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 )). 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 ).

The development of vacuum measurements down to extremely high vacuum – XHV

“…Such devices are, generally, made of a calibration chamber connected to the pumping chamber through a conductance. The pumping system is, frequently, a combination of mechanical and turbo molecular pumps and the conductance is as near as possible to a hole in a thin wall to be able to calculate its value by analytical [98,99] or by Monte Carlo methods [100,101].Those systems need to be equipped with a gas flow generating and measuring unit that is frequently based on the variable-volume and constant-pressure method [102,103]. In the simple realization the lower limit may be between 10-6 and 10-7 Pa. For lower pressure, on the historical point of view it should be mentioned the PTB molecular beam system realized several years ago by G. Messer and G. Grosse [104,95].…”

New, fully automated, primary standard for generating vacuum pressures between 10 10Pa and 3 10 2Pa with respect to residual pressure

“…The two pump orifices, 1 and 2 , were fabricated with "knife edges" with a thickness of 70 m and 40 m, respectively. The influence of both the orifice holder and the thickness (Clausing factor) were calculated by Monte Carlo simulations and are reported elsewhere in this issue [6].…”

“…With sufficient accuracy, ␥ 1 1. Factor was calculated by Monte Carlo simulations, which are reported in more detail in [6]. Therein, were calculated for the three test gauge ports, where describe the deviations of the gas density in the test ports from a gas density under ideal conditions, i.e.…”

“…The total flow is (4) and the pressure in each calibration chamber is calculated from (5) where ␥ is defined as in (1). Using these equations and dropping the assumption of isothermal conditions, we obtain (6) and (7) The first fractions on the right-hand side of ( 6) and ( 7) are the dominant factors, which describe in the numerator the approximate incoming flux and in the denominator the pumping speed, while the following two fractions describe the loss and gain of gas particles due to various backstreaming effects. The factor and the temperatures are defined as in (1) and Figure 1.…”

Theoretical and Numerical Study of Free Molecular-Flow Problems

“…F REE-MOLECULAR flows passing through small holes represent fundamental and important problems in real applications such as materials processing in high vacuum chambers [1], spacecraft design [2], and metrology of gas flow [3]. Several investigators have proposed analytical expressions for the mass flow rate near the free-molecular regime in the case of gas flow from an orifice expanding into vacuum.…”