Plant Operator Selection System Test Process Control and Instrumentation

Question 1

A thermocouple measures temperature by:

Accepted Answer

Generating a small voltage at the junction of two dissimilar metals

Answer

Thermocouples produce a millivolt signal (Seebeck effect) at the junction of two different metals that varies predictably with temperature.

Question 2

In a PID controller, the 'I' (integral) term is used to:

Accepted Answer

Eliminate steady-state offset by accumulating past error over time

Answer

The integral term sums past errors over time and applies a correction that eliminates the steady-state offset left by proportional control alone.

Question 3

A control valve with an air-to-open (ATO) fail action will:

Accepted Answer

Close fully when instrument air pressure is lost

Answer

Air-to-open (also called fail-closed) valves close when air supply is lost, which is the safe failure mode for many process streams.

Question 4

What does a differential pressure (DP) transmitter measure when installed across an orifice plate?

Accepted Answer

Flow rate based on the pressure drop across the restriction

Answer

Flow through an orifice creates a pressure drop proportional to the square of the flow rate, which the DP transmitter converts to a flow signal.

Question 5

On a Process and Instrumentation Diagram (P&ID), a dashed line connecting an instrument to a control element typically represents:

Accepted Answer

An electrical or pneumatic signal

Answer

Dashed lines on P&IDs represent signal lines (electrical, pneumatic, or digital) that transmit information between instruments and controllers.

Plant Operator Selection System Test Practice Test