IP 247: Determination of engine cleanliness and wear - Merit rating system method

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  • Method adopted/last revised: 1969
  • Method reapproved: 2001
  • REF/ISBN: IP247-2934869
  • Status: Current
  • First printed in STM books: January 1969


In compiling this rating system, several others have been considered including:

  • that published formerly in method IP124/55
  • a system devised by the Petroleum Chemistry and Technology Division, Royal Aircraft Establishment, Cobham, Surrey (formerly the Admiralty Oil Laboratory), Admiralty Oil Test Laboratory.
  • the Deposit Rating Scales published by the Co-ordinating Research Council of the USA.

It is appreciated that for practical reasons the assessments of deposits in terms of numerical ratings can, in most cases, be conveniently carried out on a demerit basis. However, it is believed that a system in which the final assessment is presented in terms of Merit Ratings gives a more tangible appreciation of the relative condition of engine components. Therefore, where it has been considered desirable, the present system makes use of a demerit scale for relating observation to numerical ratings and the final Merit Rating is obtained by subtracting the demerit ratings from the maximum possible demerit for a particular component or part of component.

In relation to the application of the merit ratings obtained, it must be emphasized that these are purely arbitrary and are intended for comparing the results obtained using the same engine type and test procedure. They will not necessarily enable direct comparisons to be made between the results from different engine test procedures, neither will they necessarily correlate with the service performance of the oils. To make such comparisons it is necessary to apply weighting factors to individual merit ratings, which can be established only by correlation programmes either between different engine test procedures or between a given engine test procedure and data obtained from a particular field.

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