Strategies for Accurate Totaliser Calculations in PI System

Strategies for Accurate Totaliser Calculations in PI System

When working with PI System, particularly in time-series data management for monitoring and control systems, users often encounter challenges with calculating accurate totals, especially in the context of flow meters. A common issue arises when totalisers trend inaccurately during periods of no flow, leading to misrepresentative cumulative totals.

Key Challenges in Totaliser Calculations

In the context of a user's PI System experience, we encounter two primary difficulties:

1. Downward Trending Totalisers: Annual totals decrease erroneously during zero-flow periods.
2. Unexpected Reset Times: Daily totalisers reset at unanticipated times (e.g., 1 AM instead of midnight).

These issues are often linked to the methodologies employed in calculating time-weighted totals and the nuances present in data interpolation processes.

Addressing Downward Totaliser Trends

The issue of downward trending totalisers is frequently a result of the underlying TagTot function, which calculates totals based on "good" data and scales it by the percentage of this data within the total time frame. This becomes problematic when discrete zero values—common in situations of no flow—affect the calculation.

Solutions for Accurate Totaliser Reporting

Several methods can mitigate inaccuracies in totaliser computations:

1. Intermediate Attributes:

   • Implement an intermediate attribute that replaces bad data with zero values.
   • This approach ensures that all data considered by the totaliser reflects actual conditions, preventing incorrect downward trends.

2. Use Alternative Functions:

   • Consider using the Total function in conjunction with RecordedValues, MapData, and FilterData to gain more explicit control over input data treatment.
   • This approach allows a more refined and accurate calculation of totals.

3. Manual Total Calculation:

   • For more granular control, manually calculate the totals by iteratively reading from and writing to a PI Point.
   • This manual method can be complex but offers heightened precision and data integrity.

4. PctGood Adjustments:

   • By incorporating the PctGood function, multiply the calculated total by PctGood/100, correcting for discrepancies due to bad data input.

5. Optimized Triggering:

   • Trigger calculations based on flow meter values rather than on an hourly basis. Further, for syncing issues, set calculations a few seconds or minutes past the hour to include relevant data inputs.
   • Implement constraints like TagTot('att', 't', PrevEvent('att, '*')) to focus strictly on intervals with available data.

Resolving Reset Timing Issues

Totalisers sometimes inexplicably reset at non-standard timings due to differences in time zone settings between the user’s local machine and the PI System analysis server.

• Ensure that time zone settings are correctly configured and synchronized across all relevant PI components to maintain alignment in system time references.

Conclusion

By addressing these common pitfalls in PI System totaliser calculations, users can achieve more accurate and reliable monitoring outputs, enhancing the effectiveness of data-driven decision-making. Implementing these strategies can significantly improve the precision of total flow measurements, ultimately leading to better operational outcomes within the PI environment.