Vacuum Technology Know How - Triumf

Pfeiffer Vacuum
Page 22
Formula 1-24
Desorption
Vacuum Technology
Vapor pressure p
10
x 1,33 mbar
3
10 2
10 1
10 0
10 -1
10 -2
10 -3
10 -4
10 -5
10 -6
10 -7
10 -8
300 800 °C 2,500 3,500
-150 -100 - 50 0 50 100 200 400 600 1,000 1,500 2,000 3,000
H 2 O
1.3.3 Desorption, diffusion, permeation and leaks
In addition to water, other substances (oil) can be adsorbed on surfaces. Substances can
also diffuse out of the metal walls, which can be evidenced in the residual gas. In the case of
particularly rigorous requirements, stainless steel vessels can be baked out under vacuum,
thus driving the majority of the volatile components out of the metal walls.
Desorption
Gas molecules, (primarily water) are bound to the interior surfaces of the vacuum chamber
through adsorption and absorption, and gradually desorb again under vacuum. The desorption
rate of the metal and glass surfaces in the vacuum system produces a gas yield that is a function
of time, however. A good approximation can be obtained by assuming that after a given
point in time t > t the reduction will occur on a linear basis over time. t is typically assumed
0 0
to be one hour.
The gas yield can thus be described as:
Q des = q des . A .
In this formula, q des is the surface-based desorption rate of the material, A the interior surface
area of the vacuum chamber, t 0 the start time and t the duration.
Hg
t
t 0
Cd
Al
Cu
Mg
Zn
In
Ag
Au
Fe
100 200 300 400 500 700 900 2,000 K 3,000 4,000
600 800 1,000
Temperature T
Melting point
Figure 1.9: Vapor pressure curves of various substances
Source: Jousten (publisher) Wutz, Handbuch Vakuumtechnik, Vieweg Verlag
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