There are two types of density associated with powders. Envelope (or bulk) density is determined for porous materials when pore space spaces within material are included in the volume measurement. Skeletal density is the ratio of the mass of solid material to the sum of the volumes of the solid material and closed (or blind) pores within the material (ASTM D3766). Micromeritics Accupyc II 1340 measures the skeletal volume of a material by gas displacement using the volume-pressure relationship of Boyle’s Law. An inert gas, typically helium, is used as the displacement medium. The sample is placed in a sealed cup of a known volume. This cup is then placed into the sample chamber. Gas is introduced to the sample chamber and then expanded into a second empty chamber with a known volume. The pressure observed after filling the sample cell and the pressure discharged into expansion chamber are measured, and then the volume is calculated. The density is determined by dividing the sample weight by the volume measured. The density measured by Helium Pycnometry is often referred to as “helium density” implying that open pores excluded in the calculation. Since He cannot access closed pores, they are included into the total volume.