsion Meter.
5.1.2.1 Torque Reaction Methods
5.1.2.2 Proney Brake

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5.1.2.3 Strain Gauges
5.1.2.4 Torsion Bars
a) Optical Method
b) Capacitive Method
c) Laser - Optic Method
d) Proxity Sensor Method
e) Stroboscope Method
f) Magnetostrictive Method :
g) Surface Acoustic Wave (SAW) Method
5.1.3 MEASUREMENT OF POWER
5.1.3.1 Mechanical Dynamometer
5.1.3.2 DC Dynamometer
5.1.3.3 Eddy Current Or Inductor Dynamometers
5.1.3.4 Hydraulic Dynamometers
5.2 MEASUREMENT OF FLOW
5.2.1 Orifice meter
5.2.2 Venturimeter
5.2.3 Rotameter
5.2.4 Pitot Tube
5.3 MEASUREMENT OF TEMPERATURE
5.3.1 Bimetallic Thermometers
5.3.2 Thermocouples
5.3.3 Electrical Thermal Resistance (Or) Resistance Thermometers (RTDs) (Or)
Electrical Resistance Thermometer
5.4 RELIABILITY & CALIBRATION / RELIABILITY & READABILITY
5.4.1 Reliability and Calibration
5.4.2 Reliability and Readability

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5.1 MEASUREMENT OF POWER: (MEASUREMENT OF FORCE, TORQUE AND POWER)
5.1.1 MEASUREMENT OF FORCE: (Mechanical, Pneumatic, Hydraulic and Electrical types)
Force: The mechanical quantity which changes or tends to change the motion or shape of a body to which it
is applied is called force.
Devices are used to measure force:
1. Direct Methods of Measurement of Force:
Involves a direct comparison with a known gravitational force on a standard
mass, say by a balance
Scale and Balance (Mechanical Based)
(ii) Analytical Balance
(iii) Unequal Arm Balance
(iv) Multiple Lever System
(v) Pendulum Scale
2. Indirect Methods of Measurement of Force:
Involves the measurement of effect of force on a body, such as acceleration of a body of
known subjected to force.
(i) Acceleration Method
(ii) Electromagnetic Balance
(iii) Using Elastic Loaded Members
(iv) Using Cantilever Elastic Member.
(v) Load Cell (Electrical Based)
a. Hydraulic Load Cell
b. Strain Gauge Load Cell
c. Pneumatic Load Cell
(vi) Elastic Force Meter
a. Proving Ring – (Mechanical Based)
b. Load Cell Type Elastic Loaded Members
c. Electronic Weighing System
5.1.1.1 DIRECT METHODS :
Basic principle of equal arm balance: It works on the principle of moment comparison. The beam of
the equal arm balance is in equilibrium when clockwise rotating moment is equal to anticlockwise rotating
moment.
Equal arm balance is the most simple force measuring system. The basic principle of operation is
based on moment comparison. The system consists of a beam pivoted on a knife edge fulcrum placed

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Fig.: 5.01 Equal Arm Balance

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(ii) Analytical Balance:
Analytical balance consists of an arm that rotates about a pivot. Two forces W1 W2 (or) weights are added at
the two ends as shown in figure.
Fig.: 5.02 Analytical Balance
Let W1 be the know force and W2 be the unknown. Let ‘G’ be the gravity center of the arm and WG
be its weight. When W1 = W2, the arm is unbalanced. This unbalance is indicated by angle the pointer
making with the vertical.
For equilibrium, the requirement is
WG.XG = W1 W1 – W2 W2
(iii) Unequal Arm Balance:
An equal arm analytical balance suffers from a major disadvantage. It requires a set of weights which are at
least as heavy as the maximum weight to be measured. In order that the heavier weights may be measured
with the help of lighter weights, balances with unequal arms are used.
The unequal arm balance uses two arms. One is called the load arm and the other is called the power arm.
The load arm is associated with load i.e., the weight force to be measured, while power arm is associated
with power i.e, the force produced by counter posing weights required to set the balance in equilibrium.
Fig. shows a typical unequal arm balance. Mass ‘m' acts as power on the beam and exerts a force of Fg due
to gravity where Fg = m x g. This force acts as counterposing force against the load which may be a test
force Ft.
The beam is pivoted on a knife edge 'q'. The test force Ft is applied by a screw or a lever through a knife
edge 'p' until the pointer indicates that the beam is horizontal.

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For balance of moments, Ft x a = Fg x b
or test force Ft = Fg x b/a
= m x g x b/a
= constant x b ( provided that g is constant)
Fig.: 5.03 Schematic of Unequal Arm Balance
Therefore the test force is proportional to the distance 'b' of the mass from the pivot. Hence, if mass 'm' is
constant and the test force is applied at a fixed distance 'a' from the knife edge 'q' (i.e., the load arm is
constant), the right hand of the beam (i.e., the power arm) may be calibrated in terms of force Ft. If the scale
is used in different gravitational fields, a correction may be made for change in value of 'g'. The set-up
shown in Fig. is used for measurement of tensile force. With suitable modifications, it can be used for
compression, shearing and bending forces. This machine can also be used for the measurement of unknown
mass. Suppose force Ft is Produced by an unknown mass mt.
Therefore f t = mt g
Hence, for balance, m1 x g x a = m x g x b
or m1 = m x b/a = a constant x b
Therefore, the power arm b may be calibrated to read the un known mass m1 directly if ‘m’ and ‘a’ are
fixed. This forms the basis of countless weighing (i.e., mass measuring) machine.

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(iv) Multiple Lever System :
Fig.: 5.04 Multi-lever System for Weighing

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(v) Pendulum Scale :
This uses the Principle of multiple leverage. The input, a direct force or a force Proportional to
weight is transmitted from a suitable agency and applied to the lord rod. As the load is applied, the sectors
rotate about A (Figure) moving the counter weights outward. This movement increases the counterweight
effective moment until the load and balance moments are equalized. Motion of the equalizer bar is converted
to indicator movement by a rack and pinion.
Fig.: 5.05 Pendulum Scale

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5.1.1.2 INDIRECT METHODS OF MEASUREMENT OF FORCE
(i) Acceleration Method:
A force will make a body accelerate. By measuring the acceleration, the force may be determined,
from the equation F=ma, when m – mass of the body used. To measure acceleration, accelerometers are
used.
gfgdfdsggfdsgdfg
Fig.: 5.06 Acceleration Method
(ii) Electromagnetic Balance:
The main parts of the electromagnetic balance are photoelectric transducer, an amplifier and a coil
suspended in a magnetic field. The coil carries a current while produces an electromagnetic torque.
Fig. :5.07 Electromagnetic Balance

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The servo system is used with the coil to balance the difference between unknown force and gravitational
force acting on a standard mass. Photo electric transducer is used to check the balance of unknown force and
the standard mass produced the electrical voltage in a resistor. It is taken as output and circuit connected to
the resistor is used to measure the unknown force. The output signal can be recorded or used for automatic
control applications.
(iii) Elastic Loaded Members:
This uses the principle of finding strain produced in a body to measure the force applied. For
measuring displacement, strain gauges are mounted as shown in figure. The body is subjected to a force and
the gauges measure the strain so produced.
Fig. :5.08 Elastic Loaded Member
From basic mechanics of materials, force F produces a displacement
Fl
AE
 
Where
l – Length of the specimen
A – Cross-sectional area
E – Young’s modulus
1 2
2 4
F
And strain ,
AE
F
,
AE
  

  
 being poison’s ratio. If the output of the circuit is e, it is given by
F
1 2 3 4
F
V.G
e = ( )
4
V.G F
e = (l )
2 AE
      
 

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(iv) Cantilever Elastic Member:
In a cantilever beam, if the point of application of load is known, the bending moment caused by it
can be interpreted as force applied.
It is established that due to force, F, deflection of a cantilever at a length ‘l’ from the point of application of
force, is given as
3
W I
3 EI
 
where E – Young’s modulus of beam material,
I – Moment of inertia of beam section =
3
bd
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Fig.: 5.09 Elastic Cantilever
From bending equation,
Moment at section x x xM x z (z-section modulus) 
2
x x
bd
M x
6
 
x
x x
1
x 2
Strain is given by
E
6.Fl
i.e.,
E.bd

  
 
Gauges R1, R3 measure tensile strain and R2, R4 measure compressive strain.
(v) LOAD CELLS:
Force transducers intended for weighing purposes are called load cells. Instead of using total deflection as
a measure of load, strain gauge load cells measure load in terms of unit strains. A load cell utilizes an elastic
member as the primary transducer and strain gauges as secondary transducer. Figure shows one such load
cell arrangement.

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Principle of working of load cells:
Force applied to the elastic member of the cell results in a proportional displacement or strain is sensed
by calibrated mechanical or electromechanical means.
Fig.: 5.10
Applications Of Electronic Weighing Machine ( working with the help of load cell) :
Platform weighing, truck weighing, crane weighing, weighing of vehicle in motion, Weigh feeders, tank &
hooper weighing, bulk load sensing as encountered in steel plants, chemical plants, petro chemical plants,
fertilizers, pharmaceuticals and textile industries, etc.

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(a) Hydraulic Load Cell:
Fig.: 5.11 Hydraulic Load cell
(b) Strain Gauge Load Cell :

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Fig.:5.12 Strain Gauge Load Cells

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In the above fig., two sets of strain gauges mounted at 90˚ to each other.
Where, - Poission ratio, - Gauge factor or gain factor, - Strain
(c) Pneumatic Load Cell :
Fig. : 5.13 Pneumatic Load Cell

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(VI) ELASTIC FORCE METER:
(a) Use of Proving Ring:
Proving rings are steel rings used for calibration of material testing machines in situations where, due
to their bulkness, dead weight standards cannot be used. P ring is a circular ring of rectangular section and
may support tensile or comprehensive force across its diameter.  the change in radius in the direction of
force, is given by,
3
K 4 F.d
16 2 EI
 
   
 
where d is the outer diameter of the ring and K is stiffness.
Fig.: 5.14 Use of Proving Ring
Deflection of the ring is measured using a precision micrometer. To get precise measurements, one edge of
the micrometer is mounted on a vibrating reed which is plucked to obtain a vibratory motion. The
micrometer contact is then moved forward until a noticeable damping of the vibration is observed.
Maximum deflection is typically of the order of 1% of the outside diameter of the ring. Proving rings are
normally used for force measurement within the range of 2 kN to 2 mN.
(b) Load Cell Type Elastic Loaded Members: Refer 5.1.1.2 (V) (b) Strain Gauge Load Cell :
(c) Electronic Weighing System :
Weighing (force) problems in industries and commercial process are solved by incorporating an electronic
system for data acquisition, processing and control purposes. The system comprises the basic load cell,
suitable signal conditioners and output recorders/indicators giving both analog and digital output for further
processing.The signal from the load cells are added and amplified to give an output 0 to 5V or 4 to 20mA for

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Fig. : 5.15

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5.1. MEASUREMENT OF POWER: ( FORCE, TORQUE AND POWER)
5.1.2 MEASUREMENT OF TORQUE:
Instruments used for the measurement of torque.
1.Optical torsion meter
2.Electrical torsion meter
3.Strain gauge torsion meter
4.Mechanical torsion meter
Basic Principle Of Mechanical Torsion Meter.
When a shaft is connected between a driving engine and driven load, a twist occurs on
the shaft between its ends. This angle of twist is measured and calibrated in terms of torque.
Torque Measurement:
5.1.2.1 TORQUE REACTION METHODS:
Fig. : 5.16

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Fig.: 5.16 (a) Cradled Power Source
(b) Cradled Power Absorber
Fig.: 5.16 Measuring Cradled Forces in Cradled Shaft Bearings
5.1.2.2 PRONEY BRAKE:

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Fig.: 5.17 Proney Brake
Fig. : 5.17

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5.1.2.3 TORQUE MEASUREMENT USING STRAIN GAUGES:
Fig.: 5.18 Strain Gauges for Shaft Torque Measurements
Slip Arrangement in Torque transducer
Fig. : 5.18
Fig. : 5.19

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Fig.: 5.19 Slip Arrangement in Torque Transducer
Strain Gauge Transducer Using Beams in Bending
Fig.: 5.20 Slip Strain Gauge Transducer Using Beams in Bending
Fig. : 5.20

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Fig.: 5.21 Block Circuit Diagram for Rotating Sensors with AC Supply
5.20

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5.1.2.4 TORQUE MEASUREMENT USING TORSION BARS:
Various methods are used measure angular twist of torsion bar
(a) Optical Method:
Fig.: 5.22 Torsion Bar Torque Transducer
Fig. : 5.22

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(b) Capacitive Method:
Fig.: 5.23 Capacitive Method
Fig. : 5.23
Fig. : 5.24

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Fig.: 5.24 (Capacitive Method) Rotating Resonant Circuit Excited by Inductive Coupling
(c) Laser - Optic Method:
Fig.: 5.25 LASER Optical Torque Measurement

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(d) Proxity Sensor Method:
Fig.: 5.26 Proximity Sensors for Torque Measurements
Fig. : 5.26

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(e) Stroboscope Method:
Fig.: 5.27 Stroboscope (for Torque Measurements)
(f) Magnetostrictive Method :
Fig. : 5.27

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Fig.: 5.28 Magnetostrictive Transducer
Fig. : 5.28

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(g) Surface Acoustic Wave (SAW) Method:
Fig. : 5.29

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Fig.: 5.29 SAW Electrode Arrangement
Fig.: 5.30 SAW Transducer (for shaft torque measurement)
Fig. : 5.30.

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5.1 MEASUREMENT OF POWER: (MEASUREMENT OF FORCE, TORQUE AND POWER)
5.1.3 MEASUREMENT OF POWER:
The force, in addition to its effect along its line of action, may exert a turning effort relative to any axis other
than those intersecting the line of action as shown in Fig. Such a turning effect is called torque or couple
Torque or couple = Fb1 - Fb3
= Fb2
Fig.: 5.31
The important reason for measuring torque is to obtain load information necessary for stress or deflection
analysis. The torque T may be computed by measuring the force F at a known radius 'r' from the following
relation T=Fr.
However, torque measurement is often associated with determination of mechanical power,
either power required to operate a machine or power developed by the machine. The power is
calculated from the relation.
P = 2 π NT
where N is the angular speed in revolutions per second.
Torque measuring devices used in this connection are commonly known as dynamometers.
There are basically three types of dynamometers.
Measurement of Power is done by the following dynamometers. They are of the following types
a) Absorption Dynamometer
b) Driving Dynamometer
c) Transmission Dynamometer
a). Absorption Dynamometers: They absorb the mechanical energy as torque is measured,

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and hence are particularly useful for measuring power or torque developed by power sources such as engines
or electric motors.
b) Driving Dynamometers: These dynamometers measure power or torque and as well provide energy to
operate the devices to be tested. They are, therefore, useful in determining performance characteristics of
devices such as pumps, compressors etc.
c) Transmission dynamometers: These are passive devices placed at an appropriate location within a
machine or in between machines to sense the torque at that location. They neither add nor subtract the
transmitted energy or power and are sometimes referred to as torque meters.
The first two types can be grouped as mechanical and electrical dynamometers.
5.1.3.1 MECHANICAL DYNAMOMETER:
Mechanical Dynamometer (Prony Brake)
These dynamometers are of absorption type. The most device is the prony brake as shown in Fig. In Prony
brake, mechanical energy is converted into heat through dry friction between the wooden brake blocks and
the flywheel (pulley) of the machine.
Two wooden blocks are mounted diametrically opposite on a flywheel attached to the rotating shaft whose
power is to be measured. One block carries a lever arm, and an arrangement is provided to tighten the rope
which is connected to the arm. The rope is tightened so as to increase the frictional resistance between the
blocks and the flywheel. The torque exerted by the prony brake is,
T = F.L
where force F is measured by conventional force measuring instruments, like balances or load cells etc. The
power dissipated in the brake is calculated by the following equation.
.
If F – Load applied and
Power dissipated
2 NT 2 NFr
P
60 60
 
 
r - Lever arm
N – Speed of flywheel (rpm)
Torque T = F.r
where force F is in Newtons, L is the length of lever arm in meters, N is the angular speed in revolution per
minute, and P in watts.
The prony brake is inexpensive, but it is difficult to adjust and maintain a specific load.

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Fig.:5.32 Schematic of Prony Brake
Limitation :
The prony brake is inherently unstable. Its capacity is limited by the following factors.
i). Due to wear of the wooden blocks, the coefficient of friction varies between the blocks and the flywheel.
This requires continuous tightening of clamp. Therefore, the system becomes unsuitable for measurement of
large powers especially when used for long periods.
ii) The use of prony brake results in excessive temperature rise which results in decrease in coefficient of
friction leading to brake failure. In order to limit the temperature rise, cooling is required. This is done by
running water into the hollow channel of the flywheel.
iii) When the machine torque is not constant, the measuring arrangement is subjected to oscillations. There
may be changes in coefficient of friction and hence the reading of force Fmay be difficult to take.
These come under the absorption type. An example for this kind is prony brake.

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5.1.3.2 DC DYNAMOMETER
Working of a D.C. Dynamometer
D.C. dynamometer is usable as an absorption as well as transmission dynamometer. So, it finds its
use in I.C. Engines, steam turbines and pumps.
A D.C. dynamometer is basically a D.C. motor with a provision to run it as a D.C. generator where the
input mechanical energy, after conversion to electrical energy, can either be dissipated through a resistance
grid or recovered for use. When used as an absorption dynamometer it acts as D.C. generator. (figure)
Cradling in trunnion bearings permits the determination of reaction torque.
Fig.:5.33 D C Electrical Dynamometer Setup
One means for measuring torque is to mount the dynamometer housing so that it is free to turn except as
restrained by a torque arm. The housing can be made free to rotate by using trunnions connected to each end
of the housing to support it in pedestal-mounted trunnion bearings. The torque arm is connected to the dyno
housing and a weighing scale is positioned so that it measures the force exerted by the dyno housing in
attempting to rotate.
The torque is the force indicated by the scales multiplied by the length of the torque arm measured from the
center of the dynamometer. A load cell transducer can be substituted for the scales in order to provide an
electrical signal that is proportional to torque.
Another means to measure torque is to connect the engine to the dynamometer through a torque sensing
coupling or torque transducer. A torque transducer provides an electrical signal that is proportional to the
torque.

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With electrical absorption units, it is possible to determine torque by measuring the current drawn (or
generated) by the absorber/driver. This is generally a less accurate method and not much practiced in
modern times, but it may be adequate for some purposes.
When torque and speed signals are available, test data can be transmitted to a data acquisition system rather
than being recorded manually. Speed and torque signals can also be recorded by a chart recorder or plotter.
Its good performance at low speeds and ease of control makes it an efficient means of torque measurement.
5.1.3.3 EDDY CURRENT OR INDUCTOR DYNAMOMETERS:
This is an example for absorption type dynamometers.
Principle: When a conducting material moves through a magnetic flux field, voltage is generated, which
causes current to flow. If the conductor is a wire forming a part of a complete circuit will be caused to flow
through that circuit, and with some form of commutating device a form of a.c. or D.C. generator may result.
(a)
(b)
Fig.: 5.34 Eddy Current Dynamometer

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An eddy current dynamometer is shown in figure. It consists of a metal disc or wheel which is
rotated in the flux of a magnetic field. The field if produced by field elements or coils excited by an external
source and attached to the dynamometer housing which is mounted in trunnion bearings. As the disc turns,
eddy currents are generated. Its reaction with the magnetic field tends to rotate the complete housing in the
trunnion bearings. Water cooling is employed.
5.1.3.4 HYDRAULIC DYNAMOMETERS:
Fig. shows a hydraulic dynometer in its simplest form which acts as a water brake. This is a power sink
which uses fluid friction for dissipation of the input energy and thereby measures the input torque-or power.
The capacity of hydraulic dynamometer is a function of two factors, speed and water level. The power
consumed is a function of cube of the speed approximately. The torque is measured with the help of a
reaction arm. The power absorption at a given speed may be controlled by adjustment of the water level in
the housing.
Fig.: 5.35 Hydraulic Dynamometer
This type of dynamometer may be made in considerably larger capacities than the simple prony brake
because the heat generated can be easily removed by circulating the water into and out of the housing.
Trunnion bearings support the dynamometer housing, allowing it a freedom to rotate except for the restraint
imposed by the reaction arm.

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In this dynamometer the power absorbing element is the housing which tends to rotate with the input shaft of
the driving machine. But, such rotation is constrained by a force-measuring device, such as some form of
scales or load cell, placed at the end of a reaction arm of radius r. By measuring the force at the known
radius, the torque T may be computed by the simple relation
Advantages of hydraulic dynamometers (over mechanical brake):
 In hydraulic dynamometer constant supply of water running through the breaking medium acts as a
coolant.
 The brake power of very large and high speed engines can be measured.
 The hydraulic dynamometer may be protected from hunting effects by means of a
dashpot damper.
 In hydraulic dynamometer there is a flexibility in controlling the operation

COIMBATORE / ME6504 / V-SEM /2015 METROLOGY & MEASUREMENTS
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5.2 MEASUREMENT OF FLOW
Quantity meter and flow meter: Quantity meter measures the rate of flow by measuring the total
quantity of fluid over a period of time and dividing it by the time considered. Flow meter measures the
actual flow rate.
5.2.1 ORIFICE METER:
Orifice meter is the most common type of head flow measuring device for medium and large pipe
sizes. When the orifice plate is placed in pipe flow, the flow rate increases but the pressure drops decreases.
The Point at which the velocity is maximum and the pressure is minimum is known as vena-contracta. So,
the flow rate is maximum at vena-contracta the orifice plate placed in the pipe flow is generally a thin metal
plate having circular opening. Depending on the hole (circular opening) placement, the orifice plates are
configured in to three types such as, concentric, eccentric and segmented. But the concentric type of orifice
plate is mainly used to measure the flow rate.
Fig.: 5.36a Orifice meter
Let a1 – Area at section I-I
a0 – Area of orifice
Cd – Discharge coefficient
Then, Flow rate
d 1 0
2 2
1 o
C a a
Q
A a
 


Three kinds of orifice plates are used: concentric, eccentric, and segmental (as shown in Figure 5.36B).
The concentric orifice plate is the most common of the three types. As shown, the orifice is equidistant
(concentric) to the inside diameter of the pipe. Flow through a sharp-edged orifice plate is characterized
by a change in velocity. As the fluid passes through the orifice, the fluid converges, and the velocity of the
fluid increases to a maximum value. At this point, the pressure is at a minimum value. As the fluid

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diverges to fill the entire pipe area, the velocity decreases back to the original value. The pressure
increases to about 60% to 80% of the original input value. The pressure loss is irrecoverable; therefore,
the output pressure will always be less than the input pressure. The pressures on both sides of the orifice
are measured, resulting in a differential pressure which is proportional to the flow rate.
Fig.: 5.36b Types of Orifice Plates
Segmental and eccentric orifice plates are functionally identical to the concentric orifice. The circular
section of the segmental orifice is concentric with the pipe. The segmental portion of the orifice eliminates
damming of foreign materials on the upstream side of the orifice when mounted in a horizontal pipe.
Depending on the type of fluid, the segmental section is placed on either the top or bottom of the
horizontal pipe to increase the accuracy of the measurement.
Eccentric orifice plates shift the edge of the orifice to the inside of the pipe wall. This design also
prevents upstream damming and is used in the same way as the segmental orifice plate.
Orifice plates have two distinct disadvantages; they cause a high permanent pressure drop (outlet pressure
will be 60% to 80% of inlet pressure), and they are subject to erosion, which will eventually cause
inaccuracies in the measured differential pressure.
Advantages of Orifice-meter:
 Initial cost is low
 Installation and replacement are easy
 It less space when compare with venturimeter.
 It can be used in wide range of pipe sizes (0.01m- 1.5 m)
Disadvantages of Orifice-meter:
 Loss of head is high
 Co efficient of discharge has a low value
 It is more susceptible to inaccuracies resulting from erosion, corrosion and scaling.

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5.2.2. VENTURIMETER:
This is just like an orifice meter. It has three distinct parts, namely convergent cone, throat and
divergent cone. A manometer measures the pressure difference between two sections as shown in figure.
The inlet cones are convergent cone tapers towards right from pipe area to throat. So, the diameter at inlet is
bigger when compare to outlet diameter of the convergent cone. Therefore, the flow rate increases from inlet
to exit. But the flow is constant throughout the throat section due to uniform diameter of throat. In the
divergent cone the diameter increases from entry to exit. So, the flow decreases at the end.
It is similar to orifice meter. It has three distinct parts, namely convergent cone, throat and divergent cone.
Manometer measures the pressure difference between two sections shown in figure.
Fig.: 5.37 Venturi meter
Let a1 - Area at the inlet (1-1)
A2 - Area at the section (2-2)
x - Pressure head difference
Cd - Discharge coefficient
Then, Q = d 1 2
2 2
1 2
C a a 2 g x
a a
 

Advantages of Venturimeter:
 Low head loss about 10% of differential pressure head.
 High co-efficient of discharge.
 Capable of measuring high flow rates in pipes having very large diameter.
 Characteristics are well established so they are extensively used in process and other industries.
 No wear and tear.
 Less likelihood of becoming clogged sediment.
Disadvantages of Venturimeter:
 It has long laying length.
 More space is required.

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 Quite expensive in installation and replacement.
 Possibility of cavitations.
5.2.3 ROTAMETER:
A rotameter is a variable area type flow meter. It consists of a vertical tapered tube with a float which is free
to move within the tube. The fluid goes from the bottom to the top. When no fluid flows, the float rests at
the bottom of the tube. The float is made of such a diameter that it completely blocks the inlet.
The tapped tube is made of glass, stainless steel or monel and floats are made of brass, Stainless steel, monel
or special plastics. Sometimes, rotameter is known as constant pressure drop meter, variable area or variable
aperture meters.
Fig.: 5.38 Rotameter
When flow starts in the pipeline and fluid reaches the float, the buoyant effect of fluid makes the float
lighter. The float passage remains closed until the pressure of the flowing material plus the buoyance effect
exceeds the downward pressure due to the float weight. Thus, depending on flow, the float assumes a
position. Thus the float gives the reading of flow rate.
Advantages :
Rota meters have several important advantages over other variable area meters.
They are easy to construct and
They are often made from inexpensive materials.
Rota meters do not require any external force aside from the substance
They are measuring and can be used in a wide variety of systems due to their portability and small design.

45
Disadvantages:
Rotameters must be made of glass or other transparent material in order to see the float in the tube.
They must also be used vertically because of their dependence on gravity.
Rotameters are only reliable for a specific substance at a specific temperature.
Therefore, multiple scales or even multiple rotameters must be used for measuring different substances
5.2.4 PITOT TUBE:
Principle: “Transformation of kinetic energy of a liquid into potential energy in the form of a static head”.
For finding the discharge of a moving liquid, two quantities are required.
1. Mean velocity of flow.
2. Area of cross section of flow.
A pitot tube is a simple device used for measuring the velocity of flow. A French scientist (Hendry Pitot)
found that when inserting a bent glass tube into the river and the water is raised into the tube to a certain
height. He is also found that this height of water is proportional to the square of velocity of flow at end
point. The velocity at the entrance point of the tube is reduced to zero (known as stagnation point). Then the
pressure at the point is increased in to the conversion of kinetic energy in to pressure energy. The velocity of
flow can be determined by measuring the increase in pressure energy at this point.
Fig.: 5.39 Pitot tube
Fig 5.39 shows a pitot tube installed in a pipeline where it acts similar to a probe. The tube consists of two
concentric tubes. The inner tube with its open ends faces the liquid. The outer tube has a closed end and it
has four to eight holes in its wall. The pressure in the outer tube is the static pressure in the line. Total
pressure is the sum of static pressure and the pressure due to the impact of fluid.

46
Pitot tube Consists of a glass tube bent at right angle. The diameter is large enough to avoid capillary effects.
The tube is kept vertically in the following stream of liquid with its open end facing the direction of flow
shown in figure 5.40. The liquid enters the tube and the level of the liquid raises in the vertical leg of the
tube. It is because of the end B of the tube being a stagnation point where the liquid has no velocity. At the
stagnation point kinetic energy is converted into pressure energy. Hence the liquid rises in the tube above the
surrounding liquid surface to a certain height which corresponds to the velocity of flow of the liquid. The
pressure at the stagnation point is known as stagnation pressure ‘Po’.
Fig.: 5.40 Pitot tube
The height of the liquid in the tube above the surrounding liquid surface gives the kinetic head of moving
liquid where the lower end of the tube kept. The outer tube has a closed end and has four to eight holes in its
wall. The pressure in the outer tube is the static pressure in the line. Total pressure is sum of static pressure
and the pressure due to the impact of fluid.
If P - Pressure at inlet (Stagnation pressure)
Ps - Static pressure
 - Density, then
The value for the coefficient of pitot tube C may be generally 0.98.
Consider a section 1 and 2 at a same level just in front of inlet of the tube
Substitute above value in Bernoulli’s
H + V12 / 2g = h + H h = V12 / 2g
V1 = √2gh
Actual velocity (V) = Cv V theoretical
V = Cv √2gh Where Cv = Coefficient of velocity
h = Rise in tube
H = head of pressure at
h + H = stagnation head
Velocity V = 02/ (P P ),  from which flow rate is determined.

47
5.3 MEASUREMENT OF TEMPERATURE
Introduction:
Purpose of temperature measurement
1. It is one of the most common and important measurements.
2. In process industries which involve chemical operations.
3. In studying the temperature of molten metal in foundries.
Instruments used to measure temperature.
1. Bimetallic thermometers
2. (Electrical) Resistance thermometers
3. Thermistors
4. Thermocouples
5. Pyrometer
Two types of hot wire anemometer.
1. Constant current type
2. Constant temperature type.
Thermistor
It is a bulk semiconductor resistance temperature sensor.
Advantages of thermistors
1. Fairly good operating range (100C to 300C).
2. Have ability to withstand electrical and mechanical stresses.
Basic principle of resistance thermometers or (RTD)
When an electric conductor is subjected to temperature change the resistance of the
conductor changes. This change in resistance of the conductor becomes a measure of the change in
temperature when calibrated.
Pyrometer
Three definitions
 “ny instrument used for measuring high temperatures by means of the radiation emitted
by a hot object
 “ thermometer designed to measure high temperatures
 “ device measuring the temperature of an object by means of the quantity and character
of the energy which it radiates
Two distinct instruments commonly referred to as pyrometers.
1. Total radiation Pyrometers.
2. Optical pyrometers.

48
5.3.1 BIMETALLIC THERMOMETERS:
Principle involved : These use the principles of metallic expansion when temperature changes.
A bimetallic strip is shown in figure which is straight initially. When temperature changes,
its shape also changes into an arc.
Fig.: 5.41 Deformation of bimetallic Strip
Two pieces of metal with different coefficient of thermal expansion are bonded together to form
the bimetallic strip shown in the figure. It is in the form of a cantilever beam. When the strip is
subjected to a temperature higher than the bonding temperature, it will be bent in one direction.
If it is subjected to a temperature lower than the bonding temperature, it will be bent in the other
direction.
Fig.: 5.42 Bi-Metallic thermometer

49
The displacement of the free end can be converted into an electric signal through use of
secondary transducers like variable resistance, inductance and capacitance transducers. Figure
shows a strip of bimetal in the form of a spiral. The curvature of the strip varies with temperature.
This causes the pointer to deflect. A scale is provided which has been calibrated to show the
temperature directly.
This kind of spiral is mostly used in devices measuring ambient temperature and air-
conditioning thermostats.
Suppose it is subjected to a temperature lower than the bonding temperature, it will be in the
other direction/
The radius of curvature r may be calculated by,
Where,
t = Combined thickness of the bonded strip in mm.
α1 = Lower co efficient of expansion per ˚C
α2 = Higher co efficient of expansion per ˚C
m = Ratio of thicknessof low to high expansion materials.
n = Ratio of modulus of elasticity of low to high expansion materials.
T = Temperature in ˚C
T0 = Initial bonding temperature in ˚C

50
Advantages:
1. Simple in construction
2. Inexpensive when compare to the other temperature measuring instruments.
3. Negligible maintenance expense.
4. It is stable operation over extended periods of time.
5. Accuracy of  2% to 5%
Limitations:
1. Not usable above 400C because of possibility of warping.
2. The permanent deformation of metallic strip may occur.
Application:
1.Frequently used in simple ON-OFF switches, control switches.
2. Refineries
3. Vulcanizers.
4. Oil burners, etc.
5.3.2 THERMOCOUPLES
 Thermocouples are based on see back effect.
 Thermocouple temperature measurement is based on creation of an EMF.
Principle:
When two dissimilar metals are joined together an e.m.f will exist between the two points A
and B, which is primarily a function of the junction temperature. The above said to be principle is
See back effect.
Fig.: 5.43 Simple Thermocouple Circuit

51
Construction:
 The Thermocouple consists of one hot junction and one cold junction
 Hot junction is inserted where temperature is measured
 Cold junction in maintained at a constant reference temperature.
Typical Thermocouple Construction:
 The leads of the Thermocouple are encased in a rigid metal sheath.
 The measuring junction is normally formed at bottom of thermocouple housing
 Magnesium oxide surrounds the Thermocouple wire to prevent vibration that could
damage the fine
 Wires and to enhance heat transfer b/w the measuring junction and the medium
surrounding the Thermocouples.
Fig.: 5.44 Construction of a Thermo Couple
Operations:
Fig.: 5.45

52
Thermocouple laws:
The thermocouple laws are classified into two
(i) Law of Intermediate Materials:
The algebraic sum of the thermoelectric forces in a circuit composed of any number of dissimilar
materials is zero, if all of the junctions are at a uniform temperature.
(ii) Law of Intermediate Temperature :

53
Fig. 5.46
Metal used for thermocouple wire.
1. Chromel - constantan
2. Iron – constantan
3. Chromel – Alumel
4. Copper – constantan
5. Platinum – Rhodium

54
5.3.3 ELECTRICAL THERMAL RESISTANCE (OR) RESISTANCE THERMOMETER
DETECTOR (RTD) (OR) ELECTRICAL RESISTANCE THERMOMETER
Principle of resistance thermometers:
When an electric conductor is subjected to temperature change the resistance of the
conductor changes. This change in resistance of the conductor becomes a measure of the change in
temperature when calibrated.
Fig.: 5.47 Construction of a Electrical Resistance Thermometer / or (RTD)

55
Resistance-Thermometer Elements:
The material should have a resistivity permitting fabrication in convenient sizes without excessive
bulk, which would degrade time response. In addition, its thermal coefficient of resistivity should
be high and as constant as possible, thereby providing an approximately linear output of
reasonable magnitude.
Undoubtedly, platinum, nickel, and copper are the materials most commonly used, although
others such as tungsten, silver and iron have also been employed. The specific choice normally
depends upon which compromises may be accepted. The temperature – resistance relation of an
RTD must be determined experimentally. For most metals, the result can be accurately
represented as
    0 o 0
R(T) R 1 A T T B T T 2    
where
R(T) = the resistance at temperature T,
R0 = the resistance at a reference temperature T0
A and B = temperature coefficients of resistance depending on material.
Over a limited temperature interval (perhaps 50C for platinum) a linear approximation to
the resistance variation may be quite acceptable.
R(T) = R0 (1+ A(T – T0))
But for the highest accuracy, a high – order polynomial fit is required.
The resistance element:
The resistance element is most often a metal wire wrapped around an electrically insulating
support of glass, ceramic or mica. The latter may have a variety of configurations, ranging from a
simple flat strip, as shown in figure to intricate bird-cage arrangement 3 . The mounted element
is then provided with a protective enclosure. When permanent installations are made and when
additional protection from corrosion or mechanical abuse is required, a well or socket may be
used, such as shown in figure.
Fig.: 5.48 Construction of a RTD

56
Limitations:
1. Self heating may occur
2. It behaves highly non-linear over its range of operation.
3. It is possible ins thermistor an increase of the resistance when the time lapses.

57
5.4 RELIABILITY & CALIBRATION / RELIABILITY & READABILITY
5.4.1 RELIABILITY AND CALIBRATION :
Definition of Reliability:
The reliability of a measurement is the extent of unsystematic variation (random error) in the quantitative
description of some characteristic of an individual when that individual is measured a number of times.
Reliability is the confidence that a measurement reflects a real or stable trait.
Reliability may also describe the ability to function at a specified moment or interval of time (Availability).
Reliability engineering represents a sub-discipline within systems engineering.
Random error is inversely related to the degree of reliability of a measuring instrument
o Random error produces random, unsystematic, fluctuations in measurement scores
o Measuring instruments always contain some random error
Calibration: It is very widely used in industries. It is the setting or correcting of a measuring device or a
base level usually by adjusting it to match or conform to a dependably known value or act of checking or
adjusting (by comparing with standard) the accuracy of a measuring instrument.
It is the procedure employed for making adjustments or checking a scale for the readings of a system
conforming to the accepted or pre defined standard i.e. to say that the system has to prove its ability to
measure reliably. Every measuring system must be provable. The procedure adopted to prove the ability of a
measuring system to measure reliably is called ‘calibration’.
The calibration of any measuring system is very important to get
meaningful results. In case where the sensing system and measuring system are different, then it
is imperative to calibrate the system as an integrated whole in order to take into account the error
producing properties of each component.
Calibration is usually carried out by making adjustments
such that readout device produces zero output for zero-measurand input, and similarly it should
display an output equivalent to the known measurand input near the full-scale input value.
It is important that any measuring system calibration should be performed under environ-
mental conditions that are as close as possible to those conditions under which actual measurements
are to be made.
It is also important that the reference measured input should be known to a much greater
degree of accuracy—usually the calibration standard for the system should be at least one order of

58
magnitude more accurate than the desired measurement system accuracy, i.e. accuracy ratio of
10: 1.
5.4.2 RELIABILITY AND READABILITY:
The reliability of a measurement is the extent of unsystematic variation (random error) in the quantitative
description of some characteristic of an individual when that individual is measured a number of times.
Reliability is the confidence that a measurement reflects a real or stable trait.
Reliability may be defined in the following ways:
 The idea that an item is fit for a purpose with respect to time
 The capacity of a designed, produced, or maintained item to perform as required over time
 The capacity of a population of designed, produced or maintained items to perform as required over
specified time
 The resistance to failure of an item over time
 The probability of an item to perform a required function under stated conditions for a specified
period of time
 The durability of an object.
Readability refers to the susceptibility of a measuring device to having its indications
converted to a meaningful number. A micrometer instrument can be made more readable by using
verniers. Very finely spaced lines may make a scale more readable when a microscope is used, but
for the unaided eye, the readability is poor.
In modern days, digital instruments are very popular and used in almost all types of measurements.
However, the analog instruments are not absolute. The analog instruments are extensively used in some
types of measurements such as weighing scale, thermometer etc.
Readability is a word which is frequently used in analog instruments. The readability depends on both the
instruments and observer. Readability is defined as the closeness with which the scale of an analog
instrument can be read.
For instance, a deflection type weighing scale with a 30cm scale span will have higher readability when
compare to a weighing scale with a 15cm scale span and this same range of measurements. For getting a
readability the instruments scale should be as high as possible. Then only the reader can observe the reading
accurately. The size of the pointer should also be larger with more accurate end conditions. i.e at the end,
pointers should be sharp. Due to this, the parallax effect should be minimized.
***************************

59
TEXT BOOKS
1. Jain. R. K., Engineering Metrology, Khanna Publishers, New Delhi, 1987
2. Gupta. R. C., Statistical Quality Control, Khanna Publishers, New Delhi, 1994
REFERENCE BOOKS
1. Alan S. Morris, “The Essence of Measurement”, Prentice Hall of India, 1997
2. Jayal A.K, “Instrumentation and Mechanical Measurements”, Galgotia Publications 2000
3. Beckwith T.G, and N. Lewis Buck, “Mechanical Measurements”, Addison Wesley, 1991
4. Donald D Eckman, “Industrial Instrumentation”, Wiley Eastern, 1985.
5. Measurement System: Application and Design by Doebelin E.O McGraw Hill Publishing Company.
6. Experimental Methods for Engineers by Holman JP McGraw Hill Publication Company.
7. Mechanical Measurement and Control by Kumar DS; Metropolitan Book Co Pvt. Ltd., New Delhi.
8. Automatic Control systems by Kuo BC; Prentice Hall.

Unit v MEASUREMENT OF POWER, FLOW AND TEMPERATURE RELATED PROPERTIES

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