The fluids to be studied are placed in a chamber (13) of variable volume delimited by a piston (5) that can be moved by hydraulic device (24) in a cavity of a body (1). An endoscope (18) allowing to view the inside of the chamber and notably to see the orifices of two diametrically opposite pipes (14, 15) that communicate the cell with the outside is placed behind a transparent porthole (8) closing the end of the chamber opposite the piston. The body (1) can swivel about a pin (26), which allows achievement of equilibrium and also to draw off a particular phase of a stratified multiphase fluid by placing the orifice of one of pipes (14, 15) within this phase. A pressure detector (19) is included in the wall of the piston so as to decrease practically any dead volume likely to distort the measurements. A single body can contain two cells placed side by side, at least one having a transparent wall. The device can be used for studying fluids, notably petroleum fluids.