Compression ratio turbomolecular pumps

The compression ratio (often also simply termed compression) of turbomolecular pumps is the ratio between the partial pressure of one gas... 

Turbomolecular pumps are highly-regarded as clean pumps. They can be regarded as having constant pumping speed when normal rotational speed has been achieved. Important characteristics of turbomolecular pumps are the compression ratio and pumping speed. The compression ratio for a particular gas (A) is given in Equation (3.15) ... 

The central transport chamber is an 80-cm-diameter stainless steel vessel, and is pumped by a 1000-1/s turbomolecular pump, which is backed by a small (501/s) turbomolecular pump to increase the compression ratio for hydrogen, and by a 16-m /h rotating-vane pump. UHV is obtained after a bake-out at temperatures above 100°C (measured with thermocouples at the outside surface) of the whole system for about a week. A pressure in the low 10 " -mbar range is then obtained. With a residual gas analyzer (quadrupole mass spectrometer, QMS) the partial pressures of various gases can be measured. During use of the system, the pressure in the central chamber is in the low 10 -mbar range due to loading of samples. Water vapor then is the most abundant species in the chamber. 

For the measurement of the compression ratio (k) of a turbomolecular pump, a test volume was attached to the pump inlet via a CF-flange. 

A mechanical pump providing even lower vacuum levels is the turbomolecular pump, in which one or more balanced rotors (turbine blades) spin at 20,000 to 50,000 rpm. At these rotation rates, the periphery moves at a speed that exceeds the mean molecular speeds of most molecules, and gas-rotor collisions impart a momentum component to the gas in the direction of the exhaust. Compression ratios up to 10 can be achieved as long as the outlet pressure is kept below about 0.1 Torr by a forepump. 

For the turbomolecular pumps mostly used at present the given rating is defined for pumping nitrogen atmosphere. The important performance data of turbo-molecular pumps is the compression ratio. The compression ratio describes the ratio of the outlet pressure (forepump) of a particular gas to the inlet pressure (MS). The turbomolecular pump gives a completely background-free high vacuum and exhibits excellent start-up properties, which is important for benchtop instruments or those for mobile use. The use of helium lowers the performance of the... 

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