OHAUS MB45, Инструкция по эксплуатации

Страница: 53 / 71

5 3
Section 8 - How to obtain the best results
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 +0.6%
1g +0.3%
2g +0.15%
5g +0.06%
10g +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.
All manuals and user guides at all-guides.com