during SE-powered operation. A makeup reservoir is a configuration in which all of the system return line fluid does not pass through the reservoir. Fluid exchange in the reservoir is limited, and results only from the changes in fluid volume that occur elsewhere in the system. You should be able to use the sampling point after an aircraft flight, without requiring the use of external SE. Taking a sample with the aircraft engines turning is satisfactory, provided no personnel hazards are involved. You should be able to use the sampling point when the system is being powered by external SE, or immediately after such an operation. The sampling point should be next, or reasonably close, to the main body or stream of fluid being sampled. A minimum amount of static fluid is acceptable; however, purge it when you start the sample flow. Do not take a sample from a point located in an area of high sedimentation. If you cannot avoid doing this, make sure sedimentation effects are minimized by discarding an initial quantity of the sample fluid drawn. Ideally, sample fluid should be obtained from turbulent high-flow areas. When you take a sample at the sampling point, do not introduce significant external contaminants into the fluid collected. If you preclean the external parts of the valve or fitting and self-flush the valve or fitting before the sample is taken, the background level attributable to the sample point itself should not exceed 10 percent of the normally observed particulate level. The internal porting of the sampling point should not impede the passage of hard particulate matter up to 500 microns in diameter. The sampling point should be accessible and convenient. There must be sufficient clearance beneath the valve or fitting to position the sample collection bottle. Under normal system operating pressure, the sample fluid flow rate should be between 100 and 1,000 milliliters per minute (approximately 3 to 30 fluid ounces). The flow rate should be manageable, and the time required to collect the required sample should not be excessive. The mechanical integrity of the sampling valve or fitting should not degrade because of repeated use. When not in use, it is mechanically secured in the closed position. ANALYSIS METHODS Learning Objective: Recognize the analysis methods used to identify and measure contamination. Contamination analysis is used to determine the particulate level of a hydraulic system and the presence of free water or other foreign substances. The methods used to identify and measure contamination are patch testing, electronic particle count analysis, and halogen testing. NOTE: The President of the United States has decreed that all production of ozone-depleting substances will cease by 31 December 1995. NAVAIRSYSCOM is striving to eliminate MIL-C-81302 (FREON) much sooner. MIL-C-81302 has already been eliminated in some of the geographical areas that the Navy presently operates within. MIL-T-81533 (TRIC) is also on the hazardous material (HAZMAT) reduction list. In the event these materials have been eliminated in your command or geographical areas, P-D-680 is the recommended solvent for performing patch tests using the tans standard. Before performing a patch test, it is imperative that you check the NAVAIR 01-1A-17, Navy directives, and the Federal and local HAZMAT regulations for the proper material to use in your command and geographical area. This note should be applied to all references to the use of these materials throughout this chapter. PATCH TESTING Patch testing is the primary contamination measurement method used at all levels of maintenance. The P/N57L414 contamination analysis kit (fig. 4-3) is used to perform patch testing. In the patch test method, a fluid sample of known volume is filtered through a filter membrane of known porosity. When the fluid passes through the filter, all particulate matter in excess of a size determined by the filter characteristics is retained on the surface of the membrane. The retention of particulate matter causes the membrane to discolor proportionally to the particulate level of the fluid sample. Free water will appear either as droplets during the fluid sample processing or as a stain on the test filter. The typical color of contamination in any given system is usually uniform. The degree of filter membrane discoloration correlates to a level of particulate contamination. By visually comparing the test filter with contamination standards that represent known contamination levels, the contaminant level of the system can be determined. 4-8