ICAR AICE-JRF/SRF Instrumentation, Sensors & Process Control: practice questions

1 · The generalised measurement system · easy

In the generalised measurement system, which functional element changes the magnitude of a signal without changing its physical nature?

1. The variable-manipulation element, for example an amplifier raising a millivolt signal to volts
2. The variable-conversion element, for example a diaphragm turning pressure into a displacement
3. The primary sensing element, for example a thermocouple junction responding to temperature
4. The data-presentation element, for example a chart recorder or digital display showing the value

2 · The generalised measurement system · easy

Which of these transducers is self-generating (active), producing an output without any external excitation supply?

1. A platinum resistance thermometer, whose resistance rises steadily with the temperature
2. A bonded foil strain gauge, whose resistance changes when the surface it is glued to strains
3. A thermocouple, which produces an emf from the temperature difference between its junctions
4. A capacitive polymer humidity sensor, whose capacitance changes as the polymer absorbs water

3 · The generalised measurement system · medium

Sixteen repeated readings of soil temperature have a standard deviation of 0.8 °C. What is the standard error of their mean?

1. 0.05 °C
2. 0.20 °C
3. 0.80 °C
4. 3.20 °C

4 · Static and dynamic characteristics of instruments · medium

A thermometer with a first-order time constant of 10 s is moved from air at 20 °C into hot water at 80 °C. What does it read 15 s later?

1. 57.9 °C
2. 62.1 °C
3. 33.4 °C
4. 66.6 °C

5 · Temperature transducers · easy

What does the designation 'Pt100' on a resistance temperature detector mean?

1. A platinum element whose resistance is 100 Ω at 100 °C
2. A platinum element whose resistance is 100 Ω at 0 °C
3. A platinum-coated thermocouple usable only up to 100 °C
4. A platinum element with a 100 µV/°C output sensitivity

6 · Temperature transducers · easy

Which thermocouple law allows a copper-wired voltmeter to be inserted into a thermocouple circuit without affecting the reading, provided both new junctions are at the same temperature?

1. The law of intermediate temperatures
2. The law of intermediate metals
3. The law of homogeneous circuits
4. The Peltier law of junction heating

7 · Pressure, flow and level measurement · medium

An orifice meter gives 16 kPa differential at 40 L/s. If flow falls to 20 L/s, what differential will it show?

1. 8 kPa
2. 2 kPa
3. 4 kPa
4. 32 kPa

8 · Signal conditioning · easy

What is the main advantage of the 'live zero' of 4 mA in a 4–20 mA signal from a field level transmitter?

1. It lets the signal travel faster than a 0–20 mA signal over long cables
2. It cancels the resistance of long cables, which a 0–20 mA loop cannot do
3. A reading of 0 mA shows a broken wire or failed transmitter, not a zero level
4. It doubles the measurement resolution of the analog-to-digital converter

9 · Data acquisition and data loggers · medium

Why is a dual-slope (integrating) ADC favoured in precision digital voltmeters used near mains equipment?

1. It converts in nanoseconds, so mains noise cannot change during the conversion
2. It has the highest resolution available because it uses one comparator per step
3. Integrating over a whole number of mains cycles averages out 50/60 Hz pick-up
4. It needs no reference voltage, so drifts in the reference are of no concern

10 · Programmable logic controllers · medium

In a ladder-logic start/stop seal-in circuit for a conveyor motor, how are the stop button and the sealing contact arranged?

1. Stop as a normally open contact in parallel with Start; the sealing contact in series with Stop
2. Stop as a normally closed contact in series; the motor's normally open contact in parallel with Start
3. Stop as a normally closed contact in parallel with Start; the sealing contact also wired as normally closed
4. Stop and Start both normally open in series; the sealing contact in series with both of them