Abstract: The invention relates to instruments for precision measurements at different pressures and temperatures of the physical properties of fluids such as isothermal and adiabatic compressibility, density, coefficient of thermal expansion, velocity and absorption of ultrasound; for determining on their base the heat capacities, other thermophysical and thermodynamic properties, functions and potentials. The apparatus comprise a casing (1), made as a pressure vessel with a measuring chamber, a piston (8), a unit (14) for excitation and reception of signals, recording and processing of data, an oscillation transducer (5) installed in the chamber and forming two measuring intercommunicating cells with a constant (6) and a variable (7) path respectively. A piston insert (17) may be installed with a slide fit between the piston and the oscillation transducer. The apparatus ensures a higher accuracy and resolving power of measurements of physical properties of fluids under various pressures, volumes and temperatures.