An Analysis of Weight Change in Filters

Due to recent repeatability problems with the gravimetric analysis procedure used for military conformance testing of military specification hydraulic fluids, an in-depth investigation was undertaken to evaluate possible sources of error. Many sources of weight change exist, including particulate and water accumulation from air, buildup of static electricity causing interference with the metal microbalance pan, and removal of extractables from filters during testing. Various procedures and filter types were investigated. Results show that the current testing procedures have several key shortcomings, and that the filters allowed for use in military conformance testing are incapable of delivering the required accuracy. By combining the application of slightly modified existing procedures with different filter materials, the required accuracy was achieved.