Here is a nifty scale for weighing fish. We mount strain gauges on a plastic tube as shown and then hang on the fish. The strain gauges have R = 150 W (unstrained) and the GF = 1.54.
(a) Which is the dummy gauge, A or B?
(b) Show how to wire the two gauges and two fixed 150 W resistors into the bridge circuit to provide temperature compensation, using the graphs given on the right. The plastic tube has an outer diameter of 3 cm and an inner diameter of 2.3 cm. The modulus of elasticity of the plastic is E = 2.45 x 108 N/m2. The weight of the fish in kilograms produces a force given by Newton's Law: F = mg, where g = 9.8 m/s2 and m is the mass in kilograms.
(c) What is the strain produced by a fish of 5 kg?
(d) How much resistance change will result from this strain? What will the bridge off-null voltage be for this strain?
(e) Find the gain and show the circuit of a differential amplifier so that a 5 kg fish will provide an output of 0.5 volts. Show how the bridge is connected to the amplifier inputs