Particle Size by Number or Volume: Keep the Weighting

Number-weighted and volume-weighted particle distributions can emphasize different parts of the same sample.

Read the Weighting Label

Malvern Panalytical explains that particle-size distributions can be weighted by number, volume or other measurement quantities. A diameter statistic needs its weighting label. Removing that label can make two valid reports look inconsistent or make unlike populations appear equivalent.

A volume contribution grows with the cube of diameter for spheres. Large particles can therefore occupy much of the volume while forming only a small fraction of the particle count. The same sample need not have the same number and volume median. Primary characterization reference.

Illustrative Population

Imagine 99 spheres of diameter 10 µm and one sphere of diameter 100 µm. The larger sphere is 1% of the count. Its volume equals 1,000 of the smaller spheres, so it contributes about 91% of their combined sphere volume.

This invented population assumes spherical particles and is not a powder-lot result. It demonstrates why few large particles can dominate a volume comparison while many small particles dominate the count. Converting measured distributions between bases can need additional assumptions.

State the Intended Comparison

Ask whether the report is intended to describe volumetric distribution, particle counts or another property. Keep technique, weighting and any conversion model with the result. Do not select whichever median supports a preferred formulation description while omitting the other report’s basis.

Use the worksheet before comparing powder characterization certificates. In a powder-filling enquiry, weighting can clarify the supplied material description; it does not determine dose consistency, dust control or passage suitability without the appropriate application evidence. HM equipment context: Dry Syrup Powder Filling Machine.