What is true density?

Answer 1

The true density is the ratio between the mass and the volume of a material. 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 by 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.