Configuring Rate of Change Alarms

A rate of change alarm detects when the absolute value of an alarm exceeds a specified limit over a measured interval. A tag is tested for a rate-of-change alarm whenever its value changes. The change rate is calculated using the previous tag value, the time of the previous change, the current tag value, and the current time.

The calculated rate of change over time is compared to a rate-of-change percentage limit specified for a tag. If the calculated rate-of-change is greater than the percentage limit, an alarm condition is set for the tag. A rate-of-change alarm remains active until the rate at which the tag is changing is less than the alarm limit.

To configure a rate of change alarm

1. Open the Tagname Dictionary.

2. Select an existing real or integer tag or create a new tag.

3. Click either Alarms or Details & Alarms at the top of the Tagname Dictionary dialog box to show the alarm details dialog box. The following figure shows only those options that apply to rate-of-change alarms.

   

4. Select the Rate of Change box.

5. In the % per box, enter the maximum allowable percentage change limit.

6. Select Sec, Min, or Hr as the time interval unit.

7. In the Priority box, type a number between 1 and 999 to set the alarm priority.

8. Optionally assign an alarm inhibitor tag for the rate of change alarm.

   1. In the Alarm Inhibitor area, click the button to show the Select Tag dialog box containing a list of defined tags.

   2. Select a tag from the list and click OK. The name of the tag you selected as the inhibitor tag appears in the Alarm Inhibitor box.

   For more information on inhibiting alarms, see Inhibiting Alarms.

9. Click Save.

10. Click Close to close the Tagname Dictionary dialog box.