What is the difference between true density and tapped density?

Answer 1

· Tap Density: The density of a powder when the volume receptacle is tapped or vibrated under specified conditions while being loaded. Each particle of a solid material has the same true density after grinding, milling or processing, but more geometric space is occupied by the material. In other words, the geometric density is less... approaching 50% less than true density if the particles are spherical. Handling or vibration of powdered material causes the smaller particles to work their way into the spaces between the larger particles. The geometric space occupied by the powder decreases and its density increases. Ultimately no further natural particle packing can be measured without the addition of pressure. Maximum particle packing is achieved. Under controlled conditions of tap rate, tap force (fall) and cylinder diameter, the condition of maximum packing efficiency is highly reproducible. This tap density measurement is formalized in the British Pharmacopoeia method for Apparent Volume, ISO 787/11 and ASTM standard test methods B527, D1464 and D4781 for tap density.

· True Density: The true density of powders often differs from that of the bulk material because the process of comminution, or grinding will change the crystal structure near the surface of each particle and therefore the density of each particle in a powder. In addition, voids at the surface of a particle, into which liquids will not penetrate, can generate apparent volume which will cause serious errors when density is measured by liquid displacement. The pycnometer G-DenPyc 2900 from Gold APP Instruments are specifically designed to measure the true volume of solid materials by employing Archimedes' principle of fluid (gas) displacement and the technique of gas expansion. True densities are measured using helium gas since it will penetrate every surface flaw down to about one Angstrom, thereby enabling the measurement of powder volumes with great accuracy. 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.