Instrumentation and Controls practice questions

Answer: C) 19.5 mV

• AIncorrect. That's 1,024 levels.
• BIncorrect. There are 256 levels.
• CCorrect. An 8-bit converter has 2⁸ = 256 levels, so 5 V ÷ 256.
• DIncorrect. That's off by a factor of 10.

Answer: A) The rate of change of the error

• ACorrect. It adds damping and anticipation.
• BIncorrect. That's the integral term.
• CIncorrect. That's the proportional term.
• DIncorrect. That isn't related.

Answer: B) The temperature difference between the two junctions

Thermocouple voltage depends on the temperature difference between the measuring and reference (cold) junctions. Cold-junction compensation measures the reference temperature and corrects for it.

• AIncorrect. A thermocouple does not measure absolute temperature. The same measuring-junction temperature gives a different voltage when the reference temperature changes.
• BCorrect. The Seebeck voltage depends on the temperature difference between the measuring and reference junctions. The reference temperature must be known, or compensated for, to find the process temperature.
• CIncorrect. Lead resistance matters little with a high-impedance voltage measurement, and it does not generate the thermoelectric voltage.
• DIncorrect. A thermocouple generates its own voltage and needs no excitation, unlike an RTD or thermistor.

Answer: B) Pressure

• AIncorrect. It responds to pressure.
• BCorrect. The curved tube straightens as pressure rises.
• CIncorrect. It responds to pressure.
• DIncorrect. It responds to pressure.

Answer: B) Compensate for temperature effects

• AIncorrect. Its purpose is compensation.
• BCorrect. Both gauges change equally with temperature.
• CIncorrect. That isn't its role.
• DIncorrect. That isn't its role.

Answer: B) 2

• AIncorrect. That's inverted.
• BCorrect. GF = (ΔR/R)/ε = 0.002/0.001.
• CIncorrect. Divide, don't multiply.
• DIncorrect. Recheck the division.

Answer: B) ±2.44 mV

Resolution (1 LSB) = full-scale range/2^n = 5/1,024 ≈ 4.88 mV. An ideal rounding ADC has a maximum quantization error of ±½ LSB ≈ ±2.44 mV.

• AIncorrect. 4.88 mV (5 V/1,024) is one full LSB, the size of one step. A rounding converter is never more than half a step off.
• BCorrect. One LSB = 5 V/2^10 ≈ 4.88 mV. Rounding limits the error to ±½ LSB ≈ ±2.44 mV.
• CIncorrect. This is a quarter LSB, which understates the worst-case rounding error by half.
• DIncorrect. This comes from using 2^9 = 512 levels (5 V/512) instead of 2^10 = 1,024. It is not the ±½ LSB bound.

Answer: B) Converts one form of energy into another

• AIncorrect. That's memory.
• BCorrect. Sensors often use transducers.
• CIncorrect. That's a generator.
• DIncorrect. That's an indicator.

Answer: B) Temperature cycles continuously in a band about the setpoint

On-off (bang-bang) control switches the heater fully on or off, so the temperature cycles above and below the setpoint. The deadband sets the band and limits how often the heater switches.

• AIncorrect. The output can only be fully on or fully off, so the heat input cannot settle at the exact level needed to hold one temperature.
• BCorrect. The heater switches on below the lower limit and off above the upper limit. This causes a sustained cycle whose size depends on the deadband and the process lag.
• CIncorrect. A steady offset with no cycling is typical of proportional-only control, not on-off control.
• DIncorrect. On-off control produces a bounded, sustained cycle, not an unstable oscillation that keeps growing.

Answer: A) The setpoint minus the measured value

• ACorrect. The controller acts to reduce it.
• BIncorrect. It's the difference from the setpoint.
• CIncorrect. That's the corrective action.
• DIncorrect. That isn't the error signal.