Natural gas measurement today is accomplished through the use of two different classifications of gas meters. The first consists of inferential type meters, including, orifice, ultra-sonic and turbine meters, and the second is the positive displacement meters, which consist of diaphragm and rotary displacement meters. The inferential type meters are so-called because rather than measuring the actual volume of gas passing through them, they infer the volume by measuring some other aspect of the gas flow and calculating the volume based on the measurements. The positive displacement type meters are so-called because they measure the actual volume of gas displaced through them