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Ensuring Precision with Excdev and Compdev

We stumbled upon a very interesting debate the other day regarding exception and compression settings on LinkedIn.

An inquiry with a specific use-case was posed relating to exception and compression settings on the PI System and what might happen.

“We are using PI historian as a data source for an MES application, and need to ensure that data (like pH and temperature) is recorded to the precision specified in the Master Batch Record.

For instance, specification is Temp >= 121.5 C.

In the MES, we read the value from PI then round to 0.1 C. Seems straightforward, I should set CompDev = 0.1 and ExcDev = 0.05.

This should ensure archiving of temperature changes greater than 0.1 C. However, if the value changes from 121.44 C (rounds to 121.4 C, not passing) to 121.45 C (rounds to 121.5 C, passing) I don’t think the 121.45 will trigger exception and archiving. ” – read more here


The proposal was to use the standard 1% and 2% but as we’ve discovered by using CompressionInsight, 1 and 2 percent settings are not always the best. You can find out more about our tool by going to www.patterndiscovery.com/compressioninsight

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