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Transducers

Classification of Transducers

SELECTING A TRANSDUCER

Strain gauges

Gauge factor

Types of Strain Gauges

Bonded Metal Foil Strain Gauges

Semiconductor Strain Gauges

Rosettes

LVDT

Linear Variable Differential Transformer – LVDT Transducer

Applications of LVDT

Measurement of Temperature

RTD - resistance temperature detector

Thermistors

Thermistors for Measurement of Temperature

Thermocouples

Photo-conductive and photo-voltaic cells

Photoconductive Cells

Photovoltaic Cells

Gauge factor

Gauge factor is defined as the ratio of per unit change in resistance to per unit change
in length

The strain is usually expressed in terms of microstrain. 1 microstrain = 1 μm/m.
If the change in the value of resistivity of a material when strained is neglected, the
gauge factor is :
Gf = 1 + 2v
The common value for Poisson's ratio for wires is 0.3.This gives a gauge factor of 1.6
for wire wound strain gauges. Poisson's ratio for all metals is between 0 & 0.5.This
gives a value of 2
Example 1: A resistance wire strain gauge uses a soft iron wire of small diameter. The gauge
factor is + 4.2. Neglecting the piezoresistive effects, calculate the Poisson's ratio.
Solution. The gauge factor is given by Eqn., Gf = 1 + 2v = 4.2
Poisson's ratio = v = (4.2 -1)/2 = 1.6
Example 2: A compressive force is applied to a structural member. The strain is 5 microstrain.
Two separate strain gauges are attached to the structural member, one is a nickel wire
strain gauge having a gauge factor of -12.1 and the other is nichrome wire strain gauge
having a gauge factor of 2. Calculate the value of resistance of the gauges after they are
strained. The resistance of strain gauges before being strained is 120 .

Strain gauges are broadly used for two major types of applications and they are :
(i) experimental stress analysis of machines and structures, and (ii) construction of force,
torque, pressure, flow and acceleration transducers.