Chairman’s Column June 2016


Everyone understands the importance that laboratories play in the development and production of lubricants, but not everyone fully appreciates that the same labs can and do struggle to replicate their test result consistently, and the consequences that this has for lubricant formulators and blenders.


Let me explain.


Lubricants are manufactured to a particular specification, and an integral part of this will include the test methods required to ensure that finished products meet the necessary performance standards. In our industry we tend to adopt the methods developed and published by the American Society for Testing and Material (ASTM). There are several hundreds of these, each describing the scope of the test method, the equipment to use, and describing exactly how the test should be carried out and subsequently reported. The test method also indicates the degree of precision that can be expected, assuming of course that the procedure is carried out correctly.


The true fact however, is that any test has some degree of uncertainty associated with it, and again this is defined and alluded to in the method.


For example:-


The low temperature viscosity of gear oils is measured using a Brookfield Viscometer, and the universally accepted test method is ASTM D2983. This method highlights the variation that can be expected when the procedure is carried out. If two identical samples are tested by the same operator, using the same equipment within a short space of time, the two results should fall within 13.5 % of their mean - If this is the case then the test is operating properly.


This is the “Repeatability” factor. On the other hand, if exactly the same material is tested in a second laboratory, the two results should fall within 18.1 % assuming of course that both laboratories have done everything correctly. This is known as “Reproducibility”.


So, why does this matter to us?


There are certain aspects of a lubricant’s performance that are so important that they are deemed to be “Critical Parameters” and the Brookfield Viscosity of gear oils is one of them. A critical parameter is defined as one where customers should expect a high degree of assurance that the true value meets or exceeds the specification limit. In practice this means that if the customer tests the product properly, a pass-result should be obtained despite some variability in the test method.


36 LUBE MAGAZINE NO.133 JUNE 2016


Andrew Goddard Chairman Verification of Lubricant Specifications


LINK www.ukla-vls.org.uk


In order to achieve this the blender will allow for the precision (or imprecision) of the test and manufacture the product well within the specification limit.


ASTM gives guidance on the limits, but in the case of Brookfield a specification limit of 150,000 cP could effectively be reduced to show a pass-result of 134,000 cP. This figure is obtained statistically but I won’t bore you with the details here.


If the concepts of test-precision and critical parameters are new to some readers, and these are certainly issues that ought to be of interest to blenders, it might be worth speaking to your technology providers who will undoubtedly provide all the necessary information.


Any sample submitted for testing under the VLS scheme will be deemed to be a fail if a critical parameter falls outside the specification limit. This possibility can be avoided if every batch of product is released with this tolerance in place. While it should now be clear that a degree of imprecision is to be expected in laboratory results (and indeed nothing can be done to prevent it completely) there is a second important factor that can influence results, and this is accuracy. Accuracy can be thought of as a lack of bias in test results and can be minimised by proper equipment calibration.


To check that results are accurate, laboratories often take part in cross-check programmes. These tend to be few and far between and can be comparatively expensive. VLS / UKLA has been canvassing members to see whether there is sufficient interest in developing our own scheme, specifically tailored to the UK lubricants industry. Ensuring our laboratories are properly calibrated and able to operate without bias, can only be advantageous to blenders and consumers alike, so I hope that any operation with a laboratory looks seriously at how participating in such a scheme might help them.


Together let’s try and eradicate bias from test results even if some test imprecision is here to stay.


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