ISO 17744:2004 describes procedures for determining the specific volume of plastics as a function of temperature and pressure in both the molten and solid states.

The standard specifies the use of a piston-equipped apparatus in which the test sample, held in a measurement cell, is pressurized by means of the piston. Measurements under conditions of constant pressure or constant temperature can be made. In the constant-pressure mode, the maximum heating and cooling rates permissible are restricted to 5 °C/min.

By using these procedures, it is possible to obtain:

• pvT diagrams that represent the relationship which exists between pressure, specific volume and temperature for a given material;
• compressibility and volumetric thermal-expansion coefficients;
• information on first-order and glass transitions as a function of temperature and pressure.