Editing Turbomolecular pump (section)

==Maximum pressure==
[[Image:Edwards turbopump.jpg|thumb|left|A turbomolecular pump with attached [[vacuum ionization gauge]] for [[pressure measurement]].]]
At atmospheric pressure, the [[mean free path]] of air is about 70&nbsp;nm. A turbomolecular pump can work only if those molecules hit by the moving blades reach the stationary blades before colliding with other molecules on their way. To achieve that, the gap between moving blades and stationary blades must be close to or less than the mean free path. From a practical construction standpoint, a feasible gap between the blade sets is on the order of 1&nbsp;mm, so a turbopump will stall (no net pumping) if exhausted directly to the atmosphere. Since the mean free path is inversely proportional to pressure, a turbopump will pump when the exhaust pressure is less than about {{convert|10|Pa|mbar|abbr=on}} where the mean free path is about 0.7&nbsp;mm.

Most turbopumps have a [[Holweck pump]] (or molecular drag pump) as their last stage to increase the maximum backing pressure (exhaust pressure) to about 1–10&nbsp;mbar. Theoretically, a centrifugal pump, a side channel pump, or a [[vacuum pump|regenerative pump]] could be used to back to atmospheric pressure directly, but currently there is no commercially available turbopump that exhausts directly to atmosphere. In most cases, the exhaust is connected to a mechanical backing pump (usually called [[roughing pump]]) that produces a pressure low enough for the turbomolecular pump to work efficiently. Typically, this backing pressure is below 0.1&nbsp;mbar and commonly about 0.01&nbsp;mbar. The backing pressure is rarely below 10<sup>−3</sup>&nbsp;mbar (mean free path ≈ 70&nbsp;mm) because the flow resistance of the vacuum pipe between the turbopump and the roughing pump becomes significant.

The turbomolecular pump can be a very versatile pump. It can generate many degrees of vacuum from intermediate vacuum (≈10<sup>−2</sup> Pa) up to [[ultra high vacuum|ultra-high vacuum]] levels (≈10<sup>−8</sup> Pa).

Multiple turbomolecular pumps in a lab or manufacturing-plant can be connected by tubes to a small backing pump. Automatic valves and [[diffusion pump]] like injection into a large buffer-tube in front of the backing pump prevents any overpressure from one pump to stall another pump.