A Rate of Change Alarm is 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 Rate of Change Alarm becomes active when the rate at which the value changes exceeds or drops below a defined limit.
A Rate of Change is 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 Rate of Change of 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.