Number of changes
Trigger an alarm when a tag changes too often (or too rarely) over a period.
A number of changes condition (also called a frequency condition) watches how often a tag changes over a window of time, rather than its value. Use it to catch equipment that’s cycling too often — a pump short-cycling, a valve hunting — or, the other way round, something that should be changing but isn’t.
Settings
| Setting | What it does |
|---|---|
| Source tag | The tag whose changes are counted. |
| Operator | How the count is compared: equal to, greater than, or less than. |
| Number of changes | The count to compare against. |
| Period | The length of the window the changes are counted over, with its time unit. |
How it triggers
THMI counts how many times the source tag changes within each period, and applies the operator to that count. For example, greater than 10 changes per 1 hour alarms when a pump starts and stops more than ten times in an hour; less than 1 change per 1 day alarms when a value that should move hasn’t moved all day.