Rate of Change Tags

Rate of Change tags are used to display how quickly another tag’s value is changing.  They can be used to watch for system problems such as leaks and blockages.

The rate of change tag calculates the first derivative of another tag’s value.  In general terms, this is defined as the change in value divided by the change in time.  A steady flow has a rate of change of zero.

 

The rate of change will be positive if the source is increasing in value, and negative if the source tag’s value is decreasing. The tag can be configured to report the absolute value instead, thus showing a positive value for any rate of change.

 

For example, the water level in a large holding tank would normally change slowly unless it started to leak.  A tank feeding a process would be expected to have a level decreasing at a minimum rate unless the outlet became blocked.  A rate of change tag can be configured for either situation in order to detect departures from an expected rate of change.  Alarms built into the tag can be triggered when the rate is too fast or too slow.

 

Note that in remote applications, the Rate of Change tag information is distributed from the server to workstations on a regular interval (default: 60 seconds) or when the value changes by a given percentage (default: 5%).  These values are controlled by variables in your configuration file.  See the Programmer’s Guide: RateOfChangeRPCThreshold and RateOfChangeRPCInterval for details.

Rate of Change Tag Type Characteristics

Tag Browser Name:   Rate of Change

Database Table Name:          RateOfChangeTag

Type:   Data Logging and Reporting Tags,  Calculation

Tag Group Memberships:      Analogs, Numeric, Trenders

Log Enabled Variables:          Value

Native Drawing Methods:    Most Analog Input drawing methods.