Editing Turbomolecular pump (section)

==Practical considerations==
Laws of [[fluid dynamics]] do not provide good approximations for the behavior of individual, highly separated, non-interacting gas molecules, like those found in [[vacuum|high vacuum]] environments. The maximum compression varies linearly with circumferential rotor speed.  In order to obtain extremely low pressures down to 1 [[pascal (unit)|micropascal]], rotation rates of 20,000 to 90,000 revolutions per minute are often necessary. Unfortunately, the compression ratio varies exponentially with the square root of the molecular weight of the gas.  Thus, heavy molecules are pumped much more efficiently than light [[molecule]]s.  Most gases are heavy enough to be well pumped but it is difficult to pump [[hydrogen]] and [[helium]] efficiently.

An additional drawback stems from the high rotor speed of this type of pump: very [[Magnetic bearing|high grade]] [[bearing (mechanical)|bearing]]s are required, which increase the cost.
[[File:Turbomolecular pump 2.jpg|thumb|right|250 px|The turbomolecular pump from ICP-MS instrument Varia.]]
Because turbomolecular pumps only work in molecular flow conditions, a pure turbomolecular pump will require a very large backing pump to work effectively. Thus, many modern pumps have a molecular drag stage such as a [[Holweck pump|Holweck or Gaede]] mechanism near the exhaust to reduce the size of backing pump required.

Much of recent turbo pump development has been focused on improvement of the effectiveness of the drag stages.   As gas is removed from a pumped space, the lighter gases hydrogen and helium become a larger proportion of the remaining gas load.  In recent years it has been demonstrated that the precise design of the surface geometry of the drag stages can have a marked effect on pumping of these light gases, improving compression ratios by up to two orders of magnitude for given pumping volume.{{Citation needed|date=November 2022}}   As a result, it is possible to use much smaller backing pumps than would be required by pure turbomolecular pumps and/or design more compact turbomolecular pumps.