The Deviation Alarm generates an alarm condition based on the difference between two tags. The classic example is the difference between Measurement and Setpoint.
Any tag in the system can be used to compare against the current tag’s value. The calculation for Deviation Alarm is:
Deviation = Current Tag – Reference Tag
A High Deviation occurs when this Tag’s value is Greater than the Reference Tag’s value.
There are separate High and Low Limits for a Deviation Alarm. There are separate High and Low Deviation Priorities. You can enable just one Deviation Alarm (for example, High Deviation Alarm only). There is a single deadband for Deviation Alarms.
The Alarm Limit is set during configuration of the tag, but can be changed in run-time VIEW while on-line from the point detail display if the user has access to the Security Level 127 configured for Tag Fields. The alarm Limit can also be changed by appending the .ADVLO or .ADVHI to the tagname in the Point Info Dialog Box (point Browser).
The relevant fields to consider when configuring a Deviation Alarm(s) are the Reference Tag, High Deviation Alarm Priority and High Deviation Limit, Low Deviation Alarm Priority, Low Deviation Limit, the Deadband, If Deadband is in Percent of Scale and the Span High and Span Low of the Tag.
To configure a Deviation Alarms see 7.4Alarm Configuration.
1. Enter a non-zero Alarm Priority by clicking on the field in the field labeled Dev Hi Priority. This is an abbreviation for High Deviation Alarm Priority.
2. Enter a numeric value for the High Deviation Alarm Limit in the field labeled Dev Hi Limit. The alarm limit is in Engineering Units.
3. Enter a non-zero Alarm Priority by clicking on the field in the field labeled Dev Lo Priority. This is an abbreviation for Low Deviation Alarm Priority.
4. Enter a numeric value for the Low Deviation Alarm Limit in the field labeled Dev Lo Limit. The alarm limit is in Engineering Units. Do not enter a minus sign.
5. Select Yes or No for field labeled Dev Percentage . If % is selected then the Deadband is in percent of scale.
6. Enter a Value for the Deadband in the Dev DB.
7. Enter the Tag Name of the Reference Tag in the field labeled Dev. Tag Name .