A theoretical calculation is made of the back-diffusion coefficient,
b, of diffusion pump, giving the formula:
b=exp(-4[ƒ(+)-ƒ(-)]
qL/λ
ON_??_),
where ƒ(+)-ƒ(-) represents the fraction of gas molecules travelling towards the fore vacuum side after the collisions with pumping fluid molecules,
q the rate of flow of the fluid vapor stream,
L the effective length of the stream,
N the molecular density at 1μHg pressure, λ
0 the mean free path of a fluid molecule in the gas at
N concentration and
v the mean molecular velocity of gas. This formula explains satisfactorily Dayton's experiments with octoil and air. The difference from the earlier theories lies in the free-molecule treatment of the pumping action whereby thorough consideration is given to the mechanism of intermolecular collisions.
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