Long-term trial of a dual energy γ-ray transmission gauge determining the ash content of washed coking coal on a conveyor belt

A dual energy γ-ray transmission technique has been used to determine the ash content of coal on a coking coal product conveyor. The gauge was operated on-line for 5 months in 1984 and 8 months in 1985. The r.m.s. difference between gauge ash and chemical assay of the 2 h composite samples for 1984 was 0.31 wt% ash for ash in the range 5.5-9 wt%, and for 1985, 0.40 wt% ash for ash in the range 5-10 wt%. The r.m.s. difference of 0.40 wt% ash for 1985 is known to include gauge errors of about 0.2 wt% ash due to ash composition changes and at least 0.14 wt% ash due to instability in measurement of count rate, and a 0.18 wt% ash error due to subsampling and chemical assay errors. The experience from this trial has led to a number of improvements in calculation methods, and to better methods of calibration of the gauge for on-line use. (author)