Microwave Determination of Moisture in Coal: Comparison of Attenuation and Phase Measurement

Abstract

The major disturbing influences on microwave determination of moisture in coal were investigated by measuring the attenuation and the phase shift of the transmission in a wide frequency range. The influence of bulk density and layer thickness can be eliminated by an additional determination of the weight per area. A comparison shows that the influence of salt content on phase shift, decreasing with increasing frequency, is negligible at frequencies above 2 GHz, whereas attenuation is influenced by salt content at frequencies up to 8 GHz. Methods using the attenuation or a combined attenuation —phase measurement therefore must operate at higher frequencies than phase methods only when the aim is to eliminate the influence of salt content. Phase shift is influenced considerably less by grain size and temperature than is attenuation. Because of the lower attenuation at lower frequencies, the phase method is best suited for moisture measurement of thick layers.