EETC Instrumentation 2 — Questions and Answers
Question 1: What is the standard live zero signal range used in industrial 4-20mA current loops?
- 0-10mA
- 4-20mA (Correct answer)
- 0-20mA
- 2-10mA
Correct answer: 4-20mA
The 4-20mA standard uses 4mA as the live zero, allowing detection of broken wires (which read 0mA, below the live zero).
Question 2: A thermocouple produces a voltage based on which physical principle?
- Piezoelectric effect
- Seebeck effect (Correct answer)
- Hall effect
- Faraday induction
Correct answer: Seebeck effect
The Seebeck effect describes the generation of an EMF at the junction of two dissimilar metals exposed to a temperature difference.
Question 3: An RTD (Resistance Temperature Detector) measures temperature by:
- Generating a voltage proportional to temperature
- Changing resistance with temperature (Correct answer)
- Varying capacitance with heat
- Emitting infrared radiation
Correct answer: Changing resistance with temperature
RTDs exploit the predictable, repeatable increase in electrical resistance of metals (typically platinum) as temperature rises.
Question 4: Which instrument measures the true RMS value of a non-sinusoidal AC waveform most accurately?
- Average-responding voltmeter
- Peak-responding voltmeter
- True RMS voltmeter (Correct answer)
- Moving-iron voltmeter
Correct answer: True RMS voltmeter
A true RMS meter computes the square root of the mean of the squared instantaneous values, correctly handling distorted waveforms.
Question 5: In a differential pressure (DP) flow transmitter, flow rate is proportional to:
- The square root of differential pressure (Correct answer)
- Differential pressure squared
- Differential pressure directly
- The cube root of differential pressure
Correct answer: The square root of differential pressure
Bernoulli's equation relates flow velocity to the square root of pressure difference, so volumetric flow = K × √ΔP.
Question 6: A loop-powered transmitter receives its operating power from:
- A separate 24VDC power supply to the transmitter terminals
- The 4-20mA signal loop itself (Correct answer)
- An internal lithium battery
- The process fluid it measures
Correct answer: The 4-20mA signal loop itself
Loop-powered (2-wire) transmitters draw their operating current (typically ≤3.6mA) from the same 4-20mA loop, requiring no separate power wiring.
Question 7: What does the term 'span' refer to in instrument calibration?
- The lowest measurable value of the instrument
- The difference between upper and lower range values (Correct answer)
- The accuracy class of the instrument
- The zero offset of the transmitter
Correct answer: The difference between upper and lower range values
Span is the algebraic difference between the upper range value (URV) and lower range value (LRV) of the calibrated range.
EETC Instrumentation 2
What is the standard live zero signal range used in industrial 4-20mA current loops?