A gas pycnometer is a laboratory device used for measuring the density — or more accurately the volume — of solids, be they regularly shaped, porous or non-porous, monolithic, powdered, granular or in some way comminuted, employing some method of gas displacement and the volume:pressure relationship known as Boyle's Law. A gas pycnometer is also sometimes referred to as a helium pycnometer.
Volume vs density
While pycnometers (of any type) are recognized as density measuring devices they are in fact devices for measuring volume only. Density is merely calculated as the ratio of mass to volume; mass being invariably measured on a discrete device, usually by weighing. The volume measured in a gas pycnometer is that amount of three-dimensional space which is inaccessible to the gas used, i.e. that volume within the sample chamber from which the gas is excluded. Therefore the volume measured considering the finest scale of surface roughness will depend on the atomic or molecular size of the gas. Helium therefore is most often prescribed as the measurement gas, not only is it of small size, it is also inert and the most ideal gas. Closed pores, i.e. those that do not communicate with the surface of the solid, are included in the measured volume. Helium may however demonstrate some measurable permeability through low density solids (polymers and cellulosic materials predominantly) thus interfering with the measurement of solid volume. In such cases larger molecule gases such as nitrogen or sulfur hexafluoride are beneficial. Adsorption of the measuring gas should be avoided, as should excessive vapor pressure from moisture or other liquids present in the otherwise solid sample.
Gas pycnometers are used extensively for characterizing a wide variety of solids such as heterogeneous catalysts, carbons, metal powders, soils, ceramics, active pharmaceutical ingredients (API's) and excipients, petroleum coke, cement and other construction materials, cenospheres/glass microballoons and solid foams.