ASRB NET Instrumentation, Sensors & Process Control: practice questions

Question 1 · easy

A greenhouse air-temperature sensor mounted in direct sunlight reads several degrees higher than the true air temperature. What is the standard remedy?

1. Mount it in a ventilated radiation shield at crop height
2. Replace it with a thermocouple, which is not affected by solar radiation
3. Move it to the coldest corner of the greenhouse near the side wall
4. Paint the sensor black so that it absorbs and emits radiation evenly

Question 2 · easy

Why is an electromagnetic flow meter unsuitable for measuring diesel fuel flow to a tractor engine on a test bed?

1. Diesel is too viscous for the magnetic field to pass through the pipe
2. The magnetic field would heat the fuel and change its density reading
3. Diesel does not conduct electricity, so no measurable emf is induced
4. Electromagnetic meters only work on gases, not on liquids of any kind

Question 3 · medium

A submersible pressure sensor at the bottom of a fertiliser tank reads 12.0 kPa gauge. The solution has a specific gravity of 1.2. What is the liquid depth?

1. 1.02 m
2. 1.22 m
3. 1.47 m
4. 12.0 m

Question 4 · easy

A hopper load cell has a rated output of 2 mV/V at its 500 kg capacity and is excited at 10 V. What output is expected with 200 kg in the hopper?

1. 8 mV
2. 20 mV
3. 0.8 mV
4. 4 mV

Question 5 · medium

1000 kg of maize is dried from 20% to 14% moisture (wet basis). How much water is removed?

1. 60.0 kg
2. 930.2 kg
3. 69.8 kg
4. 87.2 kg

Question 6 · medium

Two laboratories report different moisture contents for the same grain lot, both using correctly operated ovens. What is the most likely reason?

1. Oven methods are always random to about ±5%, so any two laboratory results on one lot will disagree
2. One laboratory reported the moisture on a wet basis and the other did too
3. They used different standard oven methods: temperature, time, ground or whole grain
4. Oven drying measures only free water, which changes daily in stored grain

Question 7 · easy

The wet bulb of a psychrometer in a seed store is not ventilated. How does this affect the relative humidity calculated from it?

1. The wet bulb reads too low, so the RH comes out too low
2. The dry bulb reads too high, so the RH comes out too low
3. The wet bulb reads too high, so the RH comes out too high
4. Nothing changes, because the wet bulb does not need airflow

Question 8 · easy

What is the correct order of a PLC scan cycle?

1. Execute program, read inputs, update outputs, then housekeeping
2. Update outputs, read inputs, execute program, then housekeeping
3. Read inputs, update outputs, execute program, then housekeeping
4. Read inputs, execute program, update outputs, then housekeeping

Question 9 · hard

During start-up of a cold grain dryer, the burner valve sits at 100% for 20 minutes while the PI controller's error stays large. Once air temperature reaches setpoint, it overshoots badly. What is the cause and remedy?

1. Derivative kick from the setpoint step; move the derivative action from the error signal onto the measurement
2. Too wide a proportional band; narrow it until the loop oscillates and leave it at that setting
3. Integral windup during saturation; add anti-windup that limits integration while the valve is saturated
4. Sensor time constant too short; fit a thicker thermowell so the reading changes more slowly

Question 10 · hard

A continuous-flow dryer controls outlet grain moisture by adjusting discharge rate. The moisture sensor at the outlet sees a change about 40 minutes after the rate changes, and the loop oscillates. Which approach is most appropriate?

1. Raise the proportional gain so the controller reacts more strongly to each reading
2. Add strong derivative action on the delayed outlet-moisture measurement signal
3. Detune the loop and add model-based prediction or feedforward from inlet moisture
4. Replace the PI loop with on-off control of the discharge auger motor speed