Rate of change alarms

A tag’s current and previous values are compared over a measured period. The rate of change of a tag’s value is calculated using the previous tag value, the time of the previous change, the current tag value, and the current time.

A tag is tested for a rate-of-change alarm whenever its value changes.The one exception is the initial tag value when an application starts running in WindowViewer. In that case, the initial value is ignored and the first rate of change comparison is made between the second and third changes in a tag’s value. Thereafter, rate of change measurements are made between consecutive tag value changes.

If the absolute difference in values between consecutive tag changes exceeds a specified limit, a rate of change alarm occurs. The rate of change limit is expressed as a percentage of the tag’s value over a time interval, which can be per second, per minute, or per hour. You can configure the value and priority of the ROC limit, and whether or not the limit is to be used.