Particle size distribution, median value, moisture content

Definition

Value for the mean particle size (50 % in weight of the dust is coarser and 50 % in weight is finer than the median value).

Determination

The particle size distribution of a dust is always determined by a screen analysis. To this effect, the dust to be tested is sieved onto standard test screens and the screen residue is weighed. If a screen analysis cannot be carried out, wet or sifting methods are used to determine particle size distribution.

If the residue in weight per cent is plotted over the particle diameter in the Rosin-Rammler-Sperling-Bennet-(RRSB) grid, a distribution curve occurs, indicating the median value of the tested dust. The median value can be used to roughly characterise the fineness of the dust sample.

The moisture content of the substance to be tested is measured as the difference between the weight of wet dust and the weight of dried dust. This is determined by carefully drying the dust until a constant weight is achieved.

Remark

The fineness and moisture content refer to the state of the dust sample as delivered respectively as tested. The "delivered" state may be identical to the tested state. The listed combustion and explosion characteristics always apply only to the dust with the conditions described in the same column.

If no data is provided on moisture content of the delivered state, it must be assumed that the substance has reached moisture saturation. Prepared testing samples, on the other hand, consist of dried dust with a residual moisture usually of <2 % in weight respectively <4 % in some cases.

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