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Page 34 of 105
7.
MOISTURE ANALYSIS TESTING
The Computrac
®
MAX
®
Series loss-on-drying moisture analysis system takes advantage
of leading edge technology to reduce test times and increase precision over standard
vacuum oven or convection oven drying procedures.
Prior to testing, a Test Program is created with the parameters to be used for testing the
sample. The instrument then controls the test using this Test Program, as explained in
more detail in the following pages. Within each Test Program, the test temperature can
be programmed between normal room temperature (25 ºC) and 225 °C (275 °C on a
MAX
®
4000XL). Using the linked test functionality available on the MAX
®
4000XL, a
multi-step temperature contour can be set up across several linked Test Programs.
In the test chamber, an aluminum sample pan rests on a pan support coupled to a
sensitive electronic force balance. The sample is added to the sample pan and the force
balance registers the initial weight of the sample before testing. The heater located on the
underside of the test chamber’s lid then heats the sample, and a Resistance Temperature
Device (RTD) monitors the temperature in the test chamber. The temperature readings
from the RTD are sent to the microprocessor, and under the microprocessor's control, the
heater element is cycled on and off to bring the test chamber to the programmed
temperature.
As the sample is heated according to the Test Program selected for the test, it loses
weight due to moisture evaporation. The balance records that weight loss and transmits
the data to the microprocessor for evaluation. The balance readings are averaged to
eliminate signal interference or erroneous data. The microprocessor interprets the
information and compares the sample's weight loss to a standard drying curve. The
sample's decreasing weight is compared to the initial sample weight and the calculated
moisture concentration appears on the display. When using the Predict Ending Criteria,
the final moisture concentration is extrapolated from the curve and results are available
within minutes.
A typical drying curve is shown at the
right.
Section (A-B) represents the sample's
weight loss as it heats from room to
testing temperature.
Section (B-C) is linear, representing
moisture loss from the interior of each
particle of the tested sample.
Section (C-D) is exponential in nature and represents the final evolution of moisture from
within the particle. The point at which no additional weight loss occurs (E) represents the
complete evolution of water from the original sample.
Section (D-E) is the longest and most time-consuming portion of the entire testing
process. The time for a sample to reach zero residual moisture in an oven can take from
one to possibly 24 hours depending upon the sample, sample preparation, and testing
temperature. When using the Predict ending criteria, the MAX
®
4000’s loss-on-drying
system calculates the sample's moisture concentration through mathematical
extrapolation from the exponential (C-D) portion of the curve.
