ISO/TR 11583:2012 describes the measurement of wet gas with differential pressure meters. It applies to two-phase flows of gas and liquid in which the flowing fluid mixture consist of gas in the region of 95 % volume fraction or more. ISO/TR 11583:2012 is an extension of ISO 5167. The ranges of gases and liquids from which the equations in ISO/TR 11583:2012 were derived are given. It is possible that the equations do not apply to liquids significantly different from those tested, particularly to highly viscous liquids.

Although the over-reading equations presented in ISO/TR 11583:2012 apply for a wide range of gases and liquids at appropriate gas-liquid density ratios, evaluating gas flow rates depends on information in addition to that required in single-phase flow: a measurement of the pressure loss can be sufficient; measurement of the liquid flow using tracers can be possible; the total mass flow rate may be known (this is more likely in a wet-steam flow than in a natural gas/liquid flow); in a wet-steam flow a throttling calorimeter can be used.

Wet-gas measurement using Venturi tubes or orifice plates is covered in ISO/TR 11583:2012.