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