The ability to accurately and precisely measure volume and flow of gases is a fundamental component in the safe practice of anaesthesia. To measure the volume of a gas in the simplest form, a chamber can be used, but due to size this is often impractical and other physical properties of the gas are used to derive measurements of volume and flow. To be able accurately to measure gas flow knowledge of the flow regime and the gas composition are required. With this knowledge the appropriate and correct choice of device for measurement of gas volume and flow can be made.

Gases are a type of fluid. A fluid is a substance that flows or deforms when a shear stress is applied to it, with the resistance to deformation being known as the viscosity. Liquids and plasmas are other fundamental states of matter that are fluids. Fluids can be further categorized into Newtonian and non-Newtonian fluids. A Newtonian fluid is one in which the viscosity of the fluid is constant when a varying amount of shear stress is applied. Gases in anaesthesia are considered as Newtonian fluids, as are many liquids including water. Non-Newtonian fluids demonstrate a change in viscosity when shear stress is applied. An example of a non-Newtonian fluid is blood, in which up to a point, viscosity decreases as shear stress increases. This phenomenon is known as shear-thinning.