<< [[Vacuum Technology MOC]] ![[_Users_isabelburgos_Documents_Obsidian_Notes_bin_Linked Files_Screen Shot 2021-07-20 at 5.49.33 PM.jpg|med right]] Gas molecules are more likely to collide with one side of a fast moving rotor, so gas is pumped in one direction. Kinetic energy is transferred from rotors to gas molecules through elastic collisions and adhesion forces. At final pressure, back diffusion of molecules equals pumping speed. The maximum compression coefficient is higher for large atomic numbers (low atomic numbers -> higher rms velocity -> more back diffusion) ![[_Users_isabelburgos_Documents_Obsidian_Notes_bin_Linked Files_Screen Shot 2021-07-20 at 5.55.50 PM.jpg|med]]![[_Users_isabelburgos_Documents_Obsidian_Notes_bin_Linked Files_Screen Shot 2021-07-20 at 5.56.59 PM.jpg|med]] This mechanism only works in the molecular flow range - collisions between gas molecules diminish pumping effect.