The maximum ratio of exhaust pressure to inlet pressure a pump can sustain for one gas with its inlet closed off. Turbomolecular pumps reach about 10⁹ or more for nitrogen but far less for light gases, often 10³ to 10⁵ for hydrogen, which is why hydrogen dominates the residual gas above a turbo. Roots pumps manage only a few tens, which is why they always need a backing pump.
Compression ratio for H2 on this little turbo is lousy, so the hydrogen peak on the RGA tracks the foreline pressure.
Also heard as
- K0
- compression
Related terms
Turbomolecular pump
High vacuum pump whose rotor blades spin at tens of thousands of rpm and bat molecules toward the exhaust.
Roots pump
Rotary lobe booster with two figure-eight rotors that adds large pumping speed between about 10 and 10⁻³ mbar.
Backing pump
Pump that keeps the exhaust of a turbo, diffusion or Roots pump below its maximum allowed pressure.
Ultimate pressure
The lowest pressure a pump can reach on a sealed, clean test dome, as stated on its data sheet.