PO Instrumentation & Measurement 2 — Questions and Answers
Question 1: What is the primary function of a control valve positioner?
- To manually operate the valve from a remote control room location
- To ensure the valve stem position accurately matches the controller output signal (Correct answer)
- To regulate only the supply air pressure to the pneumatic actuator
- To convert the electrical signal into a digital process variable value
Correct answer: To ensure the valve stem position accurately matches the controller output signal
A positioner continuously compares the actual valve stem position to the desired position commanded by the controller and adjusts actuator pressure to eliminate any deviation.
Question 2: Which type of temperature sensor generates a millivolt output based on the Seebeck thermoelectric effect?
- RTD (Resistance Temperature Detector)
- Thermistor
- Thermocouple (Correct answer)
- Filled-bulb thermometer
Correct answer: Thermocouple
Thermocouples generate a small voltage when two dissimilar metals are joined at a measurement junction and exposed to a temperature difference, exploiting the Seebeck effect.
Question 3: What is the standard current signal range used in most modern industrial field instrumentation loops?
- 0–10 mA
- 4–20 mA (Correct answer)
- 0–20 mA
- 1–5 mA
Correct answer: 4–20 mA
The 4–20 mA standard is universally used because the 4 mA 'live zero' allows detection of broken or open-circuit wiring conditions and provides excellent noise immunity over long cable runs.
Question 4: In a 4–20 mA instrumentation loop, what process condition does a reading of exactly 4 mA indicate?
- The transmitter has experienced an internal failure
- The process variable is at 0% of its calibrated span (Correct answer)
- The process variable is at 25% of its calibrated span
- The loop power supply voltage has dropped below minimum
Correct answer: The process variable is at 0% of its calibrated span
In the 4–20 mA convention, 4 mA represents 0% (minimum) of the calibrated range, while 20 mA represents 100% (maximum); 4 mA is the live zero, not a fault indication.
Question 5: What is the purpose of calibrating a process instrument?
- To physically clean the internal sensing element of contamination
- To verify and adjust the instrument output to agree with a traceable reference standard (Correct answer)
- To replace worn mechanical seals and internal components
- To extend the instrument's measurement range beyond its design specification
Correct answer: To verify and adjust the instrument output to agree with a traceable reference standard
Calibration confirms that an instrument's output accurately represents the measured variable by comparing it against a known reference and making adjustments to remove systematic error.
Question 6: Which level measurement technology uses the hydrostatic pressure at the bottom of a vessel to determine liquid level?
- Radar level transmitter
- Ultrasonic level transmitter
- Hydrostatic pressure transmitter (Correct answer)
- Capacitance level probe
Correct answer: Hydrostatic pressure transmitter
A hydrostatic pressure transmitter measures the liquid-head pressure at the vessel bottom; since pressure = density × gravity × height, the level is directly calculated from the measured pressure.
Question 7: Which instrument is specifically designed to measure the viscosity (resistance to flow) of a process fluid?
- Densitometer
- Viscometer (Correct answer)
- Refractometer
- Conductivity meter
Correct answer: Viscometer
A viscometer measures a fluid's viscosity — its internal resistance to flow — which is critical for product quality control and process optimization in many chemical and refining processes.
PO Instrumentation & Measurement 2
What is the primary function of a control valve positioner?