Why checking your data is so critical

This data logging is missing data, making the conclusions it yields inaccurate.

A compressed air expert was asked to help with some calculations of a base case for a potential project. The compressor supplier provided a free compressed air assessment where the inlet flow of the compressor was logged with an anemometer, rather than measuring the output of the compressor discharge.

The data showed fairly low flow and inefficient compressor operation. The two-week period that was captured was projected out to a full year, so an annual consumption could be estimated. The local power utility was ready to offer a significant financial incentive on the basis of this flow data — but on close inspection, the expert found there were some troubling errors in the way the assessment was done.

A close inspection of the data showed that the flow meter was not working correctly; there were at least 10 gaps in the data where the compressor was obviously putting out some compressed air, yet the data showed zero! This was not obvious when looking at the data in a spreadsheet. The missing data caused the average flow to show as fairly low, setting up the potential for the power utility to pay out too much incentive — and for the customer to not get the energy savings expected.

There was also trouble with the sampling period. If one looks at a calendar for 2019, they would find that the period measured spanned the Easter holiday break, including at least two plant production outages where the compressor was turned off completely. The power and flow profile captured during this period were not normal, and therefore cannot be used to accurately predict annual compressed air costs for a baseline.

This shows that you (or your service provider) should be careful in selecting the method of flow measurement and the sampling period. Measuring inlet air on a compressor is full of potential pitfalls. It is best to use an inline flow meter.

What’s more, never try to data log to produce a baseline when there are holidays in the sample period. Make sure the data is carefully inspected to detect errors before putting yourself out on a limb. The best case is to both measure flow with an accurate flow meter and calculate flow compressor amps as a backup check to ensure the flow meter reading is accurate.

Ron Marshall • Contributing Editor