Instrumentation and Process Control
Sensors and signals
Measurement starts with the sensor: RTDs (accurate, stable, positive temperature coefficient), thermocouples (wide range, needs cold-junction compensation, lower accuracy), strain and pressure sensors, and differential-pressure flow elements. Transmitters convert the measurement to a standard signal, most commonly 4–20 mA, chosen because a live zero distinguishes a failed loop from a legitimate zero reading and because current signals resist voltage drop and noise. Two-wire loops carry power and signal on the same pair; four-wire devices have separate supply. HART superimposes a digital signal on the 4–20 mA current without disturbing it.
Control loops
Open-loop control acts without feedback; closed-loop compares measurement to setpoint and acts on the error. PID combines proportional action (responds to present error), integral (removes steady-state offset) and derivative (responds to rate of change, and is sensitive to noise). Loop tuning trades speed against stability. Control valves are specified with fail-safe action (fail-closed or fail-open), Cv, and positioner type; air-to-open with a spring-return actuator fails closed on air loss.
Traps
Confusing accuracy (closeness to truth) with precision (repeatability); forgetting cold-junction compensation; assuming 3–15 psi and 4–20 mA are interchangeable scales without conversion; and ignoring that a 4 mA reading at the transmitter may correspond to a zero process value, not a problem.
Worked example
A differential-pressure transmitter spans 0–100 kPa over 4–20 mA. A reading of 12 mA sits at (12 − 4)/16 = 50% of span, so the measured differential pressure is 50 kPa. At 4 mA the value is 0 kPa - the loop is healthy; at 0 mA the loop has lost power or the wire is broken.