Configure rate of change alarms

A rate of change alarm identifies when an attribute value is changing too quickly over time. For example, you can set a rate of change alarm for a tank level that indicates when the pump inlet pressure increases too quickly.

Alarm limits are expressed in the Engineering Units of the attribute’s value over an interval, which can be per second, minute, hour, or day.

Rate of change is the calculated slope, which is the absolute difference between the current and previous attribute values divided by a specified interval. When the slope (positive or negative) exceeds a specified value, a rate of change alarm occurs.

As described above (see The rate of change (ROC) alarms feature) the change limits, direction of change (up or down), time interval for the change, and the evaluation interval are configurable.

Example alarm configuration:

• An ROC alarm limit is set to issue an alarm if the volume increases more than 5 liters (up limit) per second ("Charges per" <unit of time>).

• The evaluation interval is set at 5000 ms (5 seconds).

Alarm condition based on the example alarm configuration:

• After 5 seconds (the evaluation interval), the attribute value has changed from 17 to 45 liters, resulting in an increase of 28 liters (45-17=28).

• The calculated ROC over the evaluation period is 5.6 liters per second (28/5=5.6). Since this is more than the configured ROC limit (5 liters per second), an alarm condition exists.

  

You can select and configure the value and priority for the upward and downward ROC limits. You can set individual messages to be shown when the upward or downward ROC limit is exceeded.

The timestamp when a rate of change alarm becomes active or inactive is the most current timestamp of the corresponding input value. If there is no timestamp associated with the alarmed value, the AppEngine timestamp is used instead.