A Pycnometer is a laboratory instrument used for measuring the density of solids of regular shape, porous or non-porous solids, monolithic, powdered, or granular. This calculated by employing certain methods of gas displacement and analyzing the volume - pressure relationship known as Boyle's Law. Often referred to as gas pycnometer, Pycnometer is also sometimes called as a helium pycnometer.
As per its construction, a pcynometer is a type of flask with a stopper having a capillary tube through it allowing air bubbles to escape. The pycnometer is used to measure accurately the volume of solids.