A Rateof Change Alarmis commonly used to report an unusual change or lack of change in a measured value related to the speed at which the value has changed. The Rateof Change Alarmbecomes active when the rate at which the value changes exceeds or drops below a defined limit.

A Rateof Changeis measured in some time unit, such as seconds or minutes and some unit of measure such as percent or meter. For example, a tank may have a High limit for the Rateof Changeof its level (measured in meters) which would be 4 meters per minute. If the tank level changes at a rate that is greater than 4 meters per minute then the High sub state is entered.