Performance Improvement
Pumping capacity can be improved by prepumping the system by another simple and clean vacuum pump like a diaphragm pump or even a water aspirator or compressed-air venturi pump.
Sequential or multistage pumping can be used to attain lower pressures. In this case two or more pumps are connected in parallel to the vacuum vessel. Every pump has a valve to isolate it from the vacuum vessel. At the start of the pump down all valves are open. The first pump is cooled down while the others are still hot. When the first pump has reached its ultimate pressure it is closed off and the next pump is cooled down. Final pressures are in the 10-4 mbar region. What is left is mainly helium because it is almost not pumped at all. The final pressure almost equals the partial pressure of helium in air.
A sorption pump does pump all gases effectively with the exception of hydrogen, helium and neon which do not condensate at liquid nitrogen temperatures and are not efficiently adsorbed by the molecular sieves because of their small molecular size. This problem can be solved by purging the vacuum system with dry pure nitrogen before pump down. In purged system with aspirator rough pumping ultimate pressures of 10-4 mbar for a single sorption pump and 10-7 mbar for sequential pumping can be reached. A typical source of dry pure nitrogen would be a liquid nitrogen Dewar head space.
It has been suggested that by applying a dynamic pumping technique hydrogen, helium and neon can also be pumped without resorting to dry nitrogen purging. This is done by precooling the pump with the valve to the vacuum vessel closed. The valve is opened when the pump is cold and the inrush of adsorbable gases will carry all other gases into the pump. The valve is closed before hydrogen, helium or neon can back-migrate into the vacuum vessel. Sequential pumping can also be applied. No final pressures are given.
Continuous pumping may be simulated by using two pumps in parallel and letting one pump pump the system while the other pump, temporally sealed-off from the system, is in the desorption phase and venting to the atmosphere. When the pump is well desorbed it is cooled down and reconnected to the system. The other pump is sealed-off and goes into desorption. This becomes a continuous cycle.
Read more about this topic: Sorption Pump
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