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Industrial Transducers Laboratory Works
Urvish Soni
Industrial Transducers
Urvish Soni Government Polytechnic Ahmedabad
1
PRACTICAL:1
AIM: TEST & PLOT CHARACTERISTICS OF THERMOCOUPLE.
APPERANTUS: VOLTMETER, TRAINER KIT, WIRES.
THEORY:
A thermocouple consists of two conductors of different materials (usually metal
alloys) that produce a voltage in the vicinity of the point where the two conductors are in
contact. The voltage produced is dependent on, but not necessarily proportional to, the
difference of temperature of the junction to other parts of those conductors. In contrast to
most other methods of temperature measurement, thermocouples are self powered and require
no external form of excitation. The main limitation with thermocouples is accuracy; system
errors of less than one degree Celsius (C) can be difficult to achieve.
A thermocouple measuring circuit with a heat source, cold junction and a measuring
instrument
Any junction of dissimilar metals will produce an electric potential related to
temperature. Thermocouples for practical measurement of temperature are junctions of
specific alloys which have a predictable and repeatable relationship between temperature and
voltage. Different alloys are used for different temperature ranges. Properties such as
resistance to corrosion may also be important when choosing a type of thermocouple.
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Thermocouples are widely used in science and industry; applications include
temperature measurement for kilns, gas turbine exhaust, diesel engines, and other industrial
processes
TYPES:
Thermocouples with low sensitivities (B, R, and S types) have correspondingly lower
resolutions. Other selection criteria include the inertness of the thermocouple material, and
whether it is magnetic or not.
K
Type K (chromel {90% nickel and 10% chromium}—alumel {95% nickel, 2% manganese,
2% aluminium and 1% silicon}) is the most common general purpose thermocouple with a
sensitivity of approximately 41 µV/°C, chromel positive relative to alumel. It is inexpensive,
and a wide variety of probes are available in its −200 °C to +1250 °C / -330 °F to +2460 °F
range. Type K was specified at a time when metallurgy was less advanced than it is today,
and consequently characteristics may vary considerably between samples. One of the
constituent metals, nickel, is magnetic; a characteristic of thermocouples made with magnetic
material is that they undergo a deviation in output when the material reaches its Curie point;
this occurs for type K thermocouples at around 350 °C . Wire color standard is yellow (+)
and red (-).
E
Type E (chromel–constantan) has a high output (68 µV/°C) which makes it well suited to
cryogenic use. Additionally, it is non-magnetic. Wide range is −50 to 740 °C and Narrow
range is −110 to 140 °C. Wire color standard is purple (+) and red (-).
J
Type J (iron–constantan) has a more restricted range than type K (−40 to +750 °C), but
higher sensitivity of about 55 µV/°C. The Curie point of the iron (770 °C) causes an abrupt
change in the characteristic, which determines the upper temperature limit.
N
Type N (Nicrosil–Nisil) (nickel-chromium-silicon/nickel-silicon) thermocouples are suitable
for use between −270 °C and 1300 °C owing to its stability and oxidation resistance.
Sensitivity is about 39 µV/°C at 900 °C, slightly lower compared to type K.
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Platinum types B, R, and S
Types B, R, and S thermocouples use platinum or a platinum–rhodium alloy for each
conductor. These are among the most stable thermocouples, but have lower sensitivity than
other types, approximately 10 µV/°C. Type B, R, and S thermocouples are usually used only
for high temperature measurements due to their high cost and low sensitivity.
B
Type B thermocouples use a platinum–rhodium alloy for each conductor. One conductor
contains 30% rhodium while the other conductor contains 6% rhodium. These thermocouples
are suited for use at up to 1800 °C. Type B thermocouples produce the same output at 0 °C
and 42 °C, limiting their use below about 50 °C.
R
Type R thermocouples use a platinum–rhodium alloy containing 13% rhodium for one
conductor and pure platinum for the other conductor. Type R thermocouples are used up to
1600 °C.
S
Type S thermocouples are constructed using one wire of 90% Platinum and 10% Rhodium
(the positive or "+" wire) and a second wire of 100% platinum (the negative or "-" wire). Like
type R, type S thermocouples are used up to 1600 °C. In particular, type S is used as the
standard of calibration for the melting point of gold (1064.43 °C).
T
Type T (copper – constantan) thermocouples are suited for measurements in the −200 to
350 °C range. Often used as a differential measurement since only copper wire touches the
probes. Since both conductors are non-magnetic, there is no Curie point and thus no abrupt
change in characteristics. Type T thermocouples have a sensitivity of about 43 µV/°C.
C
Type C (tungsten 5% rhenium – tungsten 26% rhenium) thermocouples are suited for
measurements in the 0 °C to 2320 °C range. This thermocouple is well-suited for vacuum
furnaces at extremely high temperatures. It must never be used in the presence of oxygen at
temperatures above 260 °C.
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M
Type M thermocouples use a nickel alloy for each wire. The positive wire (20 Alloy) contains
18% molybdenum while the negative wire (19 Alloy) contains 0.8% cobalt. These
thermocouples are used in vacuum furnaces for the same reasons as with type C. Upper
temperature is limited to 1400 °C. It is less commonly used than other types.
APPLICATIONS:
Thermocouples are suitable for measuring over a large temperature range, up to 2300
°C.
Applications include temperature measurement for kilns, gas turbine exhaust, diesel
engines, and other industrial processes.
They are less suitable for applications where smaller temperature differences need to
be measured with high accuracy, for example the range 0–100 °C with 0.1 °C
accuracy.
For such applications thermistors, silicon bandgap temperature sensors and resistance
temperature detectors are more suitable.
CONCLUSION:
QUESTIONS:
1. What is thermocouple?
2. Where Thermocouples are widely used?
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PRACTICAL: 2
RESISTOR TEMPERATURE DETECTOR
AIM: TEST & PLOT CHARACTERISTICS OF RESISTANCE
TEMPERATURE DETECTOR.(PT-100,PT1000) TEMP VS
RESISTANCE IN OHMS.
APPARTUS: RTD, WIRES
THEORY:
RTD stands for Resistance Temperature Detector. RTDs are sometimes referred to
generally as resistance thermometers.
An RTD is a temperature sensor which measures temperature using the principle that
the resistance of a metal changes with temperature.
In practice, an electrical current is transmitted through a piece of metal (the RTD
element or resistor) located in proximity to the area where temperature is to be
measured.
The resistance value of the RTD element is then measured by an instrument. This
resistance value is then correlated to temperature based upon the known resistance
characteristics of the RTD element.
THE Principle Of Operation:
An RTD is a temperature sensor that operates on the measurement
principle that a materia l’s electr ical res is tance change with temperature.
The relat ionship with resis tance & surrounding temperature is highly
predictable , a l lowing for accurate & consis tent temperature measurement .
Figure 1. This two-wire connection affects measurement accuracy by adding resistance
in series with the RTD.
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Figure 2. An additional third wire to the RTD allows compensation for the wire
resistance. The only restriction is that the main connecting wires have the same
characteristics.
Figure 3. A four-wire approach enables Kelvin sensing, which eliminates the effect of
voltage drops in the two connecting wires.
You can connect a PT100 RTD to the measuring application using two wires, three wires, or
four wires (Figures 1, 2, and 3). Several analog and digital approaches are available for
compensating a PT100 RTD for nonlinearity. Digital linearization, for instance, can be
implemented with a lookup table or by implementing the previous generic equation.
A lookup table located in µP memory allows an application to convert (through interpolation)
a measured PT100 resistance to the corresponding linearized temperature. On the other hand,
the previous generic equation offers a possibility of calculating temperature values directly,
based on the actual measured RTD resistance.
A lookup table necessarily contains a limited number of resistance/temperature values, as
dictated by the required accuracy and the amount of memory available. To calculate a
specific temperature, you must first identify the two closest resistance values (those above
and below the measured RTD value), and then interpolate between them.
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Consider a measured resistance of 109.73Ω, for example. If the lookup table has a resolution
of 10°C, the two closest values might be 107.79Ω (20°C) and 111.67Ω (30°C). Interpolation
using these three values leads to:
Figure 5 shows the raw PT100 output vs. a linear approximation of the form y = ax + b, and
Figure 6 shows a linearized version of the circuit output vs. the linear approximation. Each
figure shows the calculated relationship between temperature and resistance compared to the
output calculated from the Figure 4 circuit.
Figure 5. Raw output of a normalized PT100 vs. a linear approximation to that output.
Figure 6. Analog-compensated output vs. a linear approximation to that output,
showing the error after linearization.
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RESISTANCE VALUE CHART PER 1 PT1000
TEM
P. 0 -1 -2 -3 -4 -5 -6 -7 -8 -9
-20 921.60 917.70 913.70 909.80 905.90 901.90 898.00 894.00 890.10 886.20
-10 960.90 956.90 953.00 949.10 945.20 941.20 937.30 933.40 929.50 925.50
0 1000.0
0 996.10 992.20 988.30 984.40 980.40 976.50 972.60 968.70 964.80
0 1 2 3 4 5 6 7 8 9
0 1000.0
0
1003.9
0
1007.8
0
1011.7
0
1015.6
0
1019.5
0
1023.4
0
1027.3
0
1031.2
0
1035.1
0
10 1039.0
0
1042.9
0
1046.8
0
1050.7
0
1054.6
0
1058.5
0
1062.4
0
1066.3
0
1070.2
0
1074.0
0
20 1077.9
0
1081.8
0
1085.7
0
1089.6
0
1093.5
0
1097.3
0
1101.2
0
1105.1
0
1109.0
0
1112.9
0
FEATURES OF RTD:
The platinum RTD probe has a big advantage on measuring temperature similar to
room temperature because resistance value change versus temperature is large, and
reference junctions that are necessary for thermocouple is not used for an RTD probe.
The platinum RTD probe has high long term stability.
The platinum RTD probe enables highly accurate measurement.
CONCLUSION:
QUESTIONS:
1. What is RTD?
2. What is sensing elements?
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PRACTICAL 3:
AIM:- TEST & PLOT CHARACTERISTICS OF GIVEN THERMISTOR
TEMP. VS RESISTANCE IN OHMS.
THEORY:-
A thermistor is a type of resistor whose resistance varies significantly with
temperature, more so than in standard resistors. The word is a portmanteau of thermal and
resistor. Thermistors are widely used as inrush current limiters, temperature sensors, self-
resetting overcurrent protectors, and self-regulating heating elements.
Thermistor symbol
THERMISTOR CONSTRUCTION:-
The most common form of the thermistor is a bead with two wire attached. The bead
diameter can range from about 0.5mm (0.02") to 5mm (0.2'').
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.
CHARACTERISTIC CURVE:-
The resistance versus temperature curve is one of the main characteristics that is used in
measurement, control and compensation applications using a thermistor. The characteristics
graph is shown below.
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Resistance Versus Temperature Characteristics Of Thermistor
From the characteristics graph of a typical thermistor, we can see that the resistivity
changes from 107 to 1 ohm-cm as the temperature changes from -100 degree Celsius to +400
degree Celsius. This high negative temperature coefficient of resistance makes thermistor an
ideal temperature transducer.
THRMISTOR CIRCUIT:-
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APPLICATIONS:-
PTC thermistors can be used as current-limiting devices for circuit protection, as
replacements for fuses. Current through the device causes a small amount of resistive
heating. If the current is large enough to generate more heat than the device can lose
to its surroundings, the device heats up, causing its resistance to increase, and
therefore causing even more heating. This creates a self-reinforcing effect that drives
the resistance upwards, reducing the current and voltage available to the device.
PTC thermistors were used as heater in automotive industry to provide additional heat
inside cabin with diesel engine or to heat diesel in cold climatic conditions before
engine injection.
NTC thermistors are used as resistance thermometers in low-temperature
measurements of the order of 10 K.
NTC thermistors are regularly used in automotive applications. For example, they
monitor things like coolant temperature and/or oil temperature inside the engine and
provide data to the ECU and, indirectly, to the dashboard.
NTC thermistors can be also used to monitor the temperature of an incubator.
Thermistors are also commonly used in modern digital thermostats and to monitor the
temperature of battery packs while charging.
NTC thermistors are used in the Food Handling and Processing industry, especially
for food storage systems and food preparation. Maintaining the correct temperature is
critical to prevent food borne illness.
CONCLUSION:
QUESTIONS:
1. What is thermistor ?
2. Write down types of thermistors and define them.
3. What is the relation between resistance and temperature in thermistor ?
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PRACTICAL: 4
AIM: TEST & PLOT CHARACTERISTICS OF C TYPE BOURDON
TUBE , PRESSURE VS. LINEAR DISPLACEMENT.
THEORY:
Bourdon Tubes are known for its very high range of differential pressure measurement in the
range of almost 100,000 psi (700 MPa). It is an elastic type pressure transducer.
C-shaped Bourdon Tube:
The C-shaped Bourdon tube has a hollow, elliptical cross section. It is closed at
one end and is connected to the fluid pressure at the other end.
When pressure is applied, its cross section becomes more circular, causing
the tube to straighten out, like a garden hose when the water is first turned
on, until the force of the fluid pressure is balanced by the elastic resistance of
the tube material. Since the open end of the tube is anchored in a fixed position,
changes in pressure move the closed end. A pointer is attached to the closed end of
the tube through a linkage arm and a gear and pinion assembly, which rotates the
pointer around a graduated scale. The detailed diagram of the bourdon tube is
shown below.
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WORKING:
As the fluid pressure enters the bourdon tube, it tries to be reformed and because of a
free tip available, this action causes the tip to travel in free space and the tube unwinds. The
simultaneous actions of bending and tension due to the internal pressure make a non-linear
movement of the free tip.
A lot of compound stresses originate in the tube as soon as the pressure is applied.
This makes the travel of the tip to be non-linear in nature. If the tip travel is considerably
small, the stresses can be considered to produce a linear motion that is parallel to the axis of
the link. The small linear tip movement is matched with a rotational pointer movement. This
is known as multiplication, which can be adjusted by adjusting the length of the lever. For the
same amount of tip travel, a shorter lever gives larger rotation. The approximately linear
motion of the tip when converted to a circular motion with the link-lever and pinion
attachment, a one-to-one correspondence between them may not occur and distortion results.
This is known as angularity which can be minimized by adjusting the length of the link.
Other than C-type, bourdon gauges can also be constructed in the form of a helix or a
spiral. The types are varied for specific uses and space accommodations, for better linearity
and larger sensitivity.The commonly used materials are phosphor-bronze, silicon-bronze,
beryllium-copper, inconel, and other C-Cr-Ni-Mo alloys, and so on.
In the case of forming processes, empirical relations are known to choose the tube
size, shape and thickness and the radius of the C-tube. Because of the internal pressure, the
near elliptic or rather the flattened section of the tube tries to expand as shown by the dotted
line in the figure below (a). The same expansion lengthwise is shown in figure (b). The
arrangement of the tube, however forces an expansion on the outer surface and a compression
on the inner surface, thus allowing the tube to unwind. This is shown in figure (c).
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APPLICATION:
For gaseous and liquid media which are not highly viscous ,do not crystallize and do
not attack copper alloys. For high measuring accuracy. When measuring gas or vapour, the
instruments must be used in accordance with the safety.
ADVANTAGES OF BOURDON TUBE PRESSURE GAUGE:
These Bourdon tube pressure gauges give accurate results.
Bourdon tube cost low.
Bourdon tube are simple in construction.
They can be modified to give electrical outputs.
They are safe even for high pressure measurement.
Accuracy is high especially at high pressures.
LIMITATIONS OF BOURDON TUBE PRESSURE GAUGE:
They respond slowly to changs in pressure
They are subjected to hysterisis.
They are sensitive to shocks and vibrations.
Ampilification is a must as the displacement of the free end of the bourdon tube is
low.
It cannot be used for precision measurement.
CONCLUSION:
QUESTIONS:
1. Which pressure can be measured for bourdon tube ?
2. Which materials are used to make tube ?
3. What is the range of bourdon tube pressure gauge ?
4. Draw the figure of bourdon tube and explain the working of them.
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PRACTICAL: 5
AIM: TEST & PLOT CHARACTERISTICS OF BELLOW, PRESSURE
Vs LINEAR DISPLACEMENT.
THEORY:
Bellows are also used for pressure measurement, and can be made of cascaded
capsules. The basic way of manufacturing bellows is by fastening together many individual
diaphragms. The bellows element, basically, is a one piece expansible, collapsible and axially
flexible member. It has many convolutions or fold. It can be manufactured form a single
piece of thin metal.
BELLOW:
For industrial purposes, the commonly used bellow elements are:
By turning from a solid stock of metal
By soldering or welding stamped annular rings
Rolling a tube
By hydraulically forming a drawn tubing.
WORKING:
The action of bending and tension operates the elastic members. For proper working,
the tension should be least. The design ideas given for a diaphragm is applied to bowels as
well. The manufacturer describes the bellows with two characters – maximum stroke and
maximum allowable pressure. The force obtained can be increased by increasing the
diameter. The stroke length can be increased by increasing the folds or convolutions.
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For selecting a specific material for an elastic member like bellows, the parameters to
be checked are:
Range of pressure
Hysteresis
Fatigue on dynamic operation
Corrosion
Fabrication ease
Sensitivity to fluctuating pressures
For strong bellows, the carbon steel is selected as the main element. But the material gets
easily corroded and is difficult to machine. For better hysteresis properties you can use
trumpet bass, phosphor bronze, or silicon bronze. Better dynamic performance can be
achieved by using beryllium copper. Stainless steel is corrosion resistive, but does not have
good elastic properties. For easy fabrication soft materials are sought after.
All bellow elements are used with separate calibrating springs. The springs can be aligned
in two ways – in compression or in expansion when in use. Both these types, with internal
compression springs or external tension springs, are commercially known as receiver
elements and are used universally in pneumatic control loops. The figures below show the
compressed and expanded type. Spring opposed bellows are also shown below. The open side
of a bellows element is usually rigidly held to the instrument casing and because of the rigid
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fixing, the effective or active length of the bellows element is smaller than its actual length.
This device is used in cases where the control pressure range is between 0.2 to 1 kg/cm2.
CHARACTERISTICS OF BELLOW PRESSURE Vs. LINEAR DISPLACEMENT :
MERITS OF BELLOWS:
Cost is moderate.
It is able to deliver high force.
It is adaptable for absolute and differential pressure.
It is good in the low-to-moderate pressure range.
DEMERITS OF BELLOWS:
It needs ambient temperature compensation.
It is unsuitable for high pressure.
The availability of metals and work-hardening of some of them is limited.
CONCLUSION:
QUESTIONS:
1. what is bellow ?
2. which materials can be used in bellow ?
3. which parameter can be checked for bellow ?
4. write down the types of bellows and explain it shortly.
5. What are the applications of bellws ?
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PRACTICAL: 6
AIM: TEST & PLOT CHARACTERISTICS OF DIAPHRAGM ,
PRESSURE VS LINEAR DISPLACEMENT.
THEORY:
A diaphragm pressure transducer is used for low pressure measurement. They can be
detect a pressure differential even in a range of 0 to 4 mm.
Diaphragm:
The diaphragm can be in the form of flat, corrupted or dished plates and the choice
depends on the strength and amount of deflected desired. In high precision instruments the
diaphragms are generally used in a pair, back-to-back , to form an elastic capsule.
With suitable modification, diaphragm elements can be made to cause changes in
electrical circuits, thus converting pressure movements to electrical signals which can be
transmitted to an indicating or a recording system.
The diagram of a diaphragm pressure gauge is shown below. When a force acts
against a thin stretched diaphragm, it causes a deflection of the diaphragm with its centre
deflecting the most.
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WORKING:
They are commercially available in two types – metallic and non-metallic.
Metallic diaphragms are known to have good spring characteristics and non-metallic
types have no elastic characteristics. Thus, non-metallic types are used rarely, and are usually
opposed by a calibrated coil spring or any other elastic type gauge. The non-metallic types
are also called slack diaphragm.
(a)Metallic Diaphragm:
It consists of a thin flexible diaphragm made of materials such as brass or bronze.
This type of gauge is capable of working in any position and is portable, and therefore well
adapted for use or for installation in moving equipments such as aircrafts.
(b)Slack Diaphragm Gauge:
T is more diffuclt to measure pressure below the atmospheric pressure because the
changes are small. The full range for atmospheric pressure to a perfact vacuum is only 14.7
psi (1 kg/cm2).pressure in this range can be measured with the slight modification of the
diaphragm. A diaphragm with a large area produces a large changes in force from a small
changes in pressure. A slack diaphragm can be made of rubber or other flexible materials.
A slack diaphragm gauge with a weak spring and a large area can be used over
pressure ranges as low as 0.01-0.40 mm hg.
Since the elastic limit has to be maintained, the deflection of the diaphragm must be
kept in a restricted manner. This can be done by cascading many diaphragm capsules as
shown in the figure below. A main capsule is designed by joining two diaphragms at the
periphery. A pressure inlet line is provided at the central position. When the pressure enters
the capsule, the deflection will be the sum of deflections of all the individual capsules. As
shown in figure (3), corrugated diaphragms are also used instead of the conventional ones.
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ADVANTAGES:
Their cost is moderate.
They possess high over-range characteristics.
They are adaptable to absolute and differential pressure measurement.
They have good linearity.
They are available in several materials for good corrosion resistance.
They are small in size.
They are adaptable to slurry services.
DISADVANTAGES:
They lack good vibration and shock resistance.
They are difficult to repair.
They are limited to relatively low pressure.
CONCLUSION:
QUESTIONS:
1. What is diaphragm ?
2. Which pressure can be measured from diaphragm ?
3. What are the types of diaphragm ?
4. What is the range of slack diaphragm?
5. Write down merits and demerits of diaphragm.
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PRACTICAL: 7
AIM: TEST & PLOT CHARACTERISTICS OF ORIFICE , FLOW RATE
Vs DIFFERENTIAL PRESSURE.
THEORY:
An orifice plate is a device used for measuring the flow rate. Either a volumetric or
mass flow rate may be determined, depending on the calculation associated with the orifice
plate. It uses the Bernoulli's principle which states that there is a relationship between the
pressure of the fluid and the velocity of the fluid. When the velocity increases, the pressure
decreases and vice versa.
There are four types of orifice plates which are listed below:
(1) Concentric.
(2) Segmental.
(3) Eccentric.
(4) Quadrant Edge.
(1) Concentric orifice plate:
The concentric orifice plate is the most common of the three types.it is usually
made of stainless steel and its thickness varies from 3.175 to 12.70 mm, depending on
pipe line size and flow velocity. It has a circular hole in the middle. 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. 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.
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(2) Eccentric orifice plate:
It is similar to the concentric plate except for the offset hole which is bored
tangential to the circle.concentric with the pipe and of a diameter equal to 98% of that
of the pipe. Use full for measuring containing solids, oil containing water and wet
steam. Eccentric plates can use either flange or vena contracta taps, but the tap must
be at 180º or 90º to the eccentric opening.
(3) Segmental orifice plate :
Segmental and eccentric orifice plates are functionally identical to the
concentric orifice. The circular section of the segmental orifice is concentric with the
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.
(4) Quadrant edge orifice plate :
This type of orifice plate is used for flows such as heavy crudes, syrups and
viscous flows. It is constructed in such a way that the edge is rounded to form a
quarter-circle. It may be used when the line Reynolds numbers range from 100,000 or
above, down to 3,000 to 5,000 with a coefficient accuracy of approximately 0.5%.
WORKING:
From this below figure, An orifice is placed in a pipe filled with fluid. The pressure of
the fluid is measured at two different points: 1) just upstream of the orifice and, 2) close to
the contraction of the fluid (vena contracta). The difference in these two pressures is known
as differential pressure. The differential pressure across an obstruction orifice in a pipe of
fluid is proportional to the square of the velocity of the fluid.
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Many factors associated with the pipe, orifice and fluid affect the measurement.
Satisfactory measurement requires steady-state, homogeneous, turbulent flowing fluids.
Other properties which affect the measurement include: the ratio of pipe diameter to orifice
diameter and the density, temperature, compressibility and viscosity of the fluid.
ADVANTAGES:
Orifice are small plates and easy to install/remove.
Offer very little pressure drop of which 60 to 65 % is recovered.
Easy maintenance.
Available in many materials.
DISADVANTAGES:
They cause relatively high permanent pressure loss.
hey have square root characteristics.
Their accuracy is dependent on care during installation.
They have changing characteristics because of erosion , corrosion and scalling.
CONCLUSION:
QUESTIONS:
1. Which parameter can be measured for orifice ?
2. List out the types of orifice plate.
3. Which factors are affected to the fluid flow rate ?
4. How orifice plate can be worked ?
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PRACTICAL: 8
AIM: TEST & PLOT CHARACTERISTICS OF NOZZLE , FLOW RATE
Vs PRESSURE.
THEORY:
Nozzle is a device designed to control the direction or characteristics of a fluid flow
(especially to increase velocity) as it exits an enclosed chamber or pipe via an orifice.
A nozzle is often a pipe or tube of varying cross sectional area, and it can be used to
direct or modify the flow of a fluid (liquid or gas). Nozzles are frequently used to control the
rate of flow, speed, direction, mass, shape, and/or the pressure of the stream that emerges
from them.
BASIC PRINCIPLE:
When a flow nozzle is placed in a pipe carrying whose rate of flow is to be measured,
the flow nozzle causes a pressure drop which varies with the flow rate. This pressure drop is
measured using a differential pressure sensor and when calibrated this pressure becomes a
measure of flow rate.
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The main parts of flow nozzle arrangement used to measure flow rate are as follows:
A flow nozzle which is held between flanges of pipe carrying the fluid whose flow
rate is being measured.
The flow nozzle’s area is minimum at its throat. Openings are provided at two places
1 and 2 for attaching a differential pressure sensor (u-tube manometer, differential
pressure gauge etc.,) as show in above the diagram.
OPERATION OF THE FLOW NOZZLES:
The fluid whose flow rate is to be measured enters the nozzle smoothly to the section
called throat where the area is minimum.
Before entering the nozzle, the fluid pressure in the pipe is p1. As the fluid enters the
nozzle, the fluid converges and due to this its pressure keeps on reducing until it
reaches the minimum cross section area called throat.
This minimum pressure p2 at the throat of the nozzle is maintained in the fluid for a
small length after being discharged in the downstream also. The differential pressure
sensor attached between points 1 and 2 records the pressure difference (p1-p2)
between these two points which becomes an indication of the flow rate of the fluid
through the pipe when calibrated.
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APPLICATIONS OF FLOW NOZZLE:
It is used to measure flow rates of the liquid discharged into the atmosphere.
It is usually used in situation where suspended solids have the property of settling.
Is widely used for high pressure and temperature steam flows.
ADVANTAGES OF FLOW NOZZLE:
Installation is easy and is cheaper when compared to venturi meter.
It is very compacts
High coefficient of discharge.
LIMITATIONS:
Pressure recovery is low.
Maintenance is high
Installation is difficult when compared to orifice flow meter.
CONCLUSION:
QUESTIONS:
1. What is nozzle ?
2. What is the basic principle of nozzle ?
3. Which pressure can be measured in flow nozzle ?
4. Explain the operation of flow nozzle with neat scatch.
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PRACTICAL: 9
AIM: TEST & PLOT CHARACTERISTICS OF VENTURI , FLOW
RATE Vs PRESSURE.
THEORY:
A venturi tube is used where permanent pressure loss is of prime importance, and where
maximum accuracy is desired in the measurement of high viscous fluids. The venture tubes are
usually made of cast iron or steel , and are built in several forms such as
(i) Long-form or classic venture tube
(ii) Short-form where the outlet cone is shorted
(iii) An eccentric form to minimize the build up of heavy materials, and
(iv) Rectangular form used in air duct work
VENTURI TUBE:
It consists of (i) a straight inlet section of the same diameter as the pipe in which the high
pressure tape is located,, (ii) a converging conical inlet section in which the cross-section of the
stream decreases and the velocity head decreases and the velocity head decreases of pressure head,
(iii) a cylindrical throat which provides for the low pressure tape location of the decrease pressure in
an area where flow velocity is neither increasing nor decreasing, and (iv) a diverging recovery cone in
which velocity decreases and the decreased velocity head is recovered as pressure head, as shown in
figure. The pressure taps are located one-quarter to one-half pipe diameter up-stream of the inlet cone
and at the middle of the throat section.
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WORKING:
A venturi tube is a tube or pipe which employs a temporary restriction or narrowing in its
length to reduce the pressure and increase the velocity of a fluid or gas passing through it. This
phenomenon of simultaneous pressure reduction and velocity increase is known as the venturi effect
and has a number of uses such the measurement of airflow and pumping or atomizing of a secondary
fluid. There are several types of venturi tube profiles in general use with a gradual change in profile
being the most effective.
The laws of physics dictate that a fluid or gas flowing in a tube will accelerate if that flow is
constricted. When this occurs, the pressure of the fluid in the constricted area must decrease to
conserve energy. The constriction in a tube is known as a venturi and the simultaneous increase in
flow velocity and decrease in pressure as the venturi effect. The pressure change characteristics of this
phenomenon are used to perform tasks such as the measurement of air and fuel flow in aircraft
systems and the calculation of differential pressure in meteorology.
A flow of air through a venturi meter, showing the columns connected in a U-shape (a
manometer) and partially filled with water. The meter is "read" as a differential pressure head in cm or
inches of water.
Referring to the above figure, using Bernoulli's equation in the special case of incompressible
flows (such as the flow of water or other liquid, or low speed flow of gas), the theoretical pressure
drop at the constriction is given by:
Where is the density of the fluid, is the (slower) fluid velocity where the pipe is wider,
is the (faster) fluid velocity where the pipe is narrower (as seen in the figure). This assumes the
flowing fluid (or other substance) is not significantly compressible - even though pressure varies, the
density is assumed to remain approximately constant.
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ADVANTAGES:
(i) It causes low permanent pressure loss.
(ii) It is widely used for high flow rates.
(iii) It is available in very large pipe sizes.
(iv) It has well-known characteristics.
(v) It is more accurate over wide flow ranges than orifice plates or nozzles.
(vi) It can be used at low and high beta radios.
DISADVANTAGES:
(i) Its cost is high.
(ii) It is generally not useful below 76.2 mm pipe size.
(iii) It is more difficult to inspect due to its construction.
CONCLUSION:
QUESTIONS:
1. Which materials are used to made venturi tube ?
2. Write down the types of venturi tube ?
3. Which pressure can be measured in venture tube ?
4. How to work venturi tube ?
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PRACTICAL: 10
AIM: TEST & PLOT CHARACTERISTICS OF PITOT TUBE , FLOW
RATE Vs PRESSURE.
THEORY:
A Pitot tube is a pressure measurement instrument used to measure fluid flow
velocity. Pitot tubes have the potential to measure two pressures at the same time i.e. Impact
(dynamic) and static. “The static pressure is the operating pressure in the pipe, duct, or the
environment, upstream to the Pitot tube. It is measured at right angles to the flow direction,
preferably in a low turbulence location". In a Pitot-static tube, the kinetic energy of the
flowing fluid is transformed into potential energy for measurement of fluid flow velocity.
The principle is based on the Bernoulli Equation where each term can be interpreted as a
form of pressure
P + 1/2 ρ v2 + γ h = constant along a streamline
Wherep = static pressure (relative to the moving fluid) (Pa)
Ρ = density (kg/m3) , γ = specific weight (kg/m3)
V = flow velocity (m/s) , g = acceleration of gravity (m/s2)
H = elevation height (m)
Each term of this equation has the dimension force per unit area - N/m2 or in imperial units
psi, lb/ft2.
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WORKING:
The liquid flows up the tube and when equilibrium is attained, the liquid reaches a
height above the free surface of the water stream.
Since the static pressure, under this situation, is equal to the hydrostatic pressure due
to its depth below the free surface, the difference in level between the liquid in the
glass tube and the free surface becomes the measure of dynamic pressure.
Therefore, we can write, neglecting friction,
Where p0, p and V are the stagnation pressure, static pressure and velocity
respectively at point A (Fig. 2a).
Such a tube is known as a Pitot tube and provides one of the most accurate means of
measuring the fluid velocity.
Figure 10: Simple Pitot Tube (a) Tube for measuring the Stagnation Pressure
(b) Static and Stagnation tubes together
For an open stream of liquid with a free surface, this single tube is suffcient to
determine the velocity. But for a fluid flowing through a closed duct, the Pitot tube
measures only the stagnation pressure and so the static pressure must be measured
separately.
Measurement of static pressure in this case is made at the boundary of the wall
(Fig.10). The axis of the tube measuring the static pressure must be perpendicular to
the boundary and free from burrs, so that the boundary is smooth and hence the
streamlines adjacent to it are not curved. This is done to sense the static pressure only
without any part of the dynamic pressure.
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A Pitot tube is also inserted as shown (Fig.10) to sense the stagnation pressure. The
ends of the Pitot tube, measuring the stagnation pressure, and the piezoelectric tube,
measuring the static pressure, may be connected to a suitable differential manometer
for the determination of flow velocity and hence the flow rate.
Air Flow - Velocity and Dynamic Head Chart:
Water Flow - Velocity and Dynamic Head Chart:
FEATURES:
A Pitot tube can be employed either as a permanently fixed flow sensor or as a
portable monitoring device which supplies data on a periodic basis.
Natural frequency resonant vibrations can result in the failure of a pitot tube.
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APPLICATION:
Air flow in pipes, ducts, and stacks, and
Liquid flow in pipes, weirs, and open channels
ADVANTAGES OF PITOTE TUBE:
Cost effective measurement
No moving parts
Simple to use and install
Low pressure drop
DISADVANTAGES OF PITOTE TUBE:
Poor accuracy
Unsuitable for dirty or stricy fluids
Sensitive to upstream disturbances
CONCLUSION:
QUESTIONS:
1. What is pitot tube ?
2. What is static pressure ?
3. What is the relation between flow rate and velocity in pitote tube ?
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PRACTICAL: 11
AIM: TEST & PLOT CHARACTERISTICS OF LDR , LUMINANCE Vs
RESISTANCE IN Ohms.
THEORY:
The general purpose photoconductive cell is also known as LDR – light dependent
resistor. It is a type of semiconductor and its conductivity changes with proportional change
in the intensity of light.
PRINCIPLE:
The complete principle of an LDR is as follows. In a semiconductor an energy gap
exists between conduction electrons and valence electrons. As an LDR is also known as
semiconductor photo-conductive transducer, when light is incident on it, a photon is absorbed
and thereby it excites an electron from valence band into conduction band. Due to such new
electrons coming up in conduction band area, the electrical resistance of the device decreases.
Thus the LDR or photo-conductive transducer has the resistance which is the inverse function
of radiation intensity.
Where, λ0 = threshold wavelength, in meters
e = charge on one electron, in Coulombs
Ew = work function of the metal used, in eV.
Here we must note that any radiation with wavelength greater than the value obtained
in above equation cannot produce any change in the resistance of this device.
CONSTRUCTION OF A LIGHT DEPENDENT RESISTOR:
There two common types of materials used to manufacture the photoconductive cells.
They are Cadmium Sulphide (CdS) and Cadmium Selenide (CdSe).
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The band gap energy of Cadmium Sulphide is 2.42eV and for Cadmium Selenide it is
1.74eV. Due to such large energy gaps, both the materials have extremely high resistivity at
room temperature. Hence, these materials are widely used in LDR for practical purpose.
A long, thin and narrow strip of CdS is fixed on the surface of ceramic substrate in the
form of zigzag wire as shown in following figure. This construction gives minimum area and
maximum length. Then the structure is enclosed in round metallic or plastic case and two
terminals (made up of either tin or indium) are taken out for external connections. The
structure is covered with glass sheet to protect it from moisture and dust and allows only light
to fall on it.
This is an example of a light sensor circuit :
When the light level is low the resistance of the LDR is high. This prevents current
from flowing to the base of the transistors. Consequently the LED does not light.
However, when light shines onto the LDR its resistance falls and current flows into
the base of the first transistor and then the second transistor.
The LED lights.The preset resistor can be turned up or down to increase or decrease
resistance, in this way it can make the circuit more or less sensitive.
CHARACTERISTICS OF PHOTOCONDUCTIVE CELLS:
Photoconductor Time Constant Spectral Band
Cadmium Sulphide CdS 100 milli sec 0.47 to 0.72 um
Cadmium Selenide CdSe 10 milli sec 0.6 to 0.77 um
Lead Sulphide PbS 410 micro sec 1 to 3.2 um
Lead Selenide PbSe 10.2 micro sec 1.52 o 4.2 um
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APPLICATIONS:
It is used in burglar alarm to give alarming sound when a burglar invades sensitive
premises.
It is used in street light control to switch on the lights during dusk (evening) and
switch off during dawn (morning) automatically.
It is used in Lux meter to measure intensity of light in Lux.
It is used in photo sensitive relay circuit.
CONCLUSION:
QUESTIONS:
1. What is the meaning of LDR ?
2. Which materials are used in photoconductive cell ?
3. What is the band gap energy of Cds and Cdse ?
4. What is the characteristics of LDR ?
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PRACTICAL: 12
AIM: TEST & PLOT CHARACTERISTICS OF PHOTODIODE ,
LUMINANCE Vs CURRENT.
THEORY:
A photodiode is a type of photodetector capable of converting light into either current
or voltage, depending upon the mode of operation. Many diodes designed for use specifically
as a photodiode use a PIN junction rather than a p-n junction ,to increase the speed of
response. A photodiode is designed to operate in reverse bias.
PHOTODIODE:
The mechanism of the photodiode is like that of a solar cell. Their response time is
fast, on the order of nanoseconds. As light detectors, they are reverse biased - the reverse
current is linearly proportional to the illuminance striking the diode. They are not as sensitive
as a phototransistor , but their linearity can make them useful in simple light meters.
Absorption of photons in a photodiode with a suitable bandgap energy causes an
electron to move from the valence band to the conduction band. Absorption most likely in or
near the depletion region. Generated carriers are swept out of the device to form an current in
an external circuit.
TWO MODES OF OPERATION:
Photovoltaic mode:
Photovoltaic mode, generated carriers swept out under the influence of the depletion field.
Photoconductive mode:
Photoconductive mode, generated carriers swept out under the influence of an external field.
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WORKING:
Photodiode is a pn junction diode operated in reverse bias. It has a transparent
window at the depletion region. When light of suitable energy greater than the energy gap of
the semiconductor is incident on the junction, then electron hole pairs are created due to
absorption of photons. Due to electric field of the junction, electrons and holes are separated
before they can recombine. The direction of the electric field is such that electrons reach the
n-side and holes to the p-side. This gives rise to an emf and subsequently a photocurrent.
CHARACTERISTICS OF PHOTODIODES:
Photodiodes are very small, sensitive and require little power.
Different semiconductors are sensitive to different wavelengths of light.
The reverse current through a photodiode varies linearly with illuminance once you
are significantly above the dark current region.
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Materials commonly used to produce photodiodes include:
Material
Electromagnetic spectrum
wavelength range (nm)
Silicon 190–1100
Germanium 400–1700
Indium gallium arsenide 800–2600
Lead(II) sulfide <1000–3500
APPLICATIONS OF PHOTO DIODES:
P-N photodiodes are used in similar applications to other photodetectors, such
as photoconductors, charge-coupled devices, and photomultiplier tubes. They may be
used to generate an output which is dependent upon the illumination (analog; for
measurement and the like), or to change the state of circuitry (digital; either for
control and switching, or digital signal processing).
Photodiodes are often used for accurate measurement of light intensity in science and
industry. They generally have a more linear response than photoconductors.
PIN diodes are much faster and more sensitive than p-n junction diodes, and hence are
often used for optical communications and in lighting regulation.
CONCLUSION:
QUESTIONS:
1. What is photodiode ?
2. Define photovoltaic and photoconductive mode ?
3. What is the relation between current and light intensity ?
4. What is the wave length range of silicon photodiode ?
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PRACTICAL:13
AIM: TEST & PLOT CHARACTERISTICS OF PHOTOTRANSISTOR ,
LUMINANCE Vs CURRENT.
THEORY:
A transistor is like a valve that regulates the amount of electric current that passes
through two of its three terminals. The third terminal controls just how much current passes
through the other two. Depending on the type of transistor, the current flow can be controlled
by voltage, current, or in the case of the phototransistor, by light.
PHOTOTRANSISTOR:
The drawing below shows the schematic and part drawing of the phototransistor in
your Robotics Shield Kit. The brightness of the light shining on the phototransistor’s
base (B) terminal determines how much current it will allow to pass into its collector (C)
terminal, and out through its emitter (E) terminal. Brighter light results in more current , less-
bright light results in less current.
The phototransistor looks a little bit like an LED. The two devices do have two
similarities. First, if you connect the phototransistor in the circuit backwards, it won’t work
right. Second, it also has two different length pins and a flat spot on its plastic case for
identifying its terminals. The longer of the two pins indicates the phototransistor’s collector
terminal. The shorter pin indicates the emitter, and it connects closer to a flat spot on the
phototransistor’s clear plastic case.
PHOTOTRANSISTOR CIRCUIT CONFIGURATIONS:
The phototransistor can be used in a variety of different circuit configurations. Like
more conventional transistors, the phototransistor can be used in common emitter and
common collector circuits. Common base circuits are not normally used because the base
connection is often left floating.
The choice of common emitter or common collector phototransistor circuit
configuration depends upon the requirements for the circuit. The two phototransistor circuit
configurations have slightly different operating characteristics and these may determine the
circuit used.
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Common emitter phototransistor circuit:
The common emitter phototransistor circuit configuration is possibly the most widely used,
like its more conventional straight transistor circuit. The collector is taken to the supply
voltage via a collector load resistor, and the output is taken from the collector connection on
the phototransistor. The circuit generates an output that moves from a high voltage state to a
low voltage state when light is detected.
The circuit actually acts as an amplifier. The current generated by the light affects the base
region. This is amplified by the current gain of the transistor in the normal way.
Common collector phototransistor circuit:
The common collector, or emitter follower phototransistor circuit configuration has
effectively the same topology as the normal common emitter transistor circuit - the emitter is
taken to ground via a load resistor, and the output for the circuit being taken from the emitter
connection of the device.
The circuit generates an output that moves from the low state to a high state when light is
detected.
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PHOTOTRANSISTOR CIRCUIT OPERATION:
The phototransistor circuits can be used on one of two basic modes of operation. They
are called active or linear mode and a switch mode.
Operation in the "linear" or active mode provides a response that is very broadly
proportional to the light stimulus. In reality the phototransistor does not give a particularly
linear output to the input stimulus and it is for this reason that this mode of operation is more
correctly termed the active mode.
The operation of the phototransistor circuit in the switch mode is more widely used in
view of the non-linear response of the phototransistor to light. When there is little or no light,
virtually no current will flow in the transistor, and it can be said to be in the "off" state.
However as the level of light increases, current starts to flow. Eventually a point is reached
where the phototransistor becomes saturated and the level of current cannot increase. In this
situation the phototransistor is said to be saturated. The switch mode, therefore has two
levels: - "on" and "off" as in a digital or logic system. This type of phototransistor mode is
useful for detecting objects, sending data or reading encoders, etc.
With most circuits not using the base connection (even if it is available), the only way
to change the mode of operation of the circuit is to change the value of the load resistor. This
is set by estimating the maximum current anticipated from the light levels encountered.
Using this assumption, the following equations can be used:
Active mode: VCC > RL x Ic
Switch mode: VCC < RL x Ic
Where
RL = load resistor (i.e. Rc or Re in the diagrams above).
IC = maximum anticipated current.
VCC = supply voltage.
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WHY USE PHOTOTRANSISTORS?
More sensitive than photodiodes of comparably sized area.
Available with gains form 100 to over 1500.
Moderately fast response times.
Available in a wide range of packages.
Usable with almost any visible or near infrared light source such as IREDs, lasers,
sunlight, and etc.
Same general electrical characteristics as familiar signal transistors.
PHOTOTRANSISTOR APPLICATIONS:
Computer/Business Equipment :
Write protect control – floppy driver
Margin controls – printers.
Industrial :
LED light source – light pens.
Security systems.
Consumer :
Coin counters.
Lottery card readers.
CONCLUSION:
QUESTIONS:
1. Define the terminal of phototransistor ?
2. Explain the phototransistor configuration ?
3. Why common base configuration is not more used of them ?
4. What is the response of phototransistor in active mode ?
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PRACTICAL: 14
AIM: TEST & PLOT CHARACTERISTICS OF PIEZO-ELECTRIC
CRYSTAL , VIBRATION Vs Mv.
THEORY:
A piezoelectric crystal transducer/sensor is an active sensor and it does not need the
help of an external power as it is self-generating. Piezoelectric crystals are one of many
small scale energy sources. Whenever piezoelectric crystals are mechanically deformed or
subject to vibration they generate a small voltage, commonly know as piezoelectricity. This
form of renewable energy is not ideally suited to an industrial situation.
The ability of certain crystals to generate Piezoelectricity in response to applied
mechanical stress is reversible in that piezoelectric crystals, when subjected to an externally
applied voltage, can change shape by a small amount. This deformation, though only
nanometers, has useful applications such as the production and detection of sound.
A piezoelectric disk generates a voltage when deformed (change in shape is greatly
exaggerated).
Piezoelectric Quartz Crystal:
A quartz crystal is a piezoelectric material that can generate a voltage proportional to the
stress applied upon it. For the application, a natural quartz crystal has to be cut in the shape of
a thin plate of rectangular or oval shape of uniform thickness. Each crystal has three sets of
axes – Optical axes, three electrical axes OX1, OX2, and OX3 with 120 degree with each
other, and three mechanical axes OY1,OY2 and OY3 also at 120 degree with each other. The
mechanical axes will be at right angles to the electrical axes. Some of the parameters that
decide the nature of the crystal for the application are
Angle at which the wafer is cut from natural quartz crystal
Plate thickness
Dimension of the plate
Means of mounting
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Piezoelectric Effect:
The X-Y axis of a piezoelectric crystal and its cutting technique is shown in the figure
below.
If an electric stress is applied in the directions of an electric axis (X-axis), a
mechanical strain is produced in the direction of the Y-axis, which is perpendicular to the
relevant X-axis. Similarly, if a mechanical strain is given along the Y-axis, electrical charges
will be produced on the faces of the crystal, perpendicular to the X-axis which is at right
angles to the Y-axis.
Some of the materials that inherit piezo-electric effect are quartz crystal, Rochelle
salt, barium titanate, and so on. The main advantages of these crystals are that they have high
mechanical and thermal state capability, capability of withstanding high order of strain, low
leakage, and good frequency response, and so on.
Piezoelectric Transducer:
The main principle of a piezoelectric transducer is that a force, when applied on the
quartz crystal, produces electric charges on the crystal surface. The charge thus produced can
be called as piezoelectricity. Piezo electricity can be defined as the electrical polarization
produced by mechanical strain on certain class of crystals. The rate of charge produced will
be proportional to the rate of change of force applied as input. As the charge produced is very
small, a charge amplifier is needed so as to produce an output voltage big enough to be
measured. The device is also known to be mechanically stiff. For example, if a force of 15
kiloN is given to the transducer, it may only deflect to a maximum of 0.002mm. But the
output response may be as high as 100KiloHz.This proves that the device is best applicable
for dynamic measurement.
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The figure shows a conventional piezoelectric transducer with a piezoelectric crystal
inserted between a solid base and the force summing member. If a force is applied on the
pressure port, the same force will fall on the force summing member. Thus a potential
difference will be generated on the crystal due to its property. The voltage produced will be
proportional to the magnitude of the applied force.
Piezoelectric Transducer can measure pressure in the same way a force or an
acceleration can be measured. For low pressure measurement, possible vibration of the
amount should be compensated for. The pressure measuring quartz disc stack faces the
pressure through a diaphragm and on the other side of this stack, the compensating mass
followed by a compensating quartz.
APPLICATIONS:
1. Due to its excellent frequency response, it is normally used as an accelerometer,
where the output is in the order of (1-30) mV per gravity of acceleration.
2. The device is usually designed for use as a pre-tensional bolt so that both tensional
and compression force measurements can be made.
3. Can be used for measuring force, pressure and displacement in terms of voltage.
ADVANTAGES:
1. Very high frequency response.
2. Self generating, so no need of external source.
3. Simple to use as they have small dimensions and large measuring range.
4. Barium titanate and quartz can be made in any desired shape and form. It also has a
large dielectric constant. The crystal axis is selectable by orienting the direction of
orientation.
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DISADVANTAGES:
1. It is not suitable for measurement in static condition.
2. Since the device operates with the small electric charge, they need high impedance
cable for electrical interface.
3. The output may vary according to the temperature variation of the crystal.
4. The relative humidity rises above 85% or falls below 35%, its output will be affected.
If so, it has to be coated with wax or polymer material.
CONCLUSION:
QUESTIONS:
1. What is the basic principle of piezo-electric crystal ?
2. Which materials are used to made piezo-electric crystal ?
3. What is the use of piezo-electric transduser ?
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PRACTICAL: 15
AIM: TEST & PLOT CHARACTERISTICS OF STRAIN GAUGE ,
STRAIN Vs RESISTANCE.
THEORY:
Strain Gauge is a passive transducer that converts a mechanical elongation or
displacement produced due to a force into its corresponding change in resistance R,
inductance L, or capacitance C. A strain gauge is basically used to measure the strain in a
work piece. If a metal piece is subjected to a tensile stress, the metal length will increase and
thus will increase the electrical resistance of the material. Similarly, if the metal is subjected
to compressive stress, the length will decrease, but the breadth will increase. This will also
change the electrical resistance of the conductor. If both these stresses are limited within its
elastic limit, the metal conductor can be used to measure the amount of force given to
produce the stress, through its change in resistance.
GAUGE FACTOR:
Let us consider a long straight metallic wire of length l circular cross section with
diameter d (fig. 5). When this wire is subjected to a force applied at the two ends, a strain will
be generated and as a result, the dimension will change (l changing to l + Δl ,d changing to d
+ ∆d and A changing to A + ΔA). For the time being, we are considering that all the changes
are in positive direction. Now the resistance of the wire:
TYPES OF STRAIN GAUGE:
(1) Bonded Strain Gauge
(2) Unbonded Strain Gauge
(1) Bonded Strain Gauge:
A bonded strain gauge will be either a wire type or a foil type as shown in the figure
below. It is connected to a paper or a thick plastic film support. The measuring leads are
soldered or welded to the gauge wire. The bonded strain gauge with the paper backing is
connected to the elastic member whose strain is to be measured.
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(2) Unbonded strain Gauge:
As shown in the figure below, an unbounded strain gauge has a resistance wire
stretched between two frames. The rigid pins of the two frames are insulated. When the wire
is stretched due to an applied force, there occurs a relative motion between the two frames
and thus a strain is produced, causing a change in resistance value. This change of resistance
value will be equal to the strain input.
CONSTRUCTION AND WORKING:
Below figure 9.21(b) shows a bridge circuit with four strain gauges, Rsg1, Rsg2, Rsg3
and Rsg4. Two strain gauges, Rsg1 and Rsg4 are mounted so that increasing pressure
increasing their resistance. Strain gauges Rsg2 and Rsg3, are mounted so that increasing
pressure decreases their resistance. A change in temperature affects all the four strain gauges
in the same way, resulting in no change in the pressure indication.
Figure (9.21b) Strain gauge bridge circuit
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At balance, when there is no pressure, no current flows through the galvanometer G,
and hence there will be no deflection in the galvanometer. As soon as the pressure is applied,
the strain gauge stretches or compresses accordingly and the bridge circuit is unbalanced due
to the change in resistance of the strain gauges. Thus, a current flows in the galvanometer.
These changes affect the output of the bridge circuit, which indicates a change in measured
pressure. Now, this change voltage may be calibrated for the pressure change.
ADVANTAGES:
They are small and easy to install.
They have good accuracy
Good stability
High output signal strength
High overrange capacity
No moving parts
Possess good shock and vibration characteristics
Possess fast speed of response
DISADVANTAGES:
Their cost is moderate high
Electrical readout is necessary in these transducers
They require constant voltage supply.
They require temperature compensation due to problem presented by temperature
variations
CONCLUSION:
QUESTIONS:
1. What is strain gauge ?
2. Write down the gauge factor equation.
3. What is the difference between bonded and unbounded strain gauge ?
4. Explain the bridge circuit of strain gauge.
Industrial Transducers
Urvish Soni Government Polytechnic Ahmedabad
51
PRACTICAL: 16
AIM: TEST & PLOT CHARACTERISTICS OF BIMETALLIC
THERMOMETER , TEMPERATURE Vs DISPLACEMENT.
THEORY:
Bimetallic thermometers are made up of bimetallic strips formed by joining two
different metals having different thermal expansion coefficients. Basically, bimetallic strip is
a mechanical element which can sense temperature and transform it into a mechanical
displacement. This mechanical action from the bimetallic strip can be used to activate a
switching mechanism for getting electronic output.
Basic principle:
1. A metal tends to undergo a volumetric dimensional change (expansion/contraction),
according to the change in temperature.
2. Different metals have different co-efficient of temperatures. The rate of volumetric
change depends on this co-efficient of temperature.
Bimetallic strip:
The device consists of a bimetallic strip of two different metals and they are bonded
together to form a spiral or a twisted helix. Both these metals are joined together at one end
by either welding or riveting. It is bonded so strong that there will not be any relative motion
between the two. The image of a bimetallic strip is shown below.
WORKING:
A change in temperature causes the free end of the strip to expand or contract due to
the different co-efficients of expansion of the two metals. This movement is linear to the
change in temperature and the deflection of the free end can be read out by attaching a
pointer to it. This reading will indicate the value of temperature. Bimetallic strips are
available in different forms like helix type, cantilever, spiral, and also flat type.
Industrial Transducers
Urvish Soni Government Polytechnic Ahmedabad
52
Spiral type bimetallic thermometer:
If the bimetallic element is wound in the form of a spiral, the spiral coil is tightened
with increase in temperature. As it coils, the countrepost rotates clockwise, and thus a pointer
attached to the post also moves on a calibrated temperature scale. Figure 16.2 shows a spiral
element with an attached pointer mounted in a housing with a scale. This type of temperature
indicator is often used in homes and offices for indicating ambient air temperature.
Fig. 16.2 thermometer with spiral bimetallic element
Helix type bimetallic thermometer:
The bimetal can also be used in the form of a helix to indicate temperature. T his type
of industrial bimetallic thermometer is shown in fig.16.3. It consists of a tightly wound
helical bimetallic strip located inside the stem of the thermometer with one end fastened
permanently to the outer casing. A strip is attached to a centrepost that extends from the stem
to the centre of an indicating dial. A pointer is attached to the centrepost. When the
temperature surrounding the whole stem changes, the bimetal expands and the helical Coil
winds and unwinds which rotates the centrepost. This causes the pointer to move on the dial
to indicate the measured temperature. A thermal well is used with the thermometer for
protection against correction and breakage. The material of the thermal well may be brass,
steel, stainless steel or other alloys, depending upon the requirements of the installation.
Industrial Transducers
Urvish Soni Government Polytechnic Ahmedabad
53
Fig.10.5 thermometer with helical bimetallic element
Bimetallic thermometers are inexpensive, relatively rugged, and easy to read. They
are reasonably accurate if handled carefully. They are available for the temperature range
from -103 to 1004 oF (-75 to 540 oC).
Fig: Temperature Vs Expansion For All Metals
ADVANTAGES OF BIMETALLIC THERMOMETER:
Their cost is low.
They are tough, and cannot easily be broken.
They are easily installed and maintained.
They have good accuracy relative to cost.
They have fairly wide temperature range.
Industrial Transducers
Urvish Soni Government Polytechnic Ahmedabad
54
DISADVANTAGES OF BIMETALLIC THERMOMETER:
They are limited to local mounting.
Only indicating type is available.
There is always a possibility of calibration change due to rough handling.
Their accuracy is not as high as glass stem thermometers.
CONCLUSION:
QUESTIONS:
1. What is the basic principle of bimetallic thermometer ?
2. What are the types of bimetallic thermometer ?
3. What is the temperature range of bimetallic thermometer ?
4. Explain the spiral and helix type bimetallic thermometer ?
5. What is the relation between temperature and displacement in bimetallic thermometer
?
