High-accuracy calibration in the vacuum range 0.3 Pa to 4000 Pa using the primary standard of static gas expansion

Static gas expansion provides a convenient primary standard for calibrating pressure gauges in the medium vacuum range. This paper reports on a thorough investigation of the fundamental principle, possible disturbances and achievable uncertainty of such an apparatus. The method is described and model equations for the general case of a non-isothermal expansion of a real gas are derived. As temperature effects play an essential role in the uncertainty budget, the vessels of the apparatus were immersed in a temperature-controlled circulating water bath. In addition, the inherent transient temperature effects originating from the gas expansion process itself were studied experimentally, which leads to a better understanding of these effects. As a result, uncertainties caused by temperature effects could be substantially reduced. Furthermore, deviations between different real gases were studied. The investigations led to greater accuracy of the primary standard apparatus, i.e. an expanded (k = 2) relative uncertainty of 1 × 10−3 at a pressure of 10 Pa and of 5 × 10−4 at 1000 Pa.