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The bulk density of a powder is the ratio of the mass of an undisturbed powder sample to its volume including the contribution of interparticle void volume. Bulk density is expressed in grams per mL (g/mL) although the international unit is kilograms per cubic meter (1 g/mL = 1000 kg/m3) because measurements are made using cylinders.
The bulk density of a powder is determined by measuring a volume of known weight of the powder sample, which has passed through a sieve (usually greater than or equal to 1 mm) into a graduated cylinder (Method I) or. Measuring the mass of a known volume of powder in a cup (Method II) or a measuring vessel (Method III) with a volumemeter. Method I and Method III are favored.
A commonly used 250-mL cylinder (2 mL readable) and about 100 g of test sample. If the powder concentration is too low or too high, such that the test sample has an unnecessary apparent volume of more than 250 mL or less than 150 mL, it is not possible to use a 100 g powder sample. Therefore, a different amount of powder should be selected as the test sample, such that its unused apparent volume is 150-250 mL (greater than or equal to 60% of the total volume of the cylinder); The weight of the test sample is specified in the resulting expression.
For test samples with an apparent volume between 50 mL and 100 mL, a 100-mL cylinder with a 1 mL reading may be used; The volume of the cylinder is specified in the resulting expression. The tapped concentration is obtained mechanically by tapping a graduated measuring cylinder or container containing the powder sample.
A 250-mL graduated cylinder (readable in 2 mL with a mass of 220 ± 44 g) is commonly used for TD measurements. Run 10, 500, and 1250 taps on the same powder sample and read the corresponding volumes V10, V500, and V1250 to the nearest graduated unit. If the difference between V500 and V1250 is less than or equal to 2 mL, V1250 is the tapped volume. If the difference between V500 and V1250 exceeds 2 mL, repeat in the same increments as the 1250 tap, until the difference between successful measurements is less than or equal to 2 mL. If it is not possible to use a 100-g test sample, use a smaller volume and a suitable 100-milligram graduated cylinder (readable to 1 ml) weighing 130 ± 16 g. If the difference between V500 and V1250 is less than or equal to 1 mL, V1250 is the tapped volume.
Semipressibility can be easily measured by Carr’s index and/or Hausner’s ratio using bulk and tapped density powders.
