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There are several contributions to the gas load of a system. At pressures below ~0.1 mbar, the most dominant is often ‘outgassing’.

Outgassing is the result of desorption of previously adsorbed molecules, bulk diffusion, permeation, and vaporization. Adsorption occurs via two main processes, physisorption and chemisorption, and can be described using five (or six) classifying isotherms.

Where do contributions to the gas load come from?

Looking at the desorption rate, pumping speed and re-adsorption on surfaces, the net outgassing of the system can be calculated.

As seen in Diagram 1, contributions to the gas load of a system can come from:

  1. Initial or the ‘bulk’ gas in the system
  2. Process load
  3. Back-streaming
  4. Leaks
  5. Outgassing

For a leak-tight system in High Vacuum (HV) with no process load, outgassing could contribute up to 100% of the gas load.

Turbo molecular pumper system diagram

Diagram 1: Gas loads in a vacuum system

  1. Process load
  2. Outgassing
  3. Leaks
  4. Back-streaming
  5. Initial gas

The relative contribution of different species to the gas load varies with pressure.

For many HV applications water vapour is the major concern in terms of outgassing. However, for achieving UHV in all metal systems, H2 outgassing is critical.

The table below shares typical major gas loads at various pressures.

Pressure (mbar)

Major gas load


Air (N2, O2, H2O, Ar, CO2)


Water vapour (75-95%), N2, O2


H2O, CO, CO2, N2


CO, H2, CO2, H2O


H2, CO


H2, CO

4 main mechanisms which contribute to outgassing

  1. Vapourisation of the actual surface material itself (in metals this is negligible at typical operating temperatures)
  2. Desorption: this is the reverse process of adsorption; the release of molecules bound at the surfaces of the chamber and internal fixtures
  3. Diffusion: this is the movement of molecules from the inner structure of the material to the surface
  4. Permeation: this is the movement of molecules from the external atmosphere through the bulk to the vacuum surface

The extent to which each of these contributes to outgassing depends on the composition of both the gas and the surface material (and its history). Outgassing rates are a sum of these contributions.

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