EETC Instrumentation 4 — Questions and Answers
Question 1: A transmitter with a 0.1% accuracy specification on a 0-100 psi range has a maximum error of:
- 0.1 psi (Correct answer)
- 1.0 psi
- 0.01 psi
- 10 psi
Correct answer: 0.1 psi
0.1% of full span (100 psi) = 0.1 psi maximum allowable error across the calibrated range.
Question 2: What does 'turndown ratio' describe in flow measurement?
- The ratio of pipe diameter to orifice bore
- The ratio of maximum to minimum measurable flow with acceptable accuracy (Correct answer)
- The pressure drop across the meter at full flow
- The response time for the transmitter output to change
Correct answer: The ratio of maximum to minimum measurable flow with acceptable accuracy
Turndown (rangeability) is the ratio of the maximum flow the meter measures accurately to the minimum, e.g., 10:1 means it reads correctly from 10% to 100% of full scale.
Question 3: In a PLC analog input module, a broken 4-20mA signal wire most commonly causes the reading to go to:
- 20mA (full scale high)
- 4mA (process zero)
- 0mA, triggering a below-range fault (Correct answer)
- Midscale (12mA)
Correct answer: 0mA, triggering a below-range fault
An open circuit allows no current to flow (0mA), which falls below the live zero (4mA) and should trigger a wire-break alarm in the control system.
Question 4: An ultrasonic level transmitter using the time-of-flight (TOF) method measures:
- The reflection time of a sound pulse from the liquid surface (Correct answer)
- The attenuation of ultrasound through the liquid
- The Doppler shift caused by flowing liquid
- The resonant frequency of the tank walls
Correct answer: The reflection time of a sound pulse from the liquid surface
TOF ultrasonic sensors measure the time for an emitted pulse to travel to the liquid surface and return, then calculate distance using the speed of sound.
Question 5: Which ISA instrument tag designation indicates a flow indicating controller?
- FT
- FIC (Correct answer)
- FC
- FIR
Correct answer: FIC
Per ISA-5.1, F=Flow, I=Indicating, C=Controller, so FIC is a flow instrument that both displays the measured value and provides control output.
Question 6: Intrinsically safe (IS) instrumentation limits electrical energy in hazardous areas to prevent ignition by:
- Using explosion-proof enclosures around all electronics
- Restricting voltage and current to levels below ignition thresholds (Correct answer)
- Purging the instrument enclosure with clean air or nitrogen
- Using only optical fiber signal transmission
Correct answer: Restricting voltage and current to levels below ignition thresholds
IS design ensures that under normal and fault conditions, the maximum spark energy available is insufficient to ignite the specified flammable atmosphere.
Question 7: A deadband (dead zone) in a control instrument refers to:
- The range of input signal over which output does not change (Correct answer)
- The time delay before the controller responds
- The maximum safe operating temperature range
- The calibration tolerance of the sensor
Correct answer: The range of input signal over which output does not change
Deadband is the range through which the input can change without producing a change in output, used deliberately to prevent hunting in on/off controls.
EETC Instrumentation 4
A transmitter with a 0.1% accuracy specification on a 0-100 psi range has a maximum error of: