Gaspyknometern

Gaspyknometern, or the gas pycnometer, is an instrument used to determine the true (skeletal) density of solid materials by gas displacement. It relies on the controlled measurement of gas pressure changes in a closed system, typically using helium or, less commonly, argon, because of the small molecular size and low adsorption of these gases.

Principle and method

A gas pycnometer contains a reference chamber of known volume and a sample chamber in which the test material is placed. The system is evacuated and then filled with an inert probe gas at a known temperature. When the sample chamber is closed, the gas distributes between the reference and sample volumes, and the resulting pressure change is measured. Using Boyle’s law and the known reference volumes, the volume occupied by the solid material is calculated. The gas may penetrate open pores to varying extents, so the measurement reflects the skeletal (true) density of the material rather than the bulk density. Temperature control and gas purity are important for accurate results.

Procedure and outputs

Typical procedure involves degassing the sample to remove adsorbed moisture, precisely weighing the sample, and placing it in the sample chamber. The instrument performs multiple pressure equilibrations to improve precision, and the density is reported as mass divided by the measured sample volume. Reported values are commonly described as true density, and uncertainties are generally small for well-behaved materials.

Applications and considerations

Gas pycnometry is widely used for ceramics, catalysts, metal and mineral powders, polymers, and composites, especially when porosity is present or liquid methods are unsuitable. It provides a rapid, non-destructive means to assess skeletal density, but results can be influenced by adsorption effects and the accessibility of pore spaces, requiring careful interpretation and, in some cases, correction.