Controls and Instrumentation
The loop
Every loop has a sensor, transmitter, controller, and final element. Error is setpoint minus measured value. Modes: two-position with a deadband to prevent short cycling; proportional, which produces offset (droop) corrected by reset; PI, where integral action removes offset; and PID, where derivative anticipates. Most HVAC loops are PI; derivative is reserved for fast, noisy loops and is applied to the measurement rather than the error to avoid derivative kick on setpoint changes.
Signals and devices
The 4–20 mA standard places zero at 4 mA, not 0, so a broken lead reads 0 mA and is distinguishable from a valid zero reading. Actuators are 0–10 V or floating. Valves and dampers are characterized linear, equal-percentage, or quick-opening; an equal-percentage plug matched to a linear coil gives roughly linear heat output. Check valve authority - low authority makes the published characteristic meaningless.
Sequences and networks
Know economizer changeover (dry-bulb or enthalpy), static-pressure reset for VAV, supply-air-temperature reset, and optimum start. BAS protocols are BACnet (the ASHRAE standard), Modbus, and LonWorks.
Traps
Accuracy quoted as percent of span versus percent of reading; reverse-acting versus direct-acting controllers; fail-safe (spring-return) position versus normal position; and integral windup during startup.
Worked example
A transmitter spans 50–150°F and reads 12 mA. Fraction of span = (12 − 4)/16 = 0.5, so the temperature is 50 + 0.5(100) = 100°F.