This SAE Recommended Practice describes an empirical method for determining the theoretical ash content of aviation piston engine lubricating oils by calculating the equivalent weight of metallic oxides formed at 775 °C based on the metallic elemental concentration.
The calculation method of ash determination may be used as an alternate to ASTM D 482 for application to the standards for aviation piston engine lubricating oils.
Field of Application
This procedure is recommended for use in the qualification, manufacturing, and quality assurance testing of aviation piston engine lubricating oils where the ash content is limited to a maximum of 0.011%.
The ash content as measured by ASTM D 482 has very poor precision and repeatability for lubricants having low ash content. The precision statement for the method states that the reproducibility of a sample in the range of 0.001 to 0.079% ash content is 0.005%. Further, the bias of the test cannot be determined and there is no standard reference material containing a known level of ash for this method. This poor precision has led to numerous problems concerning the actual ash content of products on many occasions. The test method is valuable when run by experienced operators, but can provide dubious information if run under the general conditions stated in the method. For example, to obtain repeatable results, a platinum crucible must always be used in place of the silica or porcelain crucibles listed as equivalent substitutes. Meticulous care and procedural knowledge must be used by experienced operators for the method to be productive.
The notes provided in ASTM D 482 also suggest that this method may not be appropriate for oils containing ashless additives or for oils containing certain phosphorous compounds which may now be in use in aviation lubricants. For oils containing additives an alternate method, ASTM D 874, is suggested. However, ASTM D 874 includes additional restrictions and reservations which question the suitability of that method as an acceptable alternate or replacement for ASTM D 482 for low ash containing lubricating oils.
These contradictions have led to the development of a "calculated ash content" method as a recommended alternative for use with aviation piston engine oils. This procedure is based on the ideal conversion of selected metallic elements to their theoretical oxide weights and then summing the components to obtain a total value. The seven metallic elements chosen were selected as being those most likely to be present in lubricant manufacturing and packaging plants. As such they would also be the most likely contaminants to be found in the aviation lubricants specified.