Method and apparatus for studying the properties of a multiphase fluid under pressure flowing in a duct, such as a petroleum stream

A method of studying the properties of a multiphase fluid flowing along a duct with a fluid confining wall, said duct having a separated first and second lengths thereof within which prevail respectively a first and second pressure and containing at least two liquid phases and a gaseous phase, in volume ratio to be determined by a sensor device, comprising the following steps:

• an extracted fluid fraction of fluid is extracted from the substantially vertical first length of the duct, within which the fluid is agitated and flows upwards, the fraction being taken by an orifice of a first tube oriented at a certain distance from the wall of said length;
• and
• the taken fluid fraction is transferred into a thermally-insulated separator receptacle, having a top portion and an end wall, the initial pressure within the separator receptacle being close to the first pressure prevailing in the first length, and the separator receptacle top portion is connected via a second tube to the second length of the duct located downstream from the first length and the pressure within the separator receptacle is slightly lower than that of the first length so that a fluid sample is obtained which preserves the volume ratio of the liquid phases, said fluid sample in said separator receptacle with a liquid-gas interface level and a liquid liquid interface level.