CAS CAS Automation Specialist Instrumentation & Sensors 1 — Questions and Answers
Question 1: What does the '4-20 mA' standard represent in industrial instrumentation?
- A voltage range for digital signals
- A current loop signal range where 4 mA represents 0% and 20 mA represents 100% of span (Correct answer)
- A resistance value for RTD calibration
- A frequency range for vibration sensors
Correct answer: A current loop signal range where 4 mA represents 0% and 20 mA represents 100% of span
The 4–20 mA current loop is the industry-standard analog signal where 4 mA = live zero (0% of range) and 20 mA = full scale (100%).
Question 2: Which type of flowmeter measures flow by detecting the frequency of vortices shed by an obstruction in the flow stream?
- Coriolis flowmeter
- Magnetic flowmeter
- Vortex flowmeter (Correct answer)
- Ultrasonic flowmeter
Correct answer: Vortex flowmeter
A vortex flowmeter uses the von Kármán effect: vortices shed from a bluff body at a frequency proportional to flow velocity.
Question 3: What is the primary advantage of a Coriolis mass flowmeter over a volumetric flowmeter?
- Lower installation cost
- It directly measures mass flow, independent of fluid density, temperature, or pressure (Correct answer)
- It works better with slurries
- It requires no upstream straight pipe runs
Correct answer: It directly measures mass flow, independent of fluid density, temperature, or pressure
Coriolis meters measure mass flow directly by detecting the Coriolis force effect, making them independent of process conditions that affect density.
Question 4: An RTD (Resistance Temperature Detector) operates on which physical principle?
- The Seebeck effect, where two dissimilar metals generate a voltage
- The predictable change in electrical resistance of a metal with temperature (Correct answer)
- The change in capacitance of a dielectric with temperature
- The resonant frequency shift of a crystal with temperature
Correct answer: The predictable change in electrical resistance of a metal with temperature
RTDs use the known, repeatable relationship between a metal's (typically platinum) electrical resistance and temperature.
Question 5: In process instrumentation, what does the term 'rangeability' refer to?
- The maximum operating pressure of an instrument
- The ratio of maximum to minimum flow that an instrument can measure accurately (Correct answer)
- The electrical signal range of a transmitter output
- The temperature range over which calibration is valid
Correct answer: The ratio of maximum to minimum flow that an instrument can measure accurately
Rangeability (or turndown ratio) is the ratio of the maximum to minimum measurable value within the instrument's specified accuracy.
Question 6: Which pressure measurement term describes pressure referenced to absolute zero (perfect vacuum)?
- Gauge pressure
- Differential pressure
- Absolute pressure (Correct answer)
- Atmospheric pressure
Correct answer: Absolute pressure
Absolute pressure is measured relative to absolute zero pressure (perfect vacuum) and equals gauge pressure plus atmospheric pressure.
CAS CAS Automation Specialist Instrumentation & Sensors 1
What does the '4-20 mA' standard represent in industrial instrumentation?