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Formula 1-8<br />
Mean free path<br />
www.pfeiffer-vacuum.net<br />
The following table shows values for selected gases.<br />
Table 1.4: Molar masses and mean thermal velocities of various gases<br />
Gas<br />
H2 He<br />
H2O N2 Air<br />
Ar<br />
CO 2<br />
1.2.5 Mean free path<br />
Molar Mass / (g / mol)<br />
2<br />
4<br />
18<br />
28<br />
29<br />
40<br />
44<br />
Mean Velocity / (m / s)<br />
1,762<br />
1,246<br />
587<br />
471<br />
463<br />
394<br />
376<br />
The mean free path is the mean path length that a molecule traverses between two successive<br />
impacts with other molecules. It depends upon molecular diameter d and temperature T<br />
m<br />
in accordance with the following equation<br />
–<br />
l . p =<br />
k . T<br />
and is of significance for the various flow types of a gas in a vacuum.<br />
Figure 1.4: Mean free paths<br />
p . 2 . 2<br />
dm<br />
The table [1] below shows the product l – . p for various gases at 0 °C.<br />
l –<br />
Mach Number<br />
5.3<br />
3.7<br />
1.8<br />
1.4<br />
1.4<br />
1.2<br />
1.1<br />
Page 13<br />
<strong>Vacuum</strong><br />
<strong>Technology</strong>
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