Pycnometers

A pycnometer is a laboratory instrument used to determine the density and volume of materials, especially solids, powders, and liquids, by measuring mass in a precisely defined volume. The term comes from the Greek pyknos, meaning dense. Pycnometers are employed to obtain true (skeletal) density, assess porosity, and, in some configurations, ascertain the density of liquids.

There are two common variants. Gas pycnometers use a sealed chamber of known internal volume and inert gas, typically helium. The chamber is evacuated and then filled with gas; a sample is introduced and the resulting pressure changes are recorded. From these measurements, the sample’s volume is computed, and its density is obtained from mass divided by volume. Gas pycnometers probe the true volume and can access closed porosity, provided temperature is controlled and the sample is dry or properly degassed.

Liquid pycnometers, sometimes referred to as specific gravity bottles or Archimedes-type arrangements, rely on the displacement of a liquid with a known density. The bottle is filled to a fixed volume with a liquid, and the mass of the liquid-filled bottle is used to determine liquid density. To measure a solid’s volume and density, the sample can be weighed in air and then in a liquid of known density, applying Archimedes’ principle to the displaced liquid. The liquid method is widely used for liquids and can be used for solids with appropriate corrections, although it generally yields the apparent density of porous materials rather than the true skeletal density.

Applications span materials science, ceramics, polymers, cement, pharmaceuticals, and soil science. Key considerations include temperature control, sample preparation, degassing, and the management of porosity and moisture, which influence measurement accuracy.