Transducers in Measurement Questions and Answers

Answer: A

The gauge is bonded first, then the bridge is nulled with no load so that the initial resistance is taken out, then the load is applied and the output read, and only then is the gauge factor used to turn that output into a strain.

Answer: B

Correct answer: 1 and 2 only are correct

Statements 1 and 2 are correct: the core does not touch the windings, so there is nothing to wear and the life is effectively unlimited, and at the null position the two secondaries cancel exactly. Statement 3 is wrong - the primary must be excited with alternating voltage, typically a few kilohertz, since a transformer cannot couple a steady current at all.

Answer: C

The first two are the fundamental facts that make the thermocouple usable and connectable. The output is tens of microvolts per degree, not volts, which is a difference of six orders of magnitude and the reason low-drift amplification is essential.

Answer: D

Correct answer: an LVDT

A strain gauge, a platinum resistance thermometer and a thermistor all report their quantity as a change of resistance, and all three are read with a bridge. An LVDT reports as an alternating voltage whose amplitude gives the magnitude of the displacement and whose phase gives its direction.

Answer: A

More excitation increases the output but also the self-heating, so the temperature error grows rather than shrinks. The other three all arrange for temperature effects to be common to arms in which they cancel, or to be small in the first place.

Answer: B

Pressure is converted first into a deflection and hence a strain, and the strain is then converted into a resistance change. Two conversions in series make the diaphragm the primary and the gauges the secondary element.

Answer: C

Correct answer: piezoelectric, its charge leaking away at low frequencies

A piezoelectric transducer produces charge rather than a steady voltage, and that charge drains away through the finite input resistance of the cable and amplifier. The arrangement therefore behaves as a high-pass filter, and the low frequency limit is set by the time constant of the crystal capacitance and that resistance - which is why a charge amplifier is used, and why a piezoelectric accelerometer cannot measure a constant acceleration at all.

Answer: D

The emf reports the difference between the junctions, so if the reference is warmer than the compensation assumes, the computed hot junction temperature comes out low by exactly that amount. Lead resistance hardly matters, since the measuring circuit draws almost no current.

Answer: A

Over a few degrees the thermistor's steep characteristic gives far more signal than platinum's slow linear change, and nonlinearity hardly matters across so small a span. For a wide range or for interchangeability the platinum element wins.

Answer: B

The gauge is a resistor cemented to the specimen, and power dissipated in it warms both, producing an apparent strain. Manufacturers quote a permissible power density that depends on how well the backing material conducts heat away.