If the difference between two consecutive values is greater than the specified percentage of full span, a Rate-Of-Change alarm will be activated.
The ROC alarm is in Percent of SPAN. Value applies to both increase and decrease in tag's value (i.e. is symmetrical). The actual rate is a function of this vale and the Scan Rate of the device. If the device scans Once per Second, then rate is change per second. If device scans at 100 milliseconds, then Rate of Change = 10 is also once per second. Rate of Change alarming is the idea that a measurement is changing too fast. An example, is change in Level of a Tank indicating flow cavitations in a pipe.
For IO Tags, ROC Alarming is Dependent on the scan rate of the communication port. For Accumulation, Calculation and Constant Point Tags, any change greater than the ROC percentage will generate a ROC alarm.
The point will remain in ROC alarm until it is acknowledged and the tag’s value changes once by a value less than the ROC percentage.
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 .ALMROC to the tagname in the Point Info Dialog Box (point Browser).
The relevant fields to consider when configuring a Low-Low Alarm are the ROC Alarm Priority, the ROC Alarm Limit, the Span High and Span Low of the Tag and the Scan Rate of the Comport the Tag is read through.
To configure a Rate-of Change (ROC) Alarm see 7.4Alarm Configuration.
1. Enter a non-zero Alarm Priority by clicking on the field in the field labeled ROC Priority. This is an abbreviation for Rate of Change Alarm Priority.
2. Enter a numeric value for the Rate of Change - ROC. The alarm limit is in engineering Units.
The ROC alarm is in Percent of SPAN (i.e. % of Span High minus Span Low). The Limit applies to both increase and decrease in tag's value (i.e. symmetrical). The actual rate is a function of this vale and the Scan Rate of the device. If the device scans Once per Second, then rate is change per second. If device scans at 100 milliseconds, then Rate of Change = 10 is also once per second. Rate of Change alarming is the idea that a measurement is changing too fast. For example, flow in a pipe causing cavitations.