Question

Strain gauge is used to measure the pressure difference of 3 (three) load has a value of 120 Ohms. In equilibrium situations or when not stretching a strain that has 120 Ohm resistance. If the strain gauge factor is 2 and e is 0.005 and the pressure difference is 100 kPa, determine the gain of the operational amplifier to obtain a 1 (one) volt voltage gain (picture of the entire set of measurement systems).

Answer 1

Assuming that a 3kPa pressure difference is required to produce a 1 volt output, (question is not clear enough to say this for sure. Different assumptions will require a different gain on the op amp)

The Strain Gauge will be placed in circuit with 3 other 120 ohm resistors in a Wheatstone bridge. The output of the wheatstone bridge will be connected to the non-inverting input of an inverting amplifier. The inverting input of that same amplifier will be provided with the output of a similar Wheatstone bridge which is always in a balanced condition.

A 3 kPa pressure difference on the strain gauge will produce a 3/100*0.005 = 1.5E-4 strain. As gauge factor is 2, this will produce a 2*1.5E-4* 120 ohm = .036 ohm change in the gauge's resistance. The resulting imbalance will produce a (1/2678) volt output in a 5V Wheatstone bridge.

Enclosed is the circuit diagram. This circuit would produce 1 V output for 3 kPa input to the strain guage.

Thus to produce a 1 volt output from the op amp, a gain of 2678 is required.