Measurements are reported of the pumping speed, for molecular flow, of a circular hole located coaxially in a diaphragm across a circular cylindrical tube. A study is specifically made of the conductance between points not immediately adjacent to the diaphragm but separated by an axial distance L.
A theoretical derivation of the conductance leads to the expression
are, respectively, the areas of the aperture and tube, D0
represents the diameter of the tube, K
, and C
is a dimensionless factor which has been evaluated as a function of relative aperture size. C
takes on values from C
=1 for A
small to C
=4/3 for A
. The experimental data indicate that the reciprocal of the conductance per unit area of the aperture does vary in an approximately linear fashion with A
, in concordance with the theoretical expression in which the slow variation of the coefficient C
is of little effect. It is suggested that knowledge of the pumping speed for the case considered may be of value in measurements of speed at other parts of a kinetic vacuum system.