Density test equipment is used to test the density of materials. Density, a measure of mass per unit of volume, is usually expressed in units of grams per milliliter (g/mL) or grams per cubic centimeter (g/cc). There are many different types of density measurements. Examples include gas density or skeletal density, bulk density or mercury density, envelope density, and tap density. Density is closely related to specific gravity or relative density, the ratio of the density of a material to the density of water.