Paper VT-TuA11
Uncertainty of UHV Flowmeter Standard Related to the Gas-Surface Interaction
Tuesday, October 20, 2015, 5:40 pm, Room 230B
Gas flows through orifices of various shapes are used in the primary metrology of vacuum in order to develop a primary standard of conductance [1]. Since the orifices are usually very think and the most part of gaseous molecules passes through the orifice without a collision with its surface, it is assumed the diffuse scattering for those particles which undergo collisions with the surface. However, many experiments, see e.g. [2], pointed out a significant deviation from the diffuse scattering especially for light gases like helium so that the conductance calculated on the basis of the diffuse scattering can be different from the real one and that leads to an additional uncertainty that is not included in the total uncertainty [1]. As was pointed out in Ref.[3], the diffuse-specular model of the gas surface interaction having just one adjustable parameter contradicts to some experimental data on the so-called thermomolecular pressure difference. At the same time, the Cercignani-Lampis (CL) model containing two adjustable parameters describes more physically the gas-surface interaction. The aim of the present work is to calculate the conductance of orifices used in the primary metrology based on the CL model using the accommodation coefficient extracted from various experimental data. A comparison of these data with those obtained for the diffuse scattering will give us the uncertainty related to the gas-surface interaction. Preliminary results show that the uncertainty is within 1%. Basing on these data, some recommendations to reduce the uncertainty will be given.[1] L. Peksa et al., Vacuum. Vol.101, P.377 (2014)
[2] B.T. Porodnov et al., J. Fluid Mech. Vol.64, P.417 (1974)
[3] F. Sharipov and V. Seleznev, J. Phys. Chem. Ref. Data. Vol.27, P.657 (1998).