Abstract: A rigid body comprising two cylindrical chambers (14, 15) with a pointed bottom for example, connected to each other by a fine line controlled by a valve V5, is fixed in a thermostat-controlled enclosure (7). Two rods (18, 17) forming pistons, provided with prestressed seals (J), can slide in these chambers, separately or coupled with one another. Their shape is suited to that of the bottom of the chambers. These rods are moved by shifting vertically hangers (12, 13) integral therewith, by means of threaded rods (8, 10) driven into rotation on demand by motors (ME1, ME2). A control unit comprising two optical display and locating blocks (19, 20) running right through the body near the bottom of the lower chamber (14), pressure, temperature and displacement detectors, under the control of a micro-computer, allows precise thermodynamic measurements to be achieved on substance samples at high pressure and temperature values. A gasometer is preferably placed in enclosure (7).