Actual Value - The actual value of the controlled condition.

Closed Loop System - Temperature detector in the extract - measures the actual temperature of the controlled condition.

Dead Zone - The interval between the values above and below the set point, at which the control unit does not take any corrective action.

Desired Value - The value of the controlled condition which the controller has to maintain.

Deviation - The difference between the measured value of the controlled condition and the set point of the controller at any instant.

Hunting - Periodical variations in the actual value of the controlled variable at a fixed reference value.

Integral Action Time - Time taken for integral action to open the valve by the same amount as initial rapid movement due to proportional action.

Mechanical Differential (intrinsic pressure difference) - The interval between the values which make the control unit move; ie the interval between making and breaking pressures for the contacts 2-1 or 2-3.

Offset - Deviation from set point

Open Loop System - Control detector downstream of the device being controlled - only measures the value of the effect the control on the device being controlled.

Proportional Band - Deviation necessary to produce full range of control action.

Repeatability - Accuracy over repeated operations.

Set Point - The value of the controlled condition to which the controller is set.

System Dead Time - The time interval between the valve moving and the detector sensing a temperature change.

Time Constant - A standardised function allowing comparison of response rates of different systems.