Standardization of negative gauge pressures is a peripheral branch of pressure metrology, but this approach is unfair. Although this pressure mode can be ensured simply by liquid columns, the utilization of pressure balances in this area brings many technically interesting solutions. They include an inverse piston-cylinder design (it is the basis of most commercial solutions), a classical pressure balance with generation of negative gauge pressure under a bell jar, a digital piston manometer and a pressure divider designed for absolute pressure mode.
The paper focuses on the methods utilized in the Czech Metrology Institute, i. e. the generation of negative gauge pressure under a bell jar and, newly, the use of the absolute pressure divider and also a digital non-rotating-piston manometer for the range of very low negative gauge pressures that are very hard to reach by other methods.
The paper introduces their principles, ways of ensuring their metrological traceability, the detailed analyses of their uncertainty budgets, practical experiences and the results of their mutual experimental comparison. Furthermore, there are described methodologies of calibration of the secondary standards of negative gauge pressure. Finally, all the methods are compared from the viewpoints of the uncertainties that can be reached and the suitability of each standard for practical calibrations.

At the end of 2006, the Pressure Group of the Centro Nacional de Metrología CENAM, Mexican National Metrology Institute, organized and piloted a proficiency test in hydraulic gauge pressure in the range from 7 MPa to 70 MPa for accredited laboratories by the Entidad Mexicana de Acreditación (ema), Mexican Accreditation Body. For the proficiency test, the pilot laboratory (CENAM) carried out three calibrations of the transfer standard; the first one at the beginning of the proficiency test, the second at the middle and the last one at the end of the program. With the measurements made by CENAM the reference values were established, including error and uncertainty. The deviations for each laboratory were compared against the reference values and the compatibility of results was calculated, for each participating laboratory, by means of the normalized error equation method. The measurements were carried out by each participant laboratory with their own resources (personnel, calibration systems, environmental conditions and in their installations). Laboratories located all around the country participated and most of the laboratories with this range of measurement participated. The proficiency test started in November 2006 and finished in March 2007.

The present paper describes the results of the bilateral comparison carried out between the CENAM (Mexico) 150 kN force standard machine and the INRiM (Italy) 1 MN Force standard machine. The two dead weight machines, their main metrological characteristics and the procedures of the comparison are described. Three different load cells (50 kN, 100 kN and 200 kN capacity) were used as transfer standards, and the test was made at 2 different load levels and at 5 angular positions for each load cell. The main results of the comparison with the relevant uncertainties from the measurements carried out at CENAM and at INRiM are presented and a final evaluation of the agreement between the two National force standard machines is given.

In order to improve the metrological characteristics of the primary force standard machines, as well as to understand the causes of anomalies and to optimise calibration methods, it is important to measure the value and the effect of the parasitic components which could influence the final value and uncertainty of the applied vertical loads.
The paper describes the measurements performed on the 150 kN CENAM (Mexico) primary force standard deadweight machine in order to evaluate the value and the direction of the parasitic components (two side forces F_X and F_Y; two bending moments F_L and F_M, and the twisting moment F_N) generated by the standard machine The influence of the parasitic components on the Force Standard Machine accuracy and the values of dynamic components during the load application transient are studied as well. This evaluation was carried out in the framework of a scientific agreement between the Istituto Nazionale di Ricerca Metrologica (INRiM, Italy) and the Centro Nacional de Metrologia (CENAM, Mexico).

The National Measurement Institute of Australia (NMIA) and the Istituto Nazionale di Ricerca Metrologia of Italy together with the University of Cassino (INRIM/UNICAS) have participated in a research project to characterise a INRIM 100 MPa free deformation piston and cylinder assembly using two different numerical procedures based on a finite element method (FEM). The pressure distortion coefficient, ? and the piston fall rates, v were calculated from the clearance profile between the piston and cylinder obtained from dimensional measurement data. Comparison of the numerical results obtained by the two groups showed a relative difference of 2 × 10^-4 in λ and 2.5 × 10^-2 in v. The numerical results were also compared to the experimental results with a relative difference of 1.9 % in λ and 16 % at 100 MPa in v. This paper presents the numerical model used for the calculations of the pressure distortion coefficient and the piston fall rates with a sensitivity analysis of the model for the estimation of the uncertainty values of these two parameters.