2-7. Practical Considerations.
a. Sample Integrity. The value of spectrometric analysis and the subsequent utilization of this analysis by
the evaluator is based on the assumption that the oil sample is representative of the system from which it was
taken. If the oil is not truly representative of the system, the analytical results are totally useless. Occasionally,
samples from one component may be erroneously substituted for another, resulting in what may at first appear to
be a developing wear condition for one of the components. Closer inspection of results will often reveal these
discrepancies. Any sudden increase of wearmetal in one component and decrease in another, within the same
weapon system/end item, should be viewed as a problem related to mislabeling of samples, i.e., misidentifying a
sample as an engine sample when it was actually a transmission, or reversing left and right engine samples.
b. Contamination. Contamination is the problem that most frequently affects sample integrity. Sharp
increases in the concentration of wearmetals, water, unusual color, and particulate matter may be indications of
contamination, and additional samples may be required to establish the true wear-metal baseline. In some
cases, systems may have to be flushed one or more times to remove contaminating substances. The most
common contamination found in lubricant systems is dirt and sand which is detected by an increase in silicon.
Silicon contamination is a common problem in dry, sandy, or dusty operational areas. Once in the component,
dirt and sand are abrasive, and may accelerate wear.
c. Type of Spectrometer. The type of instrument being used to analyze fluid samples has a direct effect on
the analytical result and must be considered. The analytical results from an inductively coupled plasma or atomic
absorption spectrophotometer will generally be lower than the value that would be given by an atomic emission
rotrode instrument for used oil analysis.
d. Calibration Standards. Calibration standards which are used to standardize the spectrometer have an
assigned shelf life. Standards which have exceeded the allowable shelf life may introduce errors into the
analytical process that may not be readily detected, particularly if all standards on hand have degraded over the
entire standard range of PPM. Calibration standards should be checked for signs of precipitation as an indication
of degradation. Refer to Volume II for detailed information concerning calibration standards.
e. Additives. New lubricating fluids normally do not contain any metallic compounds or constituents that
would interfere with spectrometric identification and measurement of the wearmetals produced by operation of the
major assembly. Occasionally, lubricant manufacturers will use a metallic compound as a fluid additive. Although
such additive compounds may only contribute a small amount of metal/chemical to the lubricant, it is necessary
for the laboratory to recognize this source of trace materials. An analysis of a sample of new fluid can be used to
establish a baseline for determining actual concentration of wearmetals.
f. Corrosion. Internal equipment corrosion may become a factor in oil analysis when water is allowed to
contaminate equipment lubricating fluid. Helicopter gearboxes are particularly susceptible to water-induced
internal corrosion because of design features that frequently do not protect against water intrusion. Evaluators
must be familiar with the corrosion mechanism because corrosion products may easily be mistaken for
impending failure indications and the equipment may be unnecessarily removed from service.
g. Fuel Dilution. Engine oil-lubricated systems using leaded gasoline sometimes become contaminated
through oil system fuel dilution. Analytical results indicating a high concentration of lead are a good indication
that the system is fuel contaminated.
h. New/Rebuilt Engines/Components. New or recently overhauled equipment tends to produce
wearmetals at an accelerated rate. During this break-in period, evaluation maybe difficult since wear-metal
production maybe higher than normal. The break-in period is about 20 hours for jets, gearboxes and constant
speed drives (CSD's) and about 100-200 hours (depending on RPM) for reciprocating engines. Curves A and B
in figure 2-2 show typical plots of operating hours versus wear-metal concentration for most new/rebuilt
equipment. After break-in is complete, an oil change may be necessary to reduce wear-metal concentration to
normal levels so evaluation criteria can be effectively utilized.