Vacuum

Pumping speed and gas load

Fig.1:Pumping speed vs. gas load illustration

Pumping speed S = dV ⁄ dt is a volume flow, i.e. the volume of gas passing through the pump; commonly used by manufacturers to characterize their pumps.

Gas load q ~ dm ⁄ dt is equivalent to a mass flow, i.e. the total number of molecules passing through the pump.

Evacuation time

Table 1:Evacuation time as a function of pumping speed S and operating pressure p

S [m³ ⁄ h]	10⁰ mbar	10⁻¹ mbar	10⁻² mbar	10⁻³ mbar	10⁻⁴ mbar
10	5 min	6 min	7 min	9 min	10 min
20	3 min	3 min	4 min	5 min	5 min
30	2 min	2 min	3 min	3 min	4 min
40	2 min	2 min	2 min	3 min	3 min
50	1 min	2 min	2 min	2 min	2 min
60	1 min	1 min	2 min	2 min	2 min
70	1 min	1 min	1 min	2 min	2 min
80	1 min	1 min	1 min	2 min	2 min
90	1 min	1 min	1 min	1 min	2 min
100	1 min	1 min	1 min	1 min	1 min
For p₀ = 1 013 mbar and V = 0.1 m³; rounded to the next integer

Effective pumping speed (S)

Pumping speed is not constant, but varies with pressure.
Different pumping speeds for different types of gases
Conductance C (how effectively the gas is transported in the system) may greatly influence the effective pumping speed:

Matching roughing pump and vacuum pump

Usually, the process defines gas load q and operating pressure p, and therefore the required effective pumping speed of the vacuum pump:

If a roughing pump is required, it has to handle the same gas load as the vacuum pump (because they are running in series).

Requirements for high vacuum

Table 2:Requirements for high vacuum

For high vacuum (and lower pressure) evacuation time and maintainable pressure are mainly determined by desorption and leakage rates.
≥ 10⁻⁴ mbar	Evacuation time is the sum of time for roughing pump and for vacuum pump (pressure may even be achieved with roughing pump only) ~ minutes
< 10⁻⁴ mbar	Evacuation is dominated by desorption fromchamber surfaceClean, smooth chamber surface is cruciale.g. turbo pump
< 10⁻⁶ mbar	Desorption of sealing material becomes relevantTurbo pump (possibly with assistance of ion getter pump)
< 10⁻⁸ mbar	Metal sealing is requiredIon getter pump is highly recommended
< 10⁻¹⁰ mbar	Leakage and permeation through chamber walls are no longer negligible