Rate of change checks

Analog and rate instruments register a value that increases or decreases depending on process conditions. The rate-of-change (ROC) is determined by normalizing the last scan value and the current value to the unit time (in seconds).

Some instruments have a manufacturer’s specification indicating that errors can occur if a certain ROC value is exceeded. At times, a monitored process variable can require a ROC limit to prevent errors based on rapid adjustments to the system. In these cases, select the Rate of Change Alarm checkbox on the Alarming tab in the analog Row Edit dialog box, and enter the maximum allowable ROC (in Engineering Units (EGUs) per second) in the Rate of Change Limit field. An alarm is generated if the calculated ROC is greater than this amount. For further information, refer to RealTime tables.

The correct limit is determined by the instrument specifications and the process limitations.