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

1 · Generalised measurement system and instrument characteristics · hard

A 12-bit analogue-to-digital converter digitises a 0–150 °C range. What is the approximate temperature resolution?

1. About 0.15 °C
2. About 0.037 °C
3. About 0.59 °C
4. About 0.012 °C

2 · Temperature measurement: thermocouples, RTDs and recorders · hard

A Pt100 (α = 0.00385 per °C) is wired with two copper leads of 0.5 Ω each in a two-wire circuit. Roughly what error do the leads cause?

1. About 2.6 °C too low
2. About 2.6 °C too high
3. About 0.4 °C too high
4. About 1.3 °C too high

3 · pH, conductivity and inline composition sensors · easy

In CIP, return-line conductivity is mainly used to:

1. Measure the flow velocity in the return pipe
2. Measure the residual protein film on surfaces
3. Count the number of cleaning cycles
4. Detect the change between rinse water and detergent

4 · Control modes: on-off, P, PI and PID · easy

A controller's proportional band is 25%. What is its gain?

1. 0.25
2. 4
3. 400
4. 25

5 · Control modes: on-off, P, PI and PID · easy

Why does a proportional-only temperature controller leave an offset after a change in milk flow?

1. It responds only to the rate of change of temperature
2. Its gain falls automatically as the load increases
3. A non-zero error is needed to hold the valve at its new opening
4. Its derivative term cancels the integral term at steady state

6 · Programmable logic controllers · easy

A PLC analogue input on a 4–20 mA loop reads 0.2 mA. This most likely means:

1. The process is above its top range value
2. A broken wire or failed transmitter
3. The loop is at half scale
4. The process is at the bottom of its range

7 · SCADA and distributed control systems · easy

What distinguishes a distributed control system (DCS) from a PLC-plus-SCADA system?

1. A DCS cannot handle analogue signals such as temperature or pressure at all
2. A DCS works only for batch processes and never for continuous ones like UHT
3. One integrated platform supplies controllers, I/O, stations and tools
4. A DCS uses no programmable controllers at any level of the system hierarchy

8 · Automation of pasteurisers, CIP and milk reception · medium

An HTST pasteuriser gives an average holding time of 16 s at 9,000 L/h. If flow rose to 10,000 L/h with the same holding tube, what would the average holding time be?

1. 17.8 s
2. 16.0 s
3. 12.8 s
4. 14.4 s

9 · Automation of pasteurisers, CIP and milk reception · medium

A CIP detergent tank holds 2,000 L of water to be made up to 1.5% w/v sodium hydroxide using a 50% w/w caustic solution. Ignoring the volume added, how much 50% solution is needed?

1. 120 kg
2. 30 kg
3. 15 kg
4. 60 kg

10 · Automation of pasteurisers, CIP and milk reception · easy

Why is an air eliminator fitted ahead of the reception flowmeter?

1. So entrained air is not measured and paid for as milk
2. So the milk is cooled before it reaches the silo
3. So the tanker empties faster through a wider pipe
4. So foam is added to protect the milk from oxygen