PerkinElmer OilExpress 4, User Manual

Page: 14 / 152

14 . OilExpress 4 System User's Guide
Figure 3 shows the example data from Figure 2 in this differential form. The heights of the
peaks due to breakdown products, contaminants and additive depletion can be measured.
4000 550 3500 3000 2500 2000 1500 1000
0.7
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
cm-1
A
Figure 3 Used-oil spectrum after unused-oil spectrum subtraction
The spectral-subtraction method works very well if the unused oil is available, but this is
often not the case, and using an incorrect reference spectrum can give misleading results.
Additionally, if the machinery has been topped up using a different oil, an appropriate
reference spectrum may not be available.
The alternative approach is to measure the peak heights directly in the used-oil spectrum. In
this case, the baseline (unused-oil) values for the measured parameters will depend on the
oil type, and the results should be trended through several measurements over the lifetime
of the oil or compared to alarm limits derived from statistical analysis of data for the
appropriate machine and lubricant type.
Standard practices and test methods
ASTM International publishes several documents discussing lubricants analysis by FT-IR,
including both direct-trending and spectral-subtraction methods
1–4
. For the Analysis methods
supported by OilExpress 4, refer to
Analysis Methods
on page 16.
A collection of direct-trending FT-IR oil analysis methods for several oil types was developed
as part of the US Army, Navy and Air Force Joint Oil Analysis Program (JOAP). These
methods are widely used and are supported by OilExpress 4; a complete list is given in
Analysis Methods on page 16.
Alarm limits
As the methods described above involve trends in absorbance values rather than direct
quantitative measurements, alarm limits should be derived from statistical analysis of oil
analysis and failure data from many machines. These limits are dependent on machinery
type and oil formulation, and a full discussion is beyond the scope of this guide. For more
information, refer to ASTM E2412-10
1
.
Water
Soot
Oxidation
Nitration
Phosphate
antiwear
Phenolic
antiwear