A few influences are responsible for possible errors and good or bad accuracy.
The coal comming from one mine only normally an accuracy of better then ± 0.5% standard error for about 10% ash also with corse coal . For ash contents lower then 10% even better results are achieved. For higher ash contents the error is bigger but not because of the principle but related to the higher error of the laboratories. The system must be calibrated with an error of 0.3% caused by the laboratoties and therefore has a restricted accuracy.
The most important possible error is caused by the coal itself: The measuring principle is based on distinguishing two components. In reality it is something like ten. Any change of the composition of the ash causes a slight error. Because the two main components do have nearly the same absorption coefficient this ratio does not cause a wrong indication. The attention must be put on changes like such in iron content. The theoretical error can be very big but is not very important in practice. Nevertheless it has to be considered.
Another error can be caused by the moisture content. Only when the ash content is about 5 - 10 % is does not cause an error because water is measured as coal. For higher contents is can cause an error. Fortunatelly this can be compensated for with an RGI micro wave moisture meter.
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