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Most turbomolecular pumps employ multiple stages, each consisting of a quickly rotating rotor blade and stationary stator blade pair. The system is an axial compressor that puts energy into the gas, rather than a turbine, which takes energy out of a moving fluid to create rotary power, thus "turbomolecular pump" is a misnomer.
The turbomolecular pump overcame many of these disadvantages. Many modern turbomolecular pumps contain built-in molecular drag stages, which allows them to operate at higher foreline pressures. As a stage in turbo molecular pumps, the most widely used design is the Holweck type, due to a significantly higher pumping speed than the Gaede design.
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Samples may become contaminated by titanium if they have line-of-sight to the pump. Also, titanium is a very hard material, so titanium film which builds up on the inside of the chamber may form flakes which fall into mechanical components (typically turbomolecular pumps and valves) and damage them. Many chambers containing TSPs also have an ...
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An oil diffusion pump is used to achieve higher vacuum (lower pressure) than is possible by use of positive displacement pumps alone. Although its use has been mainly associated within the high-vacuum range, down to 1 × 10 −9 mbar (1 × 10 −7 Pa), diffusion pumps today can produce pressures approaching 1 × 10 −10 mbar (1 × 10 −8 Pa) when properly used with modern fluids and accessories.
To achieve a higher vacuum, other techniques must then be used, typically in series (usually following an initial fast pump down with a positive displacement pump). Some examples might be use of an oil sealed rotary vane pump (the most common positive displacement pump) backing a diffusion pump, or a dry scroll pump backing a turbomolecular pump.
Generally, axial pumps tend to give much lower pressures than centrifugal pumps, and a few bars is not uncommon. Their advantage is a much higher volumetric flowrate. For this reason they are common for pumping liquid hydrogen in rocket engines, because of its much lower density than other propellants which usually use centrifugal pump designs.