Creep Detection

Creep Detection can detect calibration deterioration in analog and rate instruments. Since analog and rate instruments can creep out of their calibrated state over time, Creep Detection can be configured to measure the difference between the current value and the creep setpoint and trigger an alarm if the difference exceeds the maximum deviation value specified for that instrument.

You may need to properly calibrate analog and rate instruments to ensure that their values are accurate and do not creep out of the calibrated state. Analog and rate records have an option that allows you to store an initial value or creep setpoint, updated on startup and whenever a creep alarm is generated. This initial value can then be compared to all subsequent values. This comparison measures any creep deviation of the input value. The amount of creep is the absolute difference between the current scan value and the creep setpoint that was set when the last creep alarm occurred:

creep = currentvalue - setpoint

If the analog record is supposed to test for creeping, select the Creep Detection checkbox on the Alarming tab in the analog Row Edit dialog box. You should do this when the instrument specifications indicate a maximum raw deviation value that is acceptable before calibration deterioration occurs. Convert this raw value to the applicable Engineering Units (EGUs) for the record and enter it in the Deviation Alarm Limit field. An alarm is generated if the calculated creep exceeds this limit.