Engineers Canada Agricultural / Biosystems / Bioresource / Food Engineering examination syllabus Instrumentation, Sensors & Process Control: practice questions

Question 1 · hard

A testing station claims its draft load cell is 'traceable'. What does traceability mean in strict metrological terms?

1. It was bought from a supplier registered with the national standards body for its particular industrial sector
2. Its readings can be traced back through the logger's stored files to the original test date
3. Its calibration links through an unbroken, documented chain of comparisons to national standards and SI units
4. It has been checked against a second load cell of the same model and the readings agreed

Question 2 · medium

During calibration of a 0–100 kPa pressure transmitter, an applied 50 kPa reads 50.4 kPa when approached from below and 51.0 kPa when approached from above. What is the hysteresis as a percentage of full scale?

1. 0.6% FS
2. 1.0% FS
3. 1.2% FS
4. 0.4% FS

Question 3 · hard

Which statement about NTC thermistors compared with platinum RTDs is correct?

1. They are far more sensitive per degree but strongly non-linear over a wide range
2. They are more linear than RTDs and therefore need no calibration equation at all
3. Their resistance rises with temperature at about ten times the rate of platinum
4. They suit higher temperatures than RTDs, such as furnace and flue-gas measurement

Question 4 · medium

Why is a piezoelectric pressure transducer unsuitable for monitoring the steady pressure in an irrigation main?

1. Its output is too large and saturates any amplifier at steady line pressures
2. It can only measure absolute pressure and cannot be referenced to atmosphere
3. Its natural frequency is too low to follow any change in the line pressure
4. Its charge leaks away, so it senses changing but not constant pressure

Question 5 · medium

A dummy strain gauge, bonded to an unstressed piece of the same steel at the same temperature, is placed in the bridge arm adjacent to the active gauge. Why?

1. Temperature-induced resistance changes then appear equally in adjacent arms and cancel
2. It doubles the strain sensitivity because both gauges sense the same applied strain
3. It provides a reference voltage for the amplifier so that excitation drift cancels
4. It completes a full bridge, which makes the output four times the quarter-bridge value

Question 6 · medium

Why is derivative action in a PID controller often applied to the measured variable rather than the error, and filtered?

1. To make the derivative term remove steady-state offset, which it cannot do on the error signal
2. To slow the loop so that it never overshoots, whatever proportional gain is chosen by the user
3. To avoid a large output spike when the setpoint is stepped, and to limit amplification of noise
4. To convert the PID into an on-off controller during large setpoint changes to save energy

Question 7 · medium

A greenhouse controller increases vent opening as soon as a solar-radiation sensor shows a sudden rise, before air temperature has changed. What is this strategy called?

1. Feedforward control
2. Cascade control
3. On-off control
4. Integral control

Question 8 · medium

A process has transfer function G(s) = 5/(s + 2). Using the final value theorem, what is the steady-state output for a unit step input?

1. 5.0
2. 0.4
3. 2.0
4. 2.5

Question 9 · hard

Why does adding integral action to a proportional loop on a first-order process remove steady-state error to a step setpoint change?

1. It adds a zero at the origin, cancelling the process pole and making the loop type 0
2. It adds dead time, so the loop waits until the error has decayed on its own to zero
3. It increases the damping ratio above one, which removes both overshoot and offset
4. It adds a pole at the origin, making the loop type 1 with infinite steady-state gain

Question 10 · medium

Where a licence-free band rule limits a device to a 1% transmit duty cycle, what is the maximum total transmission time per hour?

1. 6 s
2. 60 s
3. 36 s
4. 360 s