Characterisation of the volume size distribution of respirable coalmine dust samples by Coulter Counter

The use of a Coulter counter model TA is described for measuring the volume distributions of cole-mine dusts collected by the MRE gravimetric sampler. Trisodium orthophosphate as an electrolyte has been found to give good dispersion of the dust, and a 50 μm diameter orifice gives a satisfactory size coverage for MRE dust samples. The instrument is best calibrated using suspensions with known volume concentrations. The MRE coal-dust distributions are positively skew, with modes usually between 2 and 5 μm equivalent spherical diameter. They resemble, but are consistently differnt from, log-normal distributions, and are also differnt from the negative exponential number distributions previously used to describe thermal-precipitator results. Two parameters are required to characterize the distributions. The volume median diameter, M, and the ratio, R, of third to first quartile diameters characterize the distribution reproducibility. For individual samples, M varies from about 2.5–6.5 μm, and R from 1.6 to 3.0. Small changes in the lower threshold diameter have little effect on these parameters.