The period used to determine when a Rate of Change
alarm will be triggered for a variable tag. The Rate specified in
Analog Alarm Properties is divided by this period to determine the
`maximum rate" at which the value of the variable tag can change.
At each Scan Time (see [Alarm]ScanTime), CitectSCADA checks the value of the
tag. If its rate of change is greater than the `maximum rate", a
Rate of Change Alarm is triggered.

For example, if your goal is to minimize the
likelihood a tank from filling too quickly, you might configure a
rate of change alarm, using an [Alarm]Period of 60 seconds, an
[Alarm]ScanTime of 1 second, and a Rate (see above) of 300 liters.
This means that the maximum allowable rate of change for the tank
level is 5 l/sec (300 liters / 60 seconds). CitectSCADA calculates the actual rate
of change at each ScanTime. i.e. Every second, it checks the
current level of the tank and compares it to the level recorded a
second earlier. If the actual rate of change is, say, 8 l/sec, a
Rate of Change Alarm is triggered immediately.