EETC Instrumentation 3 — Questions and Answers
Question 1: Which type of pressure measurement compares process pressure to atmospheric pressure?
- Absolute pressure
- Gauge pressure (Correct answer)
- Differential pressure
- Vacuum pressure
Correct answer: Gauge pressure
Gauge pressure (psig) uses atmospheric pressure (14.696 psia at sea level) as its reference, so a reading of 0 psig equals atmospheric.
Question 2: A pH sensor outputs approximately how many millivolts per pH unit at 25°C?
- 19.84 mV/pH
- 59.16 mV/pH (Correct answer)
- 100 mV/pH
- 0.5 mV/pH
Correct answer: 59.16 mV/pH
The Nernst equation predicts approximately 59.16mV/pH unit response for a glass electrode at 25°C (298K).
Question 3: An orifice plate in a pipeline causes flow measurement errors when:
- The upstream straight pipe run is too short (Correct answer)
- The fluid is at ambient temperature
- The orifice is installed with the bevel facing upstream
- Line pressure is above 100 psig
Correct answer: The upstream straight pipe run is too short
Insufficient straight pipe upstream causes turbulent or asymmetric velocity profiles that deviate from the conditions assumed in the orifice calibration.
Question 4: In a HART (Highway Addressable Remote Transducer) system, digital communication is superimposed on the 4-20mA signal using:
- Pulse-width modulation at 1200 baud
- FSK at 1200 baud over the analog loop (Correct answer)
- RS-485 differential signaling
- Fiber optic pulses
Correct answer: FSK at 1200 baud over the analog loop
HART uses Bell 202 FSK (Frequency Shift Keying) — 1200Hz for logic '1' and 2200Hz for logic '0' — overlaid on the DC 4-20mA signal without disrupting it.
Question 5: What is the purpose of a cold junction compensation circuit in a thermocouple instrument?
- To amplify the thermocouple millivolt signal
- To correct for the ambient temperature at the reference junction (Correct answer)
- To linearize the thermocouple output curve
- To filter electrical noise from the signal
Correct answer: To correct for the ambient temperature at the reference junction
Cold junction compensation measures the terminal block temperature and adds the equivalent voltage so the instrument reads true process temperature regardless of ambient conditions.
Question 6: A Coriolis flow meter measures which two fluid properties simultaneously?
- Volumetric flow and temperature
- Mass flow and density (Correct answer)
- Velocity and viscosity
- Differential pressure and flow
Correct answer: Mass flow and density
The Coriolis effect on vibrating tubes directly measures true mass flow rate, and tube frequency shift simultaneously yields fluid density.
Question 7: Which wiring configuration eliminates lead resistance error in RTD measurements?
- 2-wire
- 3-wire
- 4-wire (Kelvin) (Correct answer)
- Shielded 2-wire
Correct answer: 4-wire (Kelvin)
In a 4-wire (Kelvin) configuration, separate current-source and voltage-sense lead pairs ensure that no voltage drop across lead resistance is included in the measurement.
EETC Instrumentation 3
Which type of pressure measurement compares process pressure to atmospheric pressure?