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8.1.7 Lift Compensation
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LIFT COMPENSATION
Programmable lift compensation is a
feature found only in Computrac
®
instruments. It is included because of
heated air currents in the oven chamber
that apply lift to the sample pan. This
causes a falsely high apparent weight
loss. If not compensated, this error leads
to erroneously high moisture values. The
amount of lift generated by the hot
sample varies from sample to sample,
and increases with the test temperature.
In some cases, the lift is approximately 10 milligrams and is small enough that less
sensitive instruments do not provide compensation. Because the MAX
®
4000 can sense
a weight change as small as 0.1 milligrams, the lift must be compensated when the lift
value is a significant portion of the total weight change during a test.
For example, some plastic resins are controlled to a moisture content of 0.01%, so for a
30-gram sample, the weight change is only 3 milligrams. Similarly, some lyophilized
drugs are so valuable that the sample weighs only 150 milligrams. At 2% moisture, the
weight change is only 3 milligrams.
The MAX
®
4000 automatic lift compensation factor is determined at the factory. Each
instrument is characterized for lift produced under standard conditions, and for the way
that lift changes for various test temperatures. The factor is called the 100% lift
compensation value. Each sample material has a degree of similarity to the maximum lift
sample, and this is expressed as a percentage of the maximum lift. This value is entered
under LIFT COMPENSATION.
Use 100% for all samples unless it is found by
experiment that another value is more appropriate.
Contact your AZI technical sales
representative for help in determining the proper value for difficult applications.
