EUROMET project #256 was established to compare agreement between laboratory calculations of the elastic distortion that piston-cylinder assemblies of pressure balances, the primary standard for pressure measurements, undergo when subjected to pressure applied via liquid media. The following piston-cylinder assemblies were selected for comparison of the calculations: (i) PTB, 400 MPa, free-deformation type; and (ii) BNM-LNE, 200 MPa, free-deformation and controlled-clearance types. Calculations made at the IMGC using an analytical method were compared with those made at the NPL and the PTB, each using a different finite element technique.All three methods couple calculation of the elastic deformation of the piston-cylinder with calculation of the pressure distribution in the piston-cylinder clearance from a flow model. After reviewing the calculation methods, the paper presents results for each of the comparison systems. The calculated piston and cylinder distortion, annular gap profile, annular pressure distribution, piston fall rates, and pressure distortion coefficients are presented and compared with experimental measurements.A discussion is presented of the extent of the agreement between the calculation methods and what this implies for the reliability of calculated distortion coefficients. The comparison also resulted in other useful knowledge being obtained, for example on the influence of the cylinder boundary conditions and the influence of simplifications adopted in the geometrical model. This information will be of use in the application of such calculations to the design of piston-cylinder assemblies or prediction of the effect of experimentally measurable parameters.
The national metrology institutes and their partners participating in EUROMET Project 463 developed finite element methods (FEM) for calculation of the pressure distortion coefficients, including their uncertainties, of pressure balances operated at pressures up to 1 GPa and applied them to a PTB 1 GPa piston–cylinder assembly. The methods use axisymmetric models developed and analysed on the basis of the experimental data including the elastic properties of the piston–cylinder materials, pressure-dependent density and viscosity of the pressure-transmitting fluid, dimensions of the piston and cylinder and the piston–cylinder clearance as well as the conditions at the piston–cylinder boundaries. Results such as pressure distributions and radial distortions along the piston–cylinder engagement length, pressure distortion coefficients and their uncertainties as well as piston fall rates dependent on pressure are presented for the free deformation (FD) and the controlled-clearance operating modes of the assembly. The theoretical results are verified by comparing them with the distortion coefficients determined by an experimental method and with the jacket pressure distortion coefficients. The participants' results demonstrate good agreement of the distortion coefficients up to 1 GPa but rather large differences in the uncertainties of the distortion coefficients as well as in the pressure distributions, gap profiles and piston fall rate at maximum pressure. The FEM distortion coefficients obtained for the real piston–cylinder gap profile are in good agreement with the coefficients determined by the experimental method; the FEM values obtained for the ideal gap agree well with the distortion coefficients furnished by the simplified theory. For the real gap model, the uncertainty of the gap geometry is the main uncertainty source. The total standard uncertainties of the controlled-clearance distortion coefficient obtained by different methods lie between (0.078 and 0.17) × 10−6 MPa−1 at 400 MPa and between (0.04 and 0.098) × 10−6 MPa−1 at 1 GPa.
An international comparison in the pressure range 20 - 100 MPa has been carried out under the auspices of the high-pressure working group of the Comité Consultatif pour la Masse et les grandeurs apparentées (CCM) of the Comité International des Poids et Mesures (CIPM). The Standards Laboratories of 13 countries have participated in this comparison, which took place during the period 1981 - 1985. This paper presents a résumé of the comparison.The transfer standard used was an oil-operated pressure balance. Each laboratory determined the effective area of the piston-cylinder assembly of this balance in the pressure range 20 - 100 MPa. The results of the measurement of the effective area extrapolated to zero applied pressure agreed within 204 parts per million (ppm) for all 13 laboratories (9 laboratories agreed within 53 ppm). The results of the measurement of the effective area at 100 MPa agreed within 414 ppm for all 13 laboratories (8 laboratories agreed within 78 ppm).
The results obtained by five laboratories in the determination from dimensional measurements of the effective areas of two gas-operated 10 cm2 piston-cylinder assemblies are presented. These measurements were carried out as phase A1 of a key comparison in the pressure range 0.05 MPa to 1 MPa under the auspices of the Consultative Committee for Mass and Related Quantities (CCM) of the Comité International des Poids et Mesures. The participants performed diameter, straightness and roundness measurements on each piston and cylinder bore and calculated the effective area of each piston-cylinder assembly using their own methods. The differences between diameters determined by the institutes are systematic and often greater than the uncertainties claimed by the participants. Nevertheless, all calculated effective areas agree with the reference values determined within the expanded uncertainties with a coverage factor 2, most of them even within their standard uncertainties. The choice of calculation method seems to be less important than the dimensional data themselves. The effective areas determined from the dimensional measurements are compared with those obtained in cross-float experiments with national pressure standards, in a comparison referred to as phase A2 and reported in a separate paper.
Leak detection is widely used nowadays in various fields such as the automotive and refrigeration industries. The leak tightness of installations charged with refrigerants must be controlled periodically by refrigerant gas detectors, qualified by refrigerant leaks called reference leaks or 'calibrated' leaks. To this day, those refrigerant leaks are not traceable to the SI units under their operating conditions. Therefore, a project involving the LNE, the CEP and the ADEME was initiated in order to develop a national standard for calibrating refrigerant reference leaks.The method consists in measuring the accumulation of gas due to the refrigerant leak inside a closed volume at atmospheric pressure. The leak flow rate is deduced from the concentration variation, measured with a photo-acoustic spectrometer, the pressure and temperature in the accumulation volume and the capacity of the accumulation volume. The uncertainty budget thus takes into account the measurement uncertainties of these quantities. The main components of the uncertainty are due to the accumulation volume capacity and the measurement of concentration. The relative expanded uncertainty of leak flow rates between 1 g yr −1 and 50 g yr −1 is around 2%, depending on the value of the leak.
As part of a wide-ranging key comparison organized by the Consultative Committee for Mass and Related Quantities (CCM) of the Comité International des Poids et Mesures, this report gives the results of a comparison of pressure measurements in the range 0.05 MPa to 1 MPa. The two transfer standards used were pressure balances equipped with large (10 cm 2 ) effective area piston-cylinder assemblies. The scope of the comparison covered the observation of the behaviour of both piston and cylinder assemblies from two manufacturers and made from different materials. The results show agreement of all the laboratory standards within the estimated expanded uncertainties, expressed with a coverage factor . Most of the difference values (47 out of 54) are inside the standard uncertainties. These results demonstrate the coherency of the standards of the participating laboratories in the range 100 kPa to 1000 kPa for gas pressure, gauge mode.
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