Electronic particle counters, such as the HIAC Contamination Test Center, Model C-600-1, or Royco Electronic Particle Counters, are used to determine counts of the number of particles in the various size ranges. The counts obtained are compared with the maximum allowable under Navy Standard Class 5. Counts that exceed the maximum allowable in any size range make the fluid unsuitable for use in Navy aircraft.

The test results obtained by using automatic particle counters and the contamination analysis kit are not always precisely the same. Both are authorized for fleet use, and you may use either one. Automatic particle counters optically sense particles contained in the fluid sample and electronically size and count them. Most fleet equipments are calibrated so that the smallest particle counted has an effective diameter of 5 microns. Particles smaller than 5 microns, although always present, do not affect the particle count. The contamination analysis kit uses a patch-test method in which the fluid is filtered through a test-filter membrane. The sample causes the membrane to discolor proportionally to the particulate level. The test filters used have a filtration rating of 5 microns (absolute). However, they also retain a large percentage of those particles less than 5 microns in size. The contamination standards provided with the contamination analysis kit are representative of test indications that result if the fluid sample has a particle size distribution (number of particles versus size) typical of that found in the average naval aircraft. Samples from aircraft systems having typical particle size distributions will, therefore, show good correlation if tested using both particle count and patch test methods. Some operating hydraulic systems have peculiar design characteristics, so they produce a particle size distribution different from that found in typical naval aircraft. Fluid samples from these systems generally contain an abnormally large amount of siltlike particles smaller than 5 microns in size. Experience has shown that this condition results from inadequate system filtration or from using hydraulic components that have abnormally high wear rates. It is this type of fluid sample that could produce different results when tested, using both particle-counting and patch-test methods. The difference is caused by the particle counter not counting those particles smaller than 5 microns, while many of them are retained by the patch-test filter membrane, causing it to discolor proportionately. When test results conflict, the equipment tested is considered unacceptable if it fails either test method. The equipment should then be subjected to decontamination.

You need to recognize that the differing test results may indicate system deficiencies and justify a request for an engineering investigation of the equipment. Poor correlation between particle counts and patch tests can result from improper sample-taking procedures, incorrect particle counter calibration, or faulty test procedures. These possibilities must be carefully investigated if a correlation problem is encountered. 

HALOGEN TESTING

The halogen leak detector (fig. 4-5) is used to test hydraulic fluid samples for MIL-C-81302A (Freon) or other chlorinated solvents. The detector is a battery-powered, self-contained instrument. The instrument provides an audible indication, varying from a slow ticking sound to a loud squeal, to indicate the level of the vapor concentration. You can determine the acceptability of unknown hydraulic fluid samples by using the HDL-440 leak detector. To do this, you compare the vapor level of a known hydraulic fluid to that of the unknown hydraulic fluid and determine whether the unknown sample contains more or less than 200 ppm (parts per million) of chlorinated solvents. The calibration standard used in the HDL-440 is hydraulic fluid MIL-H-5606 or MIL-H-83282, which contains a known amount (200 ppm) of MIL-C-81302.

Learning Objective: Recognize decontami-nation methods used on naval aircraft and identify their purpose.

System decontamination is a maintenance operation performed when a system contains fluid that is unacceptable because of contamination. The fluid may be contaminated with foreign matter or it is not considered acceptable for service for some other reason. The purpose of decontamination is to remove foreign matter from the operating fluid or to remove the contaminated fluid itself. Before you can decontaminate an affected system, replace any failed or known contamination-generating components. Other components of the system are not to be disturbed, unless required.

Figure 4-5.HDL-440 halogen leak detector in operation.

There are four basic methods used to decontaminate aircraft hydraulic systems. The methods are recircu-lation cleaning, flushing, purging, and purifying.