Towards 7 MPa Pressure Standards with 1·10 -6 Uncertainty

To determine the Boltzmann constant by means of the dielectric-constant gas thermometer technique, pressure standards measuring absolute pressures in helium in the range of 7 MPa with a relative standard uncertainty of 1·10 -6 are required. The feasibility of such a low uncertainty with pressure balances whose piston-cylinder (p-c) assemblies are characterised by dimensional measurements is considered. It is shown that the target uncertainty can be achieved when p-c assemblies with a nominal effective area of 20 cm 2 are used as primary standards characterised by the state-of-the-art dimensional techniques available at PTB, advanced theories for the effective area calculation are applied, improved cross-float techniques are employed which involve differential pressure cells (DPCs) by means of which 2 cm 2 p-c assemblies covering the 7 MPa range can be linked to the 20 cm 2 ones, and a consistency better than 1·10 -6 within a group of p-c units can be demonstrated. Some results of dimensional and cross-float measurements as well as of a DPCs investigation are reported.