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higher at the operating tempera-
tures, providing a safeguard in case
something were to happen in terms
of higher operating temperatures
or contaminants lowering the oil's
viscosity. Reducing the operating
temperatures with coolers or other
means, or raising the ISO grade
can also help you achieve a measure
of safety.
If the viscosity index were to
drop below 108 in this example,
you would expect to see advancing
signs of wear inside the machine.
erefore, you need to know the
minimum operating viscosity to set
an alarm and limit on that value.
Most oil labs test industrial
fluids at 40 and 100 degrees C. By
using these two parameters, you
can calculate the viscosity index
and extrapolate the viscosity at
whatever in-service temperature the
compressor is operating.
You should take the approach
of setting both upper and lower
limits on viscosity, cautionary limits
at plus or minus 5 percent, and
critical limits at plus or minus 10
percent. Viscosity index could be
included as well, as there are many
reasons viscosity index could change
throughout the life of the oil.
Perhaps the most common
way viscosity index changes is by
shearing down of the viscosity index
improver additive. Over time this
additive is cut into smaller pieces
and loses its impact in the oil. You
then see a drop off in the high-
temperature viscosity. is generally
is measured or monitored by reading
the viscosity at two temperatures.
While there isn't a set level for
viscosity index across the board, you
can determine the required viscosity
index for your machine and then
set limits based on how much the
viscosity will drop at a given oper-
ating temperature. Remember, with
viscosity, in most cases it is better to
err on the side of caution and have
too much rather than not enough.
ML
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