Precision in TAN and viscosity measurements

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I estimate that our lab provides viscosity readings that are =/- 4%. While this seems pretty good, it is way off of the published accuracy for the the ASTM method. The lab uses an automated viscosimeter, but I'm not sure what brand.

Similarly, our TAN numbers seem highly variable. The numbers can vary as much as 25% with (not so) infrequent values even farther off. Again we are using an automated titrator, though I don't know which one.

Based on this precision, it is pretty difficult to evaluate the oil condition.

My questions are:

Do these precision values match your experience?

If you have "better" results, what instruments do you use?

Are there QA techniques that could improve our precision?

How do you "shop" for good lab equipment based on vendor technical data and claims.

10/12/059:08 AM

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Original Post

aaronblackMember

I am familiar with some lab accuracies, and 4% for a viscosity at a commercial lab is not bad. TAN at 25% variance is high. I would look into why the numbers fluctuate that much. If it is an automated system, you should just go step by step and see where the variable could be put into the result. Is the measuring device you are using to prepare samples functioning properly & is it tested regularly for accuracy? Start where the samples enter the system and move on from there. TAN testing is rough on electrodes - is it possible these are not working properly anymore? Are they properly cared for before and after testing? Is the software set up to read the data points properly? Start asking these kinds of questions to yourself and this should lead you to the source of the problem.

Quality control measures can ensure you are continuing to run within guidelines you set for yourself, but if your equipment cannot get you to the precision you need, they won't help.

10/13/052:21 PM

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