CAS CAS Automation Specialist Instrumentation & Sensors 2 — Questions and Answers
Question 1: What is the primary limitation of a thermocouple compared to an RTD for temperature measurement?
- Thermocouples cannot measure temperatures above 100°C
- Thermocouples require a reference junction and have lower accuracy than RTDs (Correct answer)
- Thermocouples are more expensive than RTDs
- Thermocouples cannot be used in hazardous areas
Correct answer: Thermocouples require a reference junction and have lower accuracy than RTDs
Thermocouples require cold junction (reference) compensation and typically have lower accuracy and repeatability than RTDs.
Question 2: In a differential pressure (DP) transmitter used for level measurement, what does an increase in DP typically indicate?
- A decrease in liquid level
- An increase in liquid level (Correct answer)
- A decrease in process temperature
- An increase in fluid viscosity
Correct answer: An increase in liquid level
In a DP level transmitter, higher hydrostatic head from more liquid increases the differential pressure reading, indicating higher level.
Question 3: What does HART (Highway Addressable Remote Transducer) protocol allow that a standard 4–20 mA loop does not?
- Higher current capacity for long cable runs
- Two-way digital communication superimposed on the 4–20 mA signal (Correct answer)
- Wireless communication to a control room
- Direct connection to Ethernet networks
Correct answer: Two-way digital communication superimposed on the 4–20 mA signal
HART superimposes a digital FSK signal on the 4–20 mA analog current, enabling configuration, diagnostics, and additional variables.
Question 4: Which type of level sensor uses guided microwave pulses along a probe to measure liquid level?
- Ultrasonic level sensor
- Guided Wave Radar (GWR) (Correct answer)
- Capacitance level sensor
- Float and displacer transmitter
Correct answer: Guided Wave Radar (GWR)
Guided Wave Radar (GWR) sends TDR (Time Domain Reflectometry) microwave pulses along a probe; the reflection time indicates the level.
Question 5: In an orifice plate flow measurement system, the relationship between differential pressure (ΔP) and volumetric flow rate (Q) follows which relationship?
- Q is directly proportional to ΔP
- Q is proportional to the square root of ΔP (Correct answer)
- Q is proportional to ΔP squared
- Q is inversely proportional to ΔP
Correct answer: Q is proportional to the square root of ΔP
For an orifice plate, Q ∝ √ΔP (derived from Bernoulli's principle), meaning flow varies with the square root of differential pressure.
Question 6: What is the purpose of a 'two-wire transmitter' configuration in process instrumentation?
- To provide redundant power supply paths
- To carry both power and the 4–20 mA signal on the same pair of wires (Correct answer)
- To allow two instruments to share a single cable
- To separate the sensor ground from signal ground
Correct answer: To carry both power and the 4–20 mA signal on the same pair of wires
A two-wire transmitter draws its operating power from the loop current (4–20 mA), eliminating the need for a separate power supply cable.
CAS CAS Automation Specialist Instrumentation & Sensors 2
What is the primary limitation of a thermocouple compared to an RTD for temperature measurement?