Characteristics of strain gauge.

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Aim: Study the characteristics of strain gauge
Equipment required: Strain gauge module, The signal conditioning and processing unit, Weights (50 grams, 100 grams, 200 grams).
Theory:
The strain gauge module consists of a cantilever beam which is fixed on a rigid body. Four strain gauges are placed on the surface of the beam. Two strain gauges are placed over the beam and two are beneath the beam. A D9 connector connects the strain gauges with the signal conditioning unit. There is a hook which is attached to the cantilever beam, through which the load is applied to the beam. In signal conditioning circuit there is a instrumentation amplifier and a gain amplifier with some associated circuit which calibrates the load in terms of micro strain with the help of zero and gain adjustment pot. A four digit seven segment display is provided, that displays the amount of load in terms of micro strain.
The four strain gauges which are fixed with the cantilever beam makes four arm of a wheat stone bridge, making it a full bridge. The strain gauges which are fixed above the beam detects the longitudinal strain and which are fixed below the beam detects transverse strain. When the load is applied on the end of the beam, a downward force is exerted. It tends to change the dimension of the beam. This dimensional changes are sensed by strain gauges fixed with the beam. A strain occurred on the beam is calculated by the given formula. Theoretical Strain = $\frac{6FL}{b t^2 Y}$
Where,
F = Applied load in Kg.
L = Length of the beam in cm. = 20 cm
b = Breadth of the beam in cm. = 2 cm
t = Thickness of the beam in cm. = 0.16cm
Y = Young’s modules of the material to be used in Kg/cm2. = 200Gpa.
The resistances of the strain gauges are changed due to the dimensional changes in beam when the load is applied. Due to this resistance changes an unbalanced voltage is developed in the bridge output. This unbalanced voltage are further processed and calibrated to give the strain directly on the display.

Procedure:

1. Switch ON the module.
2. Set initial(No load on cantilever beam) output value/strain as 000 by adjusting offset or zero pot.
3. Then apply full load of 500 grams and adjust the gain pot so that the display should show 500(This end value used for multiplication factor ).
4. Now remove all the loads and apply 100 gram note down the strain displayed and measure the output voltage (mV).
5. Gradually increase the load 100gm in step on the beam and note down applied load and the bridge voltage (mV).
6. Tabulate the values of applied load, theoretical strain and the bridge voltage (mV).
7. Plot a graph between theoretical strain versus bridge voltage (mV).

Sl no	Applied load(gm)	Theoretical strain(µs)	Bridge Voltage (mV)
1.
2.
3.
4.