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Design and Evaluation of Pressure Balances for the Boltzmann Constant Project
Document(s):
In support of the project for determination of the Boltzmann constant by dielectric constant gas thermometry a system of pressure balances has been designed and constructed with the goal of defining pressures in helium in gauge and absolute measurement modes up to 7 MPa with a relative standard uncertainty of 1∙10-6. The design supports this reduced uncertainty with focus on the areas of temperature stability, mass uncertainty, effective area and pressure distortion coefficient. The system includes two pressure balance platforms, three 20 cm2 and three 2 cm2 piston-cylinder assemblies (PCAs) and automated mass handlers. The design of the PCAs and of their mounting was optimized to reduce pressure distortion coefficients and mounting induced deformations. The PCAs gap width was adjusted to reach an optimal compromise of fall rate and sensitivity. In order to perform automated cross-floats with the pressure balances with a possibility of different gases and a high level of cross-float sensitivity, differential pressure cells (DPCs) were applied to indicate pressure equilibrium. The pressure balances have similar performance with both nitrogen and helium. For any pair of the PCAs, comparisons conducted by automated cross-floats demonstrate repeatability of area ratios within (0.2 to 0.5)∙10-6 up to 7 MPa. The effective area ratios obtained from cross-floats for all possible combinations of PCAs are consistent within 0.96∙10-6. Using dimensional measurements, the effective area of the 20 cm2 and 2 cm2 PCAs were determined with relative standard uncertainties of 0.7∙10-6 and 1∙10-6, respectively. Based on the currently available dimensional and cross-float data the relative standard uncertainty of the absolute pressure of 7 MPa is equal to 2.1∙10-6.