difficult. Filter bowl residues should be analyzed only
the gray standard patch. In any case, you should follow
as a means of identifying or verifying suspected
the operating instructions contained in the
component failure. Examine residue from those filter
contamination standards. Tan patches occur when rust
assemblies directly downstream from the component.
or iron chlorides are formed in the system, or the
system contains abnormal amounts of silica (sand).
ELECTRONIC PARTICLE COUNT ANALYSIS
Gray patches are typical of systems containing normal
proportions of common wear materials and external
Electronic particle counters, such as the HIAC
Contamination Test Center, Model C-600-1, or Royco
The maximum acceptable particulate level for
Electronic Particle Counters, are used to determine
naval aircraft is Navy Standard Class 5. For related SE,
counts of the number of particles in the various size
the maximum acceptable particulate level is Navy
ranges. The counts obtained are compared with the
Standard Class 3. If visible free water is present in
maximum allowable under Navy Standard Class 5.
either the sample bottle or on the surface of the test
Counts that exceed the maximum allowable in any size
filter (at completion of filtration), the system under test
range make the fluid unsuitable for use in Navy aircraft.
is rejected. A stain on the test filter membrane may be
The test results obtained by using automatic
an indication of the presence of free water. When a stain
particle counters and the contamination analysis kit are
is seen on the test filter, obtain a second fluid sample
not always precisely the same. Both are authorized for
from the system under test and process it so that water
fleet use, and you may use either one. Automatic
content can be confirmed prior to system rejection.
particle counters optically sense particles contained in
Make sure that observed water is not a result of
the fluid sample and electronically size and count them.
atmospheric condensation during the sampling process.
Most fleet equipments are calibrated so that the
If the system under test fails to meet the Navy
smallest particle counted has an effective diameter of 5
Standard Class 5 particulate requirement or if it
microns. Particles smaller than 5 microns, although
exhibits free water, the system must be decontaminated
always present, do not affect the particle count. The
according to the procedures listed in the applicable
contamination analysis kit uses a patch-test method in
MIM.
which the fluid is filtered through a test-filter
membrane. The sample causes the membrane to
Filter Bowl Contents Analysis
discolor proportionally to the particulate level. The test
filters used have a filtration rating of 5 microns
Hydraulic fluid samples obtained from filter bowls
(absolute). However, they also retain a large percentage
and/or elements cannot be used to determine system
of those particles less than 5 microns in size. The
contamination levels. The following combination of
contamination standards provided with the
factors makes the filter bowl sample useless when
contamination analysis kit are representative of test
determining the system's level of contamination:
indications that result if the fluid sample has a particle
sedimentation, functional location, and/or an inability
size distribution (number of particles versus size)
to obtain the required 100 milliliters of fluid. Filter
typical of that found in the average naval aircraft.
bowl residue analysis may be used to monitor hydraulic
Samples from aircraft systems having typical particle
system degradation, monitor for suspected impending
size distributions will, therefore, show good correlation
component failure, or isolate a cause for continued
if tested using both particle count and patch test
contaminant generation.
methods.
Evaluate filter bowl patch residues by following the
Some operating hydraulic systems have peculiar
procedures in applicable manuals. As you gain
design characteristics, so they produce a particle size
experience about normal contaminates for specific
distribution different from that found in typical naval
aircraft systems and hours of operation, you will be
aircraft. Fluid samples from these systems generally
able to evaluate filter bowl patch residue. Through
contain an abnormally large amount of silt like particles
experience, analysis of main pressure line and case
smaller than 5 microns in size. Experience has shown
drain filter bowl residues is useful in verifying failure of
that this condition results from inadequate system
the upstream hydraulic pump, as large amounts of
filtration or from using hydraulic components that have
metal usually show up in these particular assemblies.
abnormally high wear rates. It is this type of fluid
Residue in other filter assemblies is affected by so
sample that could produce different results when tested,
many other components and factors that analysis is
using both particle-counting and patch-test methods.
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