The particle density of a particulate solid or powder, is the density of the particles that make up the powder, in contrast to the bulk density, which measures the average density of a large volume of the powder in a specific medium (usually air).

Sometimes it also refers as True density which contrasts with particle density as of internal porosity.

The particle density is a relatively well-defined quantity, as it is not dependent on the degree of compaction of the solid, whereas the bulk density has different values depending on whether it is measured in the freely settled or compacted state (tap density). However, a variety of definitions of particle density are available, which differ in terms of whether pores are included in the particle volume, and whether voids are included.

 

Note:   The above text is excerpted from the Wikipedia article Particle density (packed density), which has been released under the GNU Free Documentation License.