Powerpoint presentation on variable resistance transducers eg:- sliding contact devices,thermocouples,thermistors

1. MECHANICAL VIBRATIONS
VARIABLE RESISTANCE TRANSDUCERS
BY,
GAGAN.N

2. VARIABLE RESISTANCE TRANSDUCERS
 These are one of the most commonly used types of
transducers, also called as resistive transducers or
resistive sensors.
 It used for measuring various physical quantities like
temperature, pressure, displacement, force, vibrations
etc.
 These transducers are usually used as the secondary
transducers, where the output from the primary mechanical
transducer acts as the input for the variable resistance
transducer.
 The output obtained from it is calibrated against the input
quantity and it directly gives the value of the input.
9/30/2019
Dept. of MECHANICAL ENGINEERING
MECHANICAL VIBRATIONS
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3. PRINCIPLE OF WORKING
 The variable resistance transducer elements work on the
principle that the resistance of the conductor is directly
proportional to the length of the conductor and inversely
proportional to the area of the conductor.
 Thus if L is the length of the conductor (in m) and A is its
area (in m square), R its resistance (in ohms) is given by:
R = ρL/A
 where ρ is called as resistivity of the material and it is
constant for the materials and is measured in ohm-m
 The resistance of some materials also changes with the
change in their temperature. This principle is primarily used
for the measurement of temperature.
9/30/2019
Dept. of MECHANICAL ENGINEERING
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4. SOME EXAMPLES
1) Sliding contact devices: In this type there is a long conductor
whose effective length is variable.
 One end of the conductor is fixed, while the position of the other end
is decided by the slider or the brush that can move along the whole
length of the conductor.
 The slider is connected to the body whose displacement is to be
measured.
 When the body moves the slider also moves along the conductor so
its effective length changes, due to which it resistance also changes.
 The effective resistance is measured as the resistance between the
fixed position of the conductor and the position of the sliding contact.
 The value of the resistance is calibrated against the input quantity,
whose value can be measured directly.
 One of most popular sliding contact type of variable resistance
transducer is the potentiometer.
 These devices can be used to measured linear as well as angular
displacement.
9/30/2019
Dept. of MECHANICAL ENGINEERING
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5. 2) Thermistors: Thermistors work on the principle that
resistance of some materials changes with the change in
their temperature.
 When the temperature of the material changes, its
resistance changes and it can be measured easily and
calibrated against the input quantity.
 The commonly used thermistors are made up of the
ceramic like semiconducting materials such as oxides of
manganese, nickel and cobalt.
 Thermistors can be used for the measurement of
temperature, as electric power sensing devices and also
as the controls for various processes.
9/30/2019
Dept. of MECHANICAL ENGINEERING
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6. 3) Thermocouple: The thermocouples work on the principle
of Seeback effect, Peltier effect and Thomson effect.
 The voltage output from the thermocouple changes as its
temperature changes or the temperature of the body in
whose contact it is changes.
 The voltage output is calibrated against the temperature of
the body that can be measured easily.
 Thermocouple is a very popular devise used for
measurement of temperature.
9/30/2019
Dept. of MECHANICAL ENGINEERING
MECHANICAL VIBRATIONS
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7. REFERENCES
Book: Mechanical Measurements by Thomas G.
Beckwith and N. Lewis Buck
https://www.brighthubengineering.com/hvac/53335-
variable-resistance-transducers/
www.wikipedia.com
9/30/2019
Dept. of MECHANICAL ENGINEERING
MECHANICAL VIBRATIONS
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8. 9/30/2019
Dept. of MECHANICAL ENGINEERING
MECHANICAL VIBRATIONS
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