The bulk density of a powder depends on how closely individual particles pack together. The bulk density is affected not only by the true density of the solids, but by the particle size distribution, particle shape and cohesiveness. It is an important property in packaging and powder handling.
Handling or vibration of powdered material can overcome the cohesive forces and allow particles to move relative to one another and so smaller particles can work their way into the spaces between the larger particles. The total volume occupied by the powder decreases and its density increases. Ultimately no further natural particle packing can be measured without the addition of pressure and maximum particle packing has been achieved!
Under controlled conditions of tap rate, drop height and container size, the condition of maximum packing efficiency is highly reproducible. This tap density measurement is formalized, using graduated measuring cylinders, in the British Pharmacopoeia method for Apparent Volume, ISO 787/11 and ASTM standard test methods B527, D1464 and D4781 for tap density, and others.
Automated tap density determinations are performed either by the Quantachrome Autotap or the two sample Dual Autotap.
The pycnometers from Quantachrome are specifically designed to measure the true volume of solid materials, even powders and porous solids, by employing Archimedes' principle of fluid (gas) displacement and the technique of gas expansion (Boyle's Law). True densities are usually measured using helium gas since it will penetrate very fine pores down to about two Angstroms (0.2nm), thereby enabling the measurement of true volumes with great accuracy. Also, unlike liquid-displacement techniques, the dry gas technique presents none of the problems associated with non-wetting behavior, solubility of the solid or solvent disposal.
The measurement of density by helium displacement often can reveal the presence of impurities and occluded pores which cannot be determined by any other method.
Geometric Density or Envelope Density
One of the most common density measurements involves the determination of the geometric space occupied within the envelope of a solid material... including any interior voids, cracks or pores. This is called geometric, envelope or bulk density and only equals true density when there are no internal openings in the material being measured.
The volume of irregularly shaped samples can be determined by dry powder displacement using a free-flowing powder and a measuring cylinder, and preferably an Autotap to reproducibly compact the powder around the piece being measured.