Transducers in Measurement Questions and Answers

Answer: C

The resistance changes by 0.1%.

First find the strain from Hooke's law, then multiply by the gauge factor, \(\frac{\Delta R}{R} = G\varepsilon\).

\[ \begin{aligned} \varepsilon &= \frac{\sigma}{E} = \frac{100 \times 10^{6}}{200 \times 10^{9}} = 5 \times 10^{-4} \\ \frac{\Delta R}{R} &= 2 \times 5 \times 10^{-4} = 10^{-3} \end{aligned} \]

A fractional change of \(10^{-3}\) is 0.1%. 0.01% and 1% come from slipping a factor of ten in the prefixes M and G, and 10% would mean an impossible strain for steel.

Answer: B

The maximum measurable acceleration is 1200 m/s².

The mass hits the stop when the spring is deflected by 0.02 m. At that point the spring force equals the force needed to accelerate the seismic mass:

\[ \begin{aligned} kx &= ma \\ a &= \frac{kx}{m} = \frac{3 \times 10^{3} \times 0.02}{0.05} \\ &= \frac{60}{0.05} = 1200\ \text{m/s}^{2} \end{aligned} \]

The square-root values printed with the question are not needed. 600 and 300 come from halving or quartering the displacement, and 4.9 m/s² is about half of g, which has nothing to do with the data.

Answer: C

A thermistor is suitable for low span but high sensitivity.

A thermistor's resistance changes by several percent per degree, much more than a metal resistance thermometer's, so it is very sensitive. But its resistance-temperature relation is strongly non-linear (exponential), and its usable range is limited to a few hundred degrees, so its span is small.

High span with low sensitivity describes thermocouples or platinum resistance thermometers better. A device with both high span and high sensitivity, or with both low, does not describe the thermistor's characteristic trade-off.

Answer: B

A 3 and a half digit display is the format of a digital voltmeter, so a matches 4. Pt-100 is a platinum resistance sensor that measures temperature, so b matches 3. An RTD is read through a bridge circuit, so c matches 2. Interference is reduced by guarding and shielding, so d matches 1. The code is 4 3 2 1. The code 1 2 3 4 is the reverse, and 3 1 4 2 and 3 4 1 2 mismatch the first pair. The answer is 4 3 2 1.

Answer: A

In measurement the conversion wanted is usually from some physical quantity into an electrical one, because electrical signals can be amplified, transmitted and recorded conveniently. The device producing that conversion is the transducer.

Answer: B

A thermocouple, a piezoelectric crystal and a photovoltaic cell all deliver an electrical output from the energy of the quantity being measured. A passive transducer such as a strain gauge merely changes an electrical property and needs a supply to reveal the change.

Answer: C

A strain gauge changes its resistance and nothing more; a bridge and a supply are needed before any signal appears. The other three are self-generating and therefore active, which is the standard division of transducers.

Answer: D

In a Bourdon tube fitted with an LVDT, the tube is the primary element converting pressure into displacement and the LVDT is the secondary converting displacement into voltage. Many measurements involve such a chain of two conversions.

Answer: A

A shaft encoder and a turbine flow meter with a pulse pickup are examples: the output is a count or a code rather than a magnitude. No analogue to digital conversion is needed afterwards, which is a considerable advantage in a digital system.

Answer: B

The wiper of a resistance element is moved by the quantity to be measured, so the resistance in circuit and the fraction of the supply appearing at the wiper both follow the displacement. It is simple, gives a large output and needs no amplification.