Ohaus MB45 MOISTURE ANALYZER Manual - Manual (Page 51)

MB45 49
Influence of the sample weight on the repeatability of the results
Sample weight influences the repeatability of the Moisture Analyzer. The repeatability always becomes
worse with decreasing sample weight. The relation between sample weight and repeatability is shown in
the following table:
Sample weight Repeatability
0.5g
1g
2g
5
10g
±0.6%
±0.3%
±0.15%
±0.06%
±0.03%
The preceding table is based on the assumption that the sample is ideal, homogeneous and its moisture
can always be separated completely and free from decomposition (e.g. moist sand). Deviations always
comprise the uncertainty, which depends on the sample, and the repeatability of the instrument. In
practice, measurement differences appearing within a measurement series can consequently be larger
than the values of the halogen Moisture Analyzer shown in the table.
The two examples below show how the sample weight is determined to keep the drying time to a
minimum and meet the accuracy demands of the instrument.
Example 1
The repeatability should be better than +0.15%. The accuracy table shows that a sample weight of at
least 2g is required to achieve this.
Example 2
Wet weight of sample:
10g
Mean value of the results: 15.5%
Repeatability from the table: +0.03%
Scatter of the measurement results (ideal sample) 15.47% –15.53%
8.3 Selection of the drying temperature
The drying temperature exerts a controlling influence on the measurement time. It must be selected so
that the sample neither decomposes nor changes its chemical structure. A drying temperature that is too
low can unnecessarily prolong the drying time.
Note also that certain samples can give off different amounts of moisture at different drying temperatures.
This is the case with substances in which the strength of the bonds binding the moisture varies or those
which tend to show signs of decomposition. Minimum deviations from moisture content values of the
reference method can be compensated by changing the drying temperature.