After you process the fluid sample, visually compare the test filter or patch with the contamination standards. To determine the particulate contami-nation level, compare the shade and color of the test patch with the corresponding colors of the contamination standards. If the test patch displays a rust or tan color, use the tan standard patch. If the test patch is gray, use the gray standard patch. In any case, you should follow the operating instructions contained in the contamination standards. Tan patches occur when rust or iron chlorides are formed in the system, or the system contains abnormal amounts of silica (sand). Gray patches are typical of systems containing normal proportions of common wear materials and external contaminants.

The maximum acceptable particulate level for naval aircraft is Navy Standard Class 5. For related SE, the maximum acceptable particulate level is Navy Standard Class 3. If visible free water is present in either the sample bottle or on the surface of the test filter (at completion of filtration), the system under test is rejected. A stain on the test filter membrane may be an indication of the presence of free water. When a stain is seen on the test filter, obtain a second fluid sample from the system under test and process it so that water content can be confirmed prior to system rejection. Make sure that observed water is not a result of atmospheric condensation during the sampling process.

If the system under test fails to meet the Navy Standard Class 5 particulate requirement or if it exhibits free water, the system must be decon-taminated according to the procedures listed in the applicable MIM.

Hydraulic fluid samples obtained from filter bowls and/or elements cannot be used to determine system contamination levels. The following combination of factors makes the filter bowl sample useless when determining the systems level of contamination: sedimentation, functional location, and/or an inability to obtain the required 100 milliliters of fluid. Filter bowl residue analysis may be used to monitor hydraulic system degradation, monitor for suspected impending component failure, or isolate a cause for continued contaminant generation.

Evaluate filter bowl patch residues by following the procedures in applicable manuals. As you gain experience about normal contaminates for specific aircraft systems and hours of operation, you will be able to evaluate filter bowl patch residue. Through experience, analysis of main pressure line and case drain filter bowl residues is useful in verifying failure of the upstream hydraulic pump, as large amounts of metal usually show up in these particular assemblies. Residue in other filter assemblies is affected by so many other components and factors that analysis is difficult. Filter bowl residues should be analyzed only as a means of identifying or verifying suspected component failure. Examine residue from those filter assemblies directly downstream from the component.