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Since the spectrophotometers recom-
mended in this standard are not designed
to address the specific needs of the test, a
significant portion of the varnish sub-mi-
cron particles trapped inside the membrane
volume are not visible and are often missed
during measurement.
As you see in Figure 1, the intensity of
the color when the membrane is lit from
the back leads to different results when
compared to the way it looks when lit from
the front.
In 2015, Professor Sasaki developed
a new colorimetric patch analyzer (CPA)
which can measure the color of contami-
nants on the patch surface using reflected
light and the color inside the patch by using
transmitted light.
One of the most common concerns
when using a solvent to dilute in a lab test is
that the solvent may dissolve part of what is
to be measured and distort the test results.
In 2014, Andy Sitton presented a new
index in order to address this concern in
Machinery Lubrication magazine.
In Sitton's method, the volume of oil
sample is passed through membranes in
accordance with ASTM D7843, but no
solvent is used. Although the proposed
method can take considerable time because
of the need to pass thick oil through the
membrane, it will eliminate the possible
effects of the solvent.
e varnish potential is obtained by
using a solvent in the ASTM D7843. Sitton's
new index is also measured by a spectro-
photometer and is called iMPC. e final
index will be the number obtained from the
division of iMPC into MPC.
e closer the index is to the unit, the
more likely it is to deposit varnish on the
interior surfaces of the turbine and the
greater risk it poses to its safe operation.
Soluble and insoluble
varnish contaminant:
e results from the ASTM D7843
standard do provide a useful index, but
do not provide distinct estimates of the
probable volumes of soluble and insoluble
varnishes. Because of the 72 hour oil reten-
tion time required by the ASTM D7843
and the agglomeration phenomenon,
soluble varnish particles are instead added
to the volume of insoluble varnish in the
Figure 1. Comparison of reflecting
light and transmitting light
Surface View by
reflecting light
Back View by
transmitting light