What is a Strain Gauge

A strain gauge is a passive transducer that is used to convert mechanical strain in to a corresponding electromotive force for measurement and control purposes. It consists of a two dimensional mesh of fine wiring whose resistance varies on the application of strain. This occurs due to the following two reasons:-

The strain applied varies the effective length of the mesh in the circuit. The applied strain varies the effective area of the mesh in the circuit.

In addition to that, an induced voltage is also produced in the gauge because of piezoelectric effect. In practical instrumentation, the strain gauge is used in one arm of a Wheatstone bridge. The other three arms consist of resistors whose values are determined in such a way that a null condition is achieved in the bridge i.e. the bridge is brought in to a balanced state.

The strain gauge is connected to a cantilever and the strain to be measured is applied on the free arm of that cantilever. From there, the strain gets transferred to the gauge and causes a change in the value of its resistance. This indeed disturbs the condition of balance in the bridge and the galvanometer connected between the opposite ends of the bridge shows a deflection. This deflection is already calibrated and thus the resultant electromagnetic force is measured, which indeed gives a measurement of the applied strain on the cantilever. A strain gauge is a passive transducer because it requires the presence of an external battery source to energize the bridge circuitry.

A big problem encountered in the use of strain gauges is the effect of the changes in ambient temperature on the effective resistance of the gauge. It is a well known fact that any shift in the temperature causes a change in electrical resistance. Thus such a change can affect the balance of the bridge and therefore is interpreted as output by the instrument, resulting in an error in measurement of strain. To counter this problem, a dummy gauge is used in the opposite arm of the bridge whose temperature-resistance response is same as that of the original gauge. Thus any kind of change in temperature causes an equal shift in the resistance of both the gauges present in opposite arms of the bridge and therefore the balance of the bridge is not disturbed.

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