It’s a very precisely manufactured piston-cylinder system, onto which a mass load is applied in order to generate the individual test points. The direct measurement of the pressure (p = F/A), as well as the use of high-quality materials enable a very small measurement uncertainty, in conjunction with an excellent long-term stability.
Both the piston and cylinder are manufactured from tungsten carbide. This pairing of materials, in comparison to other materials, has very low pressure and temperature coefficients of expansion, which results in a very good linearity for the effective area of the piston and a very high accuracy. Over-pressure protection is integrated, which prevents the piston from being forced out vertically and avoids damage to the piston-cylinder system in the event of masses removal under pressure.