Transducers in Measurement Questions and Answers

Answer: C

Correct answer: falls to roughly 6.5 k\( \Omega \)

A negative coefficient thermistor follows \[ R = R_0\exp\left[\beta\left(\frac{1}{T}-\frac{1}{T_0}\right)\right] = 10\,\mathrm{k}\Omega \times \exp\!\left[3500\left(\tfrac{1}{308}-\tfrac{1}{298}\right)\right] = 10 \times e^{-0.381} = 6.8\ \mathrm{k}\Omega. \] So ten degrees removes about a third of the resistance - a far larger change than a platinum sensor would give, which is the thermistor’s advantage, bought at the cost of a strongly non-linear characteristic.

Answer: D

The gauge responds to the strain of the surface it is bonded to. Stress follows only by multiplying by the modulus of elasticity of the material, so the modulus must be known and the material must be within its elastic range.

Answer: A

The charge generated does correspond to the force, but leakage through the crystal and the amplifier drains it away within seconds. A strain gauge load cell is the transducer for static weighing; the crystal is for shock and vibration.

Answer: B

It is a transformer and needs an alternating excitation, typically a few kilohertz, so that the coupling to the secondaries can induce anything at all. A phase-sensitive detector then converts the output to a signed direct voltage.

Answer: C

Correct answer: True, it generates its own emf from the junction difference

A thermocouple is self-generating: the Seebeck effect produces an emf from the temperature difference between the measuring and reference junctions, so no excitation is needed. That is what distinguishes it from the resistance devices in this topic, all of which need a current passed through them and therefore raise the question of self-heating.

Answer: D

Adjacent arms form one side of the potential divider, so equal fractional changes in both leave the ratio unaltered. Putting the dummy there makes temperature a common change and the strain of the active gauge the only difference the bridge sees.

Answer: A

Its gauge factor is far larger, not smaller, which is its whole attraction. It is avoided for precise work because that large factor is nonlinear and varies strongly with temperature, so the reason given is simply wrong.

Answer: B

The element changes by a fraction of an ohm per degree, so a few ohms of lead resistance would represent a large temperature error. Separate current and potential leads exclude it entirely, which is exactly the reason for the arrangement.

Answer: C

Each is the standard device for its quantity, and each embodies a different physical effect: electromagnetic coupling, thermoelectricity and piezoelectricity. Knowing which effect underlies a device tells you at once whether it is active or passive.

Answer: D

The strain gauge changes a resistance and needs a supply; the photovoltaic cell generates its own output; and the capacitive pickup changes a reactance and needs an alternating excitation. Each class implies a different signal conditioning circuit.