Historical Deadband - Percent

How exactly does the historical deadband work when the deadband mode is percent?
If historical deadband is 1, does it mean that the tag will register when there has been a 1% change from its previous value?


Best Regards.


Edit: Well actually no, UI videos got me confused, listen to @nminchin.

Edit 2: I've seen two UI videos talking about "the previous value", even though it doesn't make sense. The information in the videos is obviously wrong.

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The deadband % change is based on the Engineering Range of the tag.

If your engineering range is 0-200 and your deadband is percent and set to 1%, then if the tag value changes by more than or equal to 1% of 200 (2), then the value will be passed to the historian to be assessed for recording. Whether it will actually be recorded or not depends on your "Deadband Style" setting. If this is set to "Discrete", then it will be logged. If it's set to "Analogue", then it'll use the analogue algorithm to determine whether or not to store the point or not.

The same thing applies to the tag's numeric deadband as well; this uses the % of the engineering range to determine whether or not to update the tag's value.

You can demonstrate this with a memory tag and an expression tag, where the memory tag acts as the "source" tag, and the expression tag acts as for example a tag bound to an opc value:

  "name": "Deadbands",
  "tagType": "Folder",
  "tags": [
      "valueSource": "expr",
      "expression": "{[.]Source}",
      "name": "Tag",
      "deadband": 10.0,
      "deadbandMode": "Percent",
      "tagType": "AtomicTag"
      "valueSource": "memory",
      "name": "Source",
      "value": 71,
      "tagType": "AtomicTag"

The "Tag" is set to 10% deadband and it's value is tied to the "Source" tag. If you change the "Source" tag value, you'll see how the "Tag" value behaves.

This is from the tag alarm properties section in the user manual, but applies to history and numeric value as well:

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@andrew.budaiev and @nminchin, thanks for the explanation.

So it's not possible to record a value when it changed a certain percentage from its previous value?
For example, I want to log a value when it changes 1%:

  • Historical DeadBand: 1%
  • Previous Value: 5
  • Current Value: 5.06
  • Register?: ((5.06 - 5) / 5) x 100 = 1.2%.
    1.2% > 1% -> Yes

Best Regards.

No, I don't believe you can have a non-linear deadband like that, although it does sound kind of useful for high ranging values. It sounds like more load for the tag system though, constantly recalculating the deadbands of every tag. For one of our systems with 600k+ tags, that wouldn't be a good thing!