The characteristics used to describe powders include porosity, true volume, bulk volume, apparent density, true density, and bulkiness.

Where,

Porosity can be expressed as = Void × 100

Where, Void can be defined as = [{V (bulk) – V (true)} ÷ V (bulk)]

This value should be determined experimentally by measuring the volume occupied by a selected weight of a powder V (bulk) and the true volume V (true) of a powder is the space occupied by the powder exclusive of spaces greater than the intramolecular space.

Therefore, porosity is = [{V (bulk) – V} ÷ V (bulk)] × 100

And the bulk volume is = true volume + porosity.

Example

A selected powder has a true density (ρ) of 3.5 g/cc. Experimentally, 2.5 g of the powder measures 40 mL in a cylindrical graduate. Calculate the true volume, void, and porosity.

We know that,

True Density = mass (weight) / volume

Thus,

True Volume = mass (weight) / true density = 2.5 g/(3.5 g/cc) = 0.715 cc

Void = [{V (bulk) – V (true)} ÷ V (bulk)] = [(40 – 0.715) ÷ 40] = 0.982

Porosity = Void × 100 = 0.982 × 100 = 98.2%

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