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Why to calibrate strain gauges


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Hi,

I need to measure the strain in concrete beams. For this purpose I have purchased data acquisition system with a complementary 1/4 whetstone bridge circuit. The complementary circuit consists of 3 resistors of 120 ohm each. My strain gauge is 120 ohm and the gauge factor (k) is 2.11. The data acquisition system is modern and digital. Now, after connecting the strain gauge to the complementary circuit and exerts excitation (Ue) of 2.0 V (output) to the whetstone circuit and then loading the beam so that strain happens and the data acquisition system reads the voltage from the whetstone bridge (Ua)(input) that varies due to the strain in the strain gauge, my question is: Is it possible to use the formula:
strain=4 Ua/(k . Ue)
Why should I do calibration? and why the strain is not programmed to be calculated automatically to the above formula?
The data acquisition system requests the k, Ue and the whetstone bridge type (1/4, 1/2, or full) so that it has all the necessary information for calculating the strain but will not work without calibration data!

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