Deviation alarms

The current tag value is compared to a target value, and then the absolute value of the difference is compared to one or more limits, expressed as a percent of the range of the tag value.

You can individually configure values and priorities for the minor deviation limit and the major deviation limit, and indicate whether or not each limit is to be used. You can also configure a value for a deviation deadband, also expressed as a percent of the tag's range. This controls the percentage (of the total range) that the tag value must change before it is evaluated to be in alarm.

For example, you configure limits as follows:

• Range of 0 to 100

• Target of 50

• Minor deviation of 10 percent, which sets the minor deviation thresholds at 40 and 60.

• Major deviation of 20 percent, which sets the major deviation thresholds at 30 and 70.

• Deadband of 10 percent

If the tag value is 39, a minor deviation alarm occurs. However, the value must change at least to 50 (40 plus the deadband of 10) before the alarm is evaluated and cleared.

If the tag value is 72, a major deviation alarm occurs. The value drop to at least 61 before the alarm is cleared.