Alarm filter

Require consecutive out-of-limit readings before the alarm triggers.

Each alarm limit has a parameter called Alarm Filter.

Use Alarm Filter for real-world scenarios where you log a parameter frequently, but you only want an alarm after a few consecutive readings are outside the limits.

Alarm Filter is the number of consecutive out-of-limit readings required before the alarm becomes armed.

With Alarm Filter = N, the alarm triggers on the (N + 1)th consecutive out-of-limit reading.

At the start of each “out-of-limit” sequence, the system initializes an internal counter C.

The counter works like this:

• C is initially equal to the Alarm Filter value.

• C is decremented by 1 for each reading outside the limits.

• If C = 0, alarm detection is armed, and the next reading outside the limits triggers an alarm.

• After an alarm is generated, C is ignored. Further out-of-limit readings do not generate additional alarms.

• When a reading returns within the limits, C is reset to the Alarm Filter value, and alarm detection is re-enabled.

• When an alarm for the measuring point (MP) is signed, C is reset to the Alarm Filter value, and alarm detection is re-enabled.

Examples

Example 1

Alarm Filter = 0 (alarm is always armed)

Alarm Filter = 0, initial C = 0. The alarm is triggered by the first reading outside the limits.

Example 2

Alarm Filter = 2 (requires 3 consecutive out-of-limit readings)

Alarm Filter = 2, initial C = 2. The alarm is triggered by the third consecutive reading outside the limits.

Example 3

Alarm Filter = 2 (sequence is broken before the alarm triggers)

Alarm Filter = 2, initial C = 2. No alarm is triggered because the reading when C = 0 is within the limits. This resets C back to 2.

This method has the following consequences:

• If an active (not signed) alarm exists and the next reading returns within the limits, the next out-of-limit sequence can generate a new active alarm.

• If an active alarm exists and readings remain outside the limits, no new alarm is activated. If the alarm is signed while readings are still outside the limits, the alarm can be generated again once the alarm filter conditions are met.