gasspyknometer

The gasspyknometer, commonly written as gas pycnometer, is a laboratory instrument used to determine the volume and density of solid materials by measuring the displacement of a gas. It is often used with helium, due to helium's small molecular size and chemical inertness, which allows it to penetrate open porosity in many materials. This enables calculation of the skeletal (true) density of solids such as powders, pellets, catalysts, geological samples, and ceramics.

The device operates by placing a sample in a chamber connected to a reference volume. The system is evacuated and then filled with a known amount of gas at a controlled temperature. Pressure readings are taken with and without the sample, and the volume of the sample is derived from the change in pressure using Boyle's law. Because the gas can access pores connected to the surface, the resulting volume corresponds to the material's solid volume plus accessible pore volume; the density calculated from the sample mass and this volume is commonly used as the skeletal density. Measurements require careful control of temperature, gas purity, and sample dryness.

Limitations include the presence of closed porosity, gas adsorption on surfaces, and potential chemical interactions between the sample and the gas. The method is rapid, non-destructive, and requires small sample sizes, but results can depend on gas choice and measurement protocol.