Instrumentation & Process Control Flashcards

Which PID tuning method involves cycling the process at the ultimate gain to determine ultimate period and then applying tuning formulas?

Answer: Ziegler-Nichols closed-loop (ultimate cycling) method

The Ziegler-Nichols closed-loop method increases controller gain until sustained oscillation occurs, then uses the ultimate gain and ultimate period to calculate PID settings.

A thermowell is installed around a temperature sensor in a process pipe. What is the primary function of the thermowell?

Answer: To protect the sensor from process pressure, corrosion, and flow erosion while allowing removal without process shutdown

A thermowell mechanically isolates the sensor from the process fluid, protecting it from pressure and corrosion, and allowing in-service removal without process interruption.

What is 'integral windup' in a PID controller, and when does it typically occur?

Answer: Accumulation of integral action during periods when the output is saturated, causing overshoot when the constraint is removed

Integral windup occurs when the controller output saturates (e.g., valve fully open/closed) while the integral term keeps accumulating error, resulting in large overshoot when the process variable returns to range.

For a vortex flowmeter to operate correctly, which condition is most important regarding the process fluid?

Answer: The Reynolds number must be sufficiently high (turbulent flow)

Vortex meters require a minimum Reynolds number (typically >10,000–20,000) to ensure the Kármán vortex street is well-established and measurable.

A 4-wire RTD is preferred over a 2-wire RTD in precision temperature measurement because:

Answer: It eliminates lead wire resistance error from the measurement

A 4-wire RTD uses two wires to supply excitation current and two separate wires to sense voltage, so lead resistance does not contribute to the measured value.

Which control action is added to a PI controller to improve response to rapidly changing process variables?

Answer: Derivative (rate) action

Derivative action anticipates future error by responding to the rate of change of the process variable, improving transient response when added to a PI controller.

In a HART communication system, what information is transmitted alongside the analog 4-20 mA signal?

Answer: Digital data including device diagnostics, configuration, and additional process variables superimposed on the analog signal

HART superimposes a frequency-shift-keyed (FSK) digital signal on the 4-20 mA loop, carrying device diagnostics, configuration parameters, and secondary variables without interrupting the analog signal.