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"Calc Failed" is usually written to the PI Data Archive from a Performance Equation (PE) whenever the PE includes a mathematical error, or points used in the equation have values like "Bad" or "No Data." In order to prevent these "Calc Failed" messages from being archived, check the syntax of your PE and include a check for source point error values in your PE (KB00498 has an example on how to do this).
The difference in results that you get when you click the "Evaluate" button and when the PE is triggered is due to the input values it uses at that moment. When you click the "Evaluate" button, the PE uses the most recently archived values for the tags included in the PE at that moment. When it is triggered by the PE Scheduler Service, it uses the snapshot values at the time that it is triggered.
My suspicion is that at the moments that the PE outputs "Calc Failed," one of the input tags has a value that doesn't allow the PE to be evaluated properly.
Here are some first troubleshooting steps to take to diagnose this issue:
- Check the syntax for your PE to see if the expression is valid.
- Check when it's scheduled to evaluate - is it triggered by a tag, or is it clock-based scheduled?
- Check that the tags used in the PE have a healthy value at the time the PE was evaluating.
There was an issue in the triggered tag and once that's corrected the PE tag working fine.