CANAL

Certain  factors are responsible for possible errors and good or bad accuracy.

The  coal coming out of a mine normally gives an accuracy of better than ± 0.5% standard error for about 10% ash, even with hard coal. Even better results are achieved with ash contents below 10%. With higher ash contents, the error is greater, but not because of the principle, but because of the higher error of the laboratories.

The  system must be calibrated with a laboratory error of 0.3% and is therefore of limited accuracy.

The  most important possible error is caused by the coal itself: the measuring principle is based on the distinction between two components. In reality, it's about ten.

If  the composition of the ash changes, it causes a minor error. however, since the two main components have almost the same absorption coefficient, this ratio does not lead to an incorrect display.

Attention  must be drawn to such changes in iron content. The theoretical error can be very large, but is not very important in practice. Nevertheless, it must be observed.

Another  mistake can be caused by the moisture content and only if the ash content is approx. 5 - 10% will no error be caused since water is measured as coal, however an error can occur with higher contents, but this can be compensated with an RGI microwave moisture meter.

To  measure the radiation, it has to be absorbed, where as denser the absorber material is, the higher the absorption will be.

There  are gas-filled ionization chambers or Geiger tubes, but much more sensitive is a NaJ crystal with a density of 3.7 g / cm 3, which generates small flashes of light when exposed to radiation that must be detected by a light amplifier called a photomultiplier.

The   entire sensor must be installed in a mechanical protective and light-tight housing and stabilized against interference.

Both radiation sources must be shielded in lead containers so that the rays cannot spread in all directions, but a thin channel ensures that the useful beam can only exit in one direction.

The radiation is measured by a scintillation counter, which is the most sensitive detector for industrial applications, where both - the source and the detector - form a measurement path, which is required to determine the ash content.

It  would be possible to use only one detector for both radiations, but since each radiation physically influences the other in the detector, it is more reliable to use separate ones.

The source or detector can be installed above or below the conveyor belt. The measuring paths should be at least 1 m apart.

A solid mechanical frame is required to prevent changes in the distance between the source and detector: it must be absolutely constant to avoid measurement errors.