Monte Carlo study of a new PTB primary standard for very low pressures

Abstract

The Physikalisch-Technische Bundesanstalt (PTB), Germany, has recently constructed a new primary standard of low pressures based on the continuous expansion method. This paper is concerned with the properties of the gas flow in one of the calibration chambers of this standard. To describe these properties it is essential to know, with the smallest possible uncertainty, the conductance of the orifice and also the local gas densities at the points where the tested gauges are installed. The problem was investigated using the Monte Carlo method. The gas flux density on the chamber walls and the correction factors for each gauge port were determined (the highest value of the factor was -0.0033 with an expanded uncertainty of 0.000 26 (3 σ)). The conductance of the orifice system was also examined under free molecular conditions by simulating the movements of over 109 molecules. The value obtained, referred to the conductance of an ideal orifice C0 was Ceff/C0 = 0.997 03 with an expanded uncertainity of 0.000 06 (3 σ).