A deviation Alarm is commonly used to report an excess deviation between a desired setpoint level of a process value and an actual measurement of that value. The deviation Alarm becomes active when the deviation exceeds or drops below a defined limit.
For example, if a setpoint had a value of 10, a high deviation Alarm limit of 2 and a low deviation Alarm limit of -1 then the low sub state is entered if the process value drops below 9; the high sub state is entered if the process value raises above 12. If the setpoint is changed to 11 then the new deviation values would be 10 and 13 respectively. The setpoint can be fixed by a configuration, adjusted by an Operator or it can be adjusted by an algorithm, the actual functionality exposed by the setpoint is application specific. The deviation Alarm can also be used to report a problem between a redundant data source where the difference between the primary source and the secondary source exceeds the included limit. In this case, the SetpointNode would point to the secondary source.
The LowLimit and LowLowLimit shall be negative, indicating a number below the target value and the HighLimit and HighHighLimit shall be positive, indicating a number above the target value. If provided, the limits shall not be zero and shall follow these rules:
For example, if the LowLimit is -2 then a LowLowLimit of -1 would not be allowed, but a LowLowLimit of -3 would be allowed.