Transducers in Measurement Questions and Answers

Answer: A

Nothing touches, so the device has an indefinite mechanical life and adds no friction to the mechanism. The costs are the need for an alternating excitation and a phase-sensitive detector, though both are now packaged with the device.

Answer: B

All three quantities appear in the expression for capacitance, so any of them may carry the measurement. Varying the spacing gives a large but nonlinear response, varying the overlap area gives a linear one, and a moving dielectric slab suits level measurement.

Answer: C

Capacitance is proportional to the reciprocal of the distance, so equal increments of displacement give unequal increments of capacitance. The sensitivity is very high at small spacings, which is why the arrangement is used for tiny displacements despite the curvature.

Answer: D

The capacitance being measured may be a few picofarads while the leads contribute tens, so screening and guarding are essential and the electronics is often built into the transducer housing. A moving hand nearby can otherwise change the reading.

Answer: A

Moving an iron armature alters the air gap in a magnetic circuit, and the inductance of the coil round that circuit changes accordingly. It is rugged and needs no sliding contact, and it is the principle of many pressure and vibration pickups.

Answer: B

Deforming certain crystals separates charge across their faces in proportion to the deformation, and hence to the force producing it. Quartz and various ceramics are used, and the same effect run backwards makes such a crystal a source of ultrasound.

Answer: C

The charge is generated by the deformation and would persist, but the input resistance of any amplifier and the leakage of the crystal itself drain it away with a time constant of seconds at best. Such transducers are therefore used for vibration and shock rather than for static weighing.

Answer: D

A voltage amplifier would see the crystal's charge divided by the crystal capacitance plus the cable capacitance, so changing the cable would change the calibration. A charge amplifier holds its input at virtual earth and measures the charge itself, so the cable no longer matters.

Answer: A

Two dissimilar conductors joined at two junctions at different temperatures drive a current round the loop, and the emf depends on the pair of metals and on the two temperatures. The Peltier and Thomson effects are the reverse and complementary phenomena.

Answer: B

This is what allows copper leads, solder and instrument terminals to be introduced without upsetting the measurement, provided each pair of junctions is isothermal. Without it a thermocouple could not be connected to anything.