An instrument for measuring the pore volume, and hence the porosity, of a core sample. The term is also used for some instruments that actually measure grain volume, such as the Boyle?s Law Double-Cell method. Pore volume is then obtained from the difference between bulk volume and grain volume.

Pore volume is most commonly measured directly by Boyle's Law Single-Cell method, summation of fluids or liquid saturation. Bulk volume is most commonly measured by buoyancy, mercury displacement or a physical measurement of size (calipering); grain volume by Boyle?s law Double-Cell method or disaggregation of the sample.

Except for disaggregation, all techniques determine the effective porosity, in the sense of all but the isolated pores.