AN ANALYSIS OF WEIGHT CHANGE IN FILTERS

Abstract 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. Introduction Gravimetric analysis is used to detect and measure the presence of insoluble particulate contaminants in hydraulic fluid. This is essential for the performance and longevity of equipment because particulate contaminants can cause filter failure and damage other components of the hydraulic system, resulting in excessive wear and even system failure. For this reason, accuracy is of the utmost importance. United States military hydraulic fluids must pass conformance tests on each batch purchased, as described in the military specifications (MIL-PRF-5606, MIL-PRF-83282, MIL-PRF-87257). Fluids exceeding the specified gravimetric increase in weight, after a fluid is filtered, are rejected. Because of discrepancies in experimental results using the military’s currently prescribed methodology for gravimetric analysis of hydraulic fluids, a more reliable, accurate, and reproducible method is needed to ensure proper acceptance of hydraulic fluids. Based on this work MIL-PRF-5606 and MIL-PRF-87257 specifications have been changed to require two stacked polypropylene filters. In this investigation, a step-by-step approach was taken to isolate possible sources of error and develop alternate methods to eliminate them. The response of filters to various environmental conditions and solvent interaction has been examined in detail. Experiments were devised to better understand the lack of repeatability of the method. This would allow the military hydraulic fluid specifications to require specific practices so that various fluid suppliers and the military laboratories can improve reproducibility. The primary purpose of these experiments was to develop accurate, precise methods for