Gas-Based Pressure Fixed Points

Abstract

In Chapters 7 and 8 we discussed the pressure measurements in gaseous media from 100 Pa to 100 MPa, as well as the appropriate primary and secondary pressure standards and the problems connected with their use at the lowest uncertainty level. The basic definition of the pressure is given, for the pressure range considered in this book, in terms of a force exerted on a known area or of the height of a liquid column. The relationship between well-defined states and pressure values has proved very useful from the metrological standpoint, particularly in view of the possible use of such states as transfer standards, for the verification of pressure values obtained from the measurement of force per unit area. The discussion in the present book being limited to gaseous substances, the establishment of a pressure fixed point involves the definition of a pressure-to-temperature relation generally occurring during a phase transition (triple point, melting or freezing curve, vapor-pressure equilibrium, critical point,...;), which is intrinsically based on some invariant properties of the substance (see Chapter 2).