Calculation and uncertainty evaluation of conductance of a precise orifice for orifice-flow method

For the realization of an accurate vacuum standard by the orifice-flow method in high and ultra-high vacuum, it is important to determine the conductance of an orifice accurately. Since the shape of the edge of the orifice significantly influences the conductance, a new orifice with thin tapered edge was designed and fabricated to measure its dimension and shape precisely with microscopy techniques.The area of the aperture was determined with the relative standard uncertainty, u_r, of 0.011% from the diameter measurement. The transmission probability, W, was estimated with u_r of 0.039% using a Monte Carlo calculation from the geometry. Influences of a deviation from cosine distribution of incident molecules due to the geometry of a vacuum chamber and the specular reflection at the orifice edge on the value of W were also discussed.As a result, the conductance was calculated with u_r of 0.041%. The main source of the uncertainty was that of W and originated from its geometry, which verifies that the measurement of orifice edge is important for the accurate evaluation of the conductance.