different from those measured. Sampling is where a sample of the wet gas is removed from the pipe line for analysis to determine the constituent components. It is important that a sample is collected that is representative of both the gas and liquid phase fractions and that there is no mass transfer between the phases during sampling. Tracer method involves injecting a tracer dye into the wet gas flow and then sampling at a specific distance down stream to measure the dilution of the dye. The dilution of the dye in the liquid phase is used to calculate the liquid flow rate. This technique can be rather difficult to apply as it can be difficult gaining access to the correct points needed to conduct this test. Microwave technology utilises the higher permittivity of water than hydrocarbons to detect the overall water fraction in the liquid and gas phases. As it only detects and measures the water component, the liquid hydrocarbon component must be measured by another method. Overall pressure loss across different pressure meters using a Venturi tube to measure flow causes a pressure drop in the flow which is partly recovered downstream of the meter. In dry gas flows the recovery is greater than in wet gas flows due to the liquid component. This difference can be exploited to provide a measurement for the liquid fraction. This involves adding a second pressure tapping downstream of the Venturi to provide a measure of the partly recovered pressure drop. This method can be affected by changes in the system pressure and gas velocity. Advanced signal processing is used where a liquid phase has an effect on the measurement signal such as pressure fluctuations in a DP flow meter or shift in the speed of sound in ultrasonic flow meter. Complex analysis and modelling of these signals can determine the liquid and gas flows. There are a number of commercially available wet gas flow meters. Most of the meters use differential pressure for the gas phase and a form of liquid detection or wet gas density measurement for the liquid phase normally utilising one of the techniques listed above. See also Natural gas condensate Hydrocarbon dew point Flow measurement Two-phase flow Multiphase flow Multiphase flow meter Fluid dynamics References Natural gas