Vacuum Technology Know How - Triumf

Formula 7-10
Diffusion coefficient (T)
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The reason for this is that the escaping gases are not only bound on the surface, but must
also diffuse out of the interior of the seal. With extended pumping times, desorption from
plastics can therefore dominate desorption from the metal surfaces. The outgassing rate of
plastics is calculated in accordance with:
Q desK = q desK . Ad
t
t 0
(Formula 1-25).
We use Q desK = S . p desK and obtain the following for p b4 = 10 - 8 mbar: t 4 = 459 10 6 s = 1,277 h.
In this connection, t 0 = 3,600 s and the associated value q desK is read from the diagram for
FPM [19]. It can be seen that the contribution to pump-down time made by desorption of the
cold-state seal is on a similar order of magnitude as that of the metal surface.
Since the diffusion of the gases released in the interior of the seal will determine the behavior
of the desorption gas flow over time, the dependence of diffusion coefficient D upon temperature
will have a crucial influence on pumping time:
E dif
D = D 0 exp (- )
R . T
As temperature rises, the diffusion coefficient increases, as well; however it will not rise as
much as the desorption rate of the metal surface. We thus see that elastomer seals can have
a pronouncedly limiting effect on base pressure due to their desorption rates, which is why
they are not suitable for generating ultra high vacuum.
Leakage rate and permeation rate
The gas flow that flows into the vacuum system through leaks is constant and results in
pressure . A system is considered to be sufficiently tight when this pressure is less
than 10 % of working pressure. Leakage rates of 10 - 8 mbar . Ql p = l S
l / s are usually easy to attain and
are also required for this system. This results in a pressure proportion of the leakage rate of
p l = 1.46 . 10 -11 mbar. This value is not disturbing and can be left out of consideration.
Permeation rates through metal walls do not influence the ultimate pressure that is required
in this example; however diffusion through elastomer seals can also have a limiting effect on
base pressure in the selected example.
Summary
Pressures of up to 10 - 7 mbar can be attained in approximately one day in clean vessels
without the need for any additional measures.
If pressures of up to 10 - 4 mbar are to be attained, the pump-down times of the backing
pump and the turbopump must be added together. In the above-mentioned case, this is
approximately 200 s. At pressures of less than 10 - 6 mbar, a high turbomolecular pump
pumping speed will be required, in particular in order to pump down the water adsorbed
on the metal walls.
This will only be possible by additionally baking out the vacuum vessel (90 to 400 °C) if the
required base pressure p b of 10 - 8 mbar is to be attained within a few hours. The heater is
turned off after 100 times the value of the desired pressure has been attained. The base
pressure will then be reached after cool-down of the vacuum vessel.
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