Intermediate pressure range measurements in the range of few pascals up to few kilopascals are very important for many production lines in the industry. This range of pressure suffers from a lack of pressure standards suitable to measure with low uncertainty. This research aims at filling this gap by establishing a suitable pressure standard such as Force-Balanced Piston Gauge (FPG) and to characterise it as a primary pressure standard measuring this range of pressure with sufficiently low uncertainty. FPG is a pressure standard working in the intermediate pressure range of 1 Pa to 15 kPa in both absolute and gauge pressure operating modes. FPG uses a special Piston Cylinder Assembly (PCA) with a bi-conically shaped inner surface of the cylinder. The PCA was measured dimensionally for straightness, roundness, and diameter. A 3D model was generated by filtering the raw measurement data using a Gaussian filter and applying the least-squares method to minimise the discrepancies between dimensional data of different types. It was challenging to determine the effective area using the dimensional properties of PCA, considering the effect of the gas flow, and calculating the pressure distribution in the piston-cylinder gap with boundary conditions presented by the three pressures at the top, middle, and bottom of PCA. Different mathematical models (ideal and real gas flow as well as kinetic model) were considered to achieve an effective area of PCA with low uncertainty. The uncertainty of this realization depends significantly on the geometric imperfectness of the PCA, i.e. deviations of the piston and cylinder bore from the ideal cylindrical form. In the case of axially non-symmetrical PCAs, the latter was a major uncertainty factor. The effective area was determined with a 2D flow model, which considers PCA radius variations in both tangential and axial directions. A single effective area value for the entire PCA was obtained and the contribution of uncertainty due to the axial non-symmetry of the PCA was excluded. With this procedure, the effective area of PCA was determined traceable to the SI unit metre. Additionally, all other parameters affecting the pressure measured by the FPG were analysed to get an accurate result with decreasing uncertainties and traceable to the SI base units. The results were validated by determining the effective area of the FPG taking it as a secondary standard and comparing it to a classical deadweight pressure balance. Moreover, the FPG was compared with two other primary pressure standards such as a mercury manometer and a static expansion system to validate FPG's measurement capability over the whole operation range of the FPG.

PTB Th-13