The degree of normalisation required to reset an active alarm state, measured from the alarm activation limit. Normally expressed in terms of a fraction (%) of the operating range.
An adjustable band through which the process variable must pass before an alarm/limit will change state. This Hysteresis is only applicable to alarm/limits based on the Process Value or Control Deviation, as illustrated below.
The band is always on the “safe” side of an alarm/limit point, e.g. a high alarms hysteresis band is below the high alarm value, and a low alarms hysteresis is above the low alarm value.