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 )). The conductance of the orifice system was also examined under free molecular conditions by simulating the movements of over 10 9 molecules. The value obtained, referred to the conductance of an ideal orifice , was 0.997 03 with an expanded uncertainty of 0.000 06 (3 ).
Different methods, based both on analytical considerations and on the Monte Carlo evaluation, were applied to the calculation of the correction factor, , of the conductance of the orifice of the new Istituto di Metrologia "G. Colonnetti" (CNR-IMGC) dynamic system. The results are compared and the uncertainty with which is determined is discussed. The data from the McCulloh method and from Monte Carlo simulations are compared. The latter method, while taking into account the conductance of all the geometrical parts, offers the possibility of evaluating the uncertainty without any approximation.
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