ICAR AIEEA-PG: Dairy Science (group 16) and Dairy Technology (group 17) · India · NTA for ICAR
Original practice questions written to the published syllabus · not past papersMeasuring and controlling temperature, flow, pressure and level in a milk plant, from sensors to PLC and SCADA.
In the exams: ICAR AICE subject 38 Unit 7; ICAR AIEEA-PG group 17 Unit I (computerisation and automatic process control in milk processing)
How our questions for ICAR AIEEA-PG spread across the units of Instrumentation, Process Control & Automation. Difficulty: 16 easy, 1 medium, 0 hard.
| Generalised measurement system and instrument characteristics | 4 | |
| Temperature measurement: thermocouples, RTDs and recorders | 3 | |
| Control modes: on-off, P, PI and PID | 3 | |
| Pressure, flow and level measurement | 2 | |
| pH, conductivity and inline composition sensors | 1 | |
| Final control elements: valves and drives | 1 | |
| Programmable logic controllers | 1 | |
| SCADA and distributed control systems | 1 | |
| Automation of pasteurisers, CIP and milk reception | 1 |
Choose an answer, then open the working. Each explanation says why the right answer is right and why the others are not.
A thermometer gives readings that agree closely with each other but are all 0.6 °C above the reference. The instrument is:
A. Precise but not accurate
Close agreement between readings is precision (repeatability); a constant offset from the true value is a lack of accuracy, which calibration corrects. Averaging does not remove a systematic bias.
The change of an instrument's output per unit change of its input is called its:
C. Sensitivity
Sensitivity is output change per unit input, such as millivolts per degree. Resolution is the smallest detectable change, hysteresis the rising–falling difference, and dead band the input range giving no output change.
Why is a three-wire connection used for an RTD read in a bridge?
A. Lead resistances sit in adjacent bridge arms and so cancel
In a three-wire bridge, one lead sits in each adjacent arm, so lead resistance and its temperature changes largely cancel. RTDs need no cold-junction compensation, and the third wire neither doubles sensitivity nor carries a chart signal.
In the regenerator of an HTST pasteuriser, why is pasteurised milk kept at a higher pressure than raw milk?
C. So that any plate leak lets pasteurised milk into raw milk, not the reverse
If a pinhole develops, flow goes from high to low pressure; keeping the pasteurised side higher means safe milk leaks into raw milk, which is then pasteurised. The differential has nothing to do with deaeration, pump power or recorder accuracy.
In CIP, return-line conductivity is mainly used to:
D. Detect the change between rinse water and detergent
Detergents conduct far more than water, so the return conductivity shows when full-strength detergent arrives and when rinse water replaces it. It does not measure soil films, cycle counts or velocity.
A controller's proportional band is 25%. What is its gain?
B. 4
Gain = 100 ÷ PB = 100 ÷ 25 = 4. 0.25 is PB as a fraction; 25 is PB itself; 400 multiplies instead of dividing.
Why does a proportional-only temperature controller leave an offset after a change in milk flow?
C. A non-zero error is needed to hold the valve at its new opening
With P-only control, output changes only when there is error, so a new steady load needs a steady error. P-only has no integral or derivative terms, its gain is fixed, and it responds to error, not rate.
A pneumatic valve that is spring-closed and opens when air is applied fails in which position?
C. Closed
Air-to-open valves are held shut by the spring, so on loss of air they close. Air-to-close valves fail open; ordinary spring-return valves do not hold mid or last position.
A PLC analogue input on a 4–20 mA loop reads 0.2 mA. This most likely means:
B. A broken wire or failed transmitter
A live zero of 4 mA means any reading well below it signals a fault such as an open circuit. The bottom of the range is 4 mA, not near zero.
On an HTST pasteuriser, the flow diversion valve sends milk forward only when:
B. The holding-tube outlet temperature is at or above the legal cut-in value
The recorder-controller at the holding-tube outlet decides forward flow; below the cut-out it diverts. Balance-tank level, cooling and homogeniser pressure are not the forward-flow condition.
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