8 Tips for Setting Flagging Limits

Our customers often ask us to display the condemning limits for the tests performed on their sample, on the actual sample report they receive. Believe it or not, there is a reason why we exclude the limits from the report. It’s not a simple case of we are unable to, or we do not want to. The real reason relates to the complexity of using the correct limits for each parameter, based on several pieces of equipment and application information; as well as looking at trends of historical samples from the same unit, rate of change and any other applicable information relating to the sample and component. These differentiating factors result in limits changing dynamically on a sample by sample case, dependent on this information.

When setting limits, the following information must be taken into consideration on each sample, below are some examples:

  1. Equipment Type: (e.g. hydraulic)
  2. Specific Application: (e.g. Injection Moulding Machine)
  3. Equipment Manufacturer: (e.g. ARBURG)
  4. Equipment Model: (e.g. 1120H)
  5. Industry Type: (e.g. industrial manufacturing)
  6. Filter Type: (e.g. INLINE)
  7. Filter Micron Rating: (e.g. 10 micron)
  8. Sump Capacity: (e.g. 1000 litres)

Once limits are set, it’s best not to use them exactly. If for example, the iron ppm condemning limit for a hydraulic oil sample is set at 30ppm, customers may not expect the iron result to be flagged until it exceeds 30ppm. However, if historical samples from the same unit have consistently shown iron results of less than 5ppm, and on the latest sample the results are 22ppm, this is considered an abnormal trend, and exceeds expected Rate of Change values. In this instance, the result of 22ppm would be flagged as abnormal for these reasons, but is still below the statistical condemning limit level of 30ppm. So, if we did display this limit of 30ppm on the sample report, the customer may ask why the result has been flagged at 22ppm as it is under the threshold, which would in turn lead to more confusion and questions. Another important point to note is that combinations of metals present also affect the limits used, as they may indicate abnormal wear to a specific component.

Most OEM’s will provide some form of condemning limits in their technical bulletins or operational guides, but often come with the caveat that ‘these fixed limits should only be used if the laboratory cannot provide limits based on robust statistical analysis of previous results’, which further confirms the need for an accurate statistical model to be used on all samples tested.

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Published April 24, 2018