The off-delay will delay clearing the alarm once the measurement has returned to normal by the delay time entered. When the process exceeds the alarm limit, the alarm will be activated, but will not clear until the process stays below or above the alarm limit by the amount of the delay time setting. This will not resolve the activation of a false alarm, but will reduce the number of alarm activations for a noisy measurement. The advantage of an off-delay is immediate notification of a real alarm to the operator. Figure 4 shows an oscillating process (blue) that would give 11 alarms (orange) where the time delay off (green) indicates only one alarm. In this demonstration, the off-delay would reduce the false positive alarms by 10, but create a standing alarm during false
High alarm setpoint
Deadband
Deadband
Low alarm setpoint
Alarm state
DEADBAND SLOWS RESPONSE Figure 2: Used properly, deadband can reduce frequency of alarms without slowing activation, but it can hide a real alarm if the deadband level is not cleared before the next activation.
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