Positive displacement flow meters measure the volumetric flow rate of a moving fluid or gas by way of precision-fitted gears or rotors containing cavities through which precisely known volumes of fluid pass. Positive displacement flow meters are very accurate and have high turndown. They work best with clean, non-corrosive, and non-erosive liquids and gases, although some models will tolerate some impurities. They require no straight runs of pipe for fluid flow stream conditioning though pressure drop can be an issue. They are widely used in custody transfer and are applied on residential home natural gas and water metering.