About rate-of-change (ROC) alarm checking

Limit checking for each ROC alarm is as follows.

• If the rate evaluation period has expired, then calculate the ROC as:

  • (PV - last PV ) / (current time - last time)

  • convert rate-of-change to target units (/sec, /min, /hour or /day)

• If rate-of-change < -Decreasing Hi Limit, then set the Decreasing High condition. Else, clear the Decreasing High condition.

• If rate-of-change > Increasing Hi Limit, then set the Increasing High condition. Else, clear the Increasing High condition.

Rate-of-change alarms are used to detect and report a condition when the process value (PV) increases or decreases at too fast of a rate. You can specify a limit for both the increasing and decreasing directions. For the increasing limit, if the PV increases at a rate greater than the limit, an alarm is generated. Similarly, for the decreasing limit, if the PV decreases at a rate greater than the limit, an alarm is generated. The limit value in either case is a positive number.

In addition to specifying the rate limit value, the time dimension for the rate must be specified depending on the time dynamics of the variable being measured. The choices are per second, per minute, per hour, and per day. Finally, for calculation of the rate itself, the time interval at which the rate (or slope between points) is calculated is also specified as a time interval in milliseconds.

So, for example, if you want to put a rate limit on the rate at which a number of gallons accumulates in a tank as 200 gallons per minute, and you want to specify the calculation to be performed every 5 seconds, then specify:

IncreasingHi.Limit = 200

Rate.Units = /Min

EvalPeriod = 5000