EETC Instrumentation 5 — Questions and Answers
Question 1: What is the primary purpose of a signal isolator in an instrumentation loop?
- To amplify weak millivolt signals to 4-20mA
- To break ground loops and provide electrical isolation between circuits (Correct answer)
- To convert 4-20mA to a digital HART signal
- To provide redundant power to the transmitter
Correct answer: To break ground loops and provide electrical isolation between circuits
Isolators break conductive paths between circuits, eliminating ground loop currents that would otherwise cause measurement offset or noise.
Question 2: A vortex flow meter operates on the principle that vortices shed by a bluff body occur at a frequency that is:
- Proportional to fluid viscosity
- Independent of flow velocity
- Proportional to fluid velocity (Correct answer)
- Proportional to fluid density
Correct answer: Proportional to fluid velocity
The Strouhal number relates vortex-shedding frequency to fluid velocity and bluff-body width: f = St × V / d, making frequency a direct velocity indicator.
Question 3: During a field calibration of a pressure transmitter, 'zero suppression' means:
- Setting the 4mA output to a pressure above zero absolute
- Raising the lower range value above zero so the 4mA point is above zero (Correct answer)
- Removing zero offset error from a faulty transmitter
- Calibrating without applying any pressure to the transmitter
Correct answer: Raising the lower range value above zero so the 4mA point is above zero
Zero suppression sets the 4mA (LRV) point above zero, such as calibrating 50-150 psig, so the instrument ignores pressure below 50 psig.
Question 4: What does SIL (Safety Integrity Level) classify in process instrumentation?
- The accuracy class of the measurement device
- The electromagnetic compatibility rating of the instrument
- The reliability level required for a safety function to reduce risk (Correct answer)
- The ingress protection rating of the enclosure
Correct answer: The reliability level required for a safety function to reduce risk
SIL (defined in IEC 61511) quantifies the required probability of failure on demand (PFD) for a safety instrumented function to reduce process risk to tolerable levels.
Question 5: A magnetic (electromagnetic) flow meter requires the process fluid to be:
- Non-conductive
- Electrically conductive (minimum ~5 µS/cm) (Correct answer)
- At least 50% liquid by volume
- Free of suspended solids
Correct answer: Electrically conductive (minimum ~5 µS/cm)
Mag meters induce a voltage in the flowing fluid per Faraday's law, which requires the fluid to be conductive enough to complete the circuit between the electrodes.
Question 6: In process control, hysteresis in a control valve causes problems because:
- The valve responds faster than the controller expects
- The valve position lags behind the controller signal, causing control offsets (Correct answer)
- It increases the process gain unpredictably
- It improves stability in cascaded loops
Correct answer: The valve position lags behind the controller signal, causing control offsets
Hysteresis (stiction and backlash) means the valve does not move until the signal overcomes internal friction, creating a dead zone and limit cycling in control loops.
Question 7: What is the function of a thermowell in temperature measurement installations?
- To amplify the thermocouple signal before it reaches the transmitter
- To protect the temperature sensor from process pressure, flow, and corrosion (Correct answer)
- To provide cold junction compensation for the thermocouple
- To linearize the RTD resistance-temperature curve
Correct answer: To protect the temperature sensor from process pressure, flow, and corrosion
A thermowell is a closed-end tube inserted into the process that allows the temperature sensor to be installed, removed, or replaced without shutting down or depressurizing the process.
EETC Instrumentation 5
What is the primary purpose of a signal isolator in an instrumentation loop?