Pfeiffer Vacuum

2.3.2 Application notes

Their hydrocarbon-free suction chambers make diaphragm pumps particularly well suited as dry backing pumps for turbomolecular pumps with Holweck stage. Even two-stage diaphragm pumps that can reach an ultimate pressure of approximately 5 mbar can be used as backing pumps for Holweck turbopumps. Their clean vacuum is particularly valued for analytical applications. Diaphragm pumps, too, do not displace water vapor without gas ballast. Even the low volumes of water vapor that desorb from the walls of high vacuum equipment can allow the ultimate pressure of a diaphragm pump to increase dramatically. However some diaphragm pumps are equipped with a gas ballast valve that operates in accordance with a patented process.

For this purpose, gas is admitted into the connection channel between the first and second stages of two-stage diaphragm pumps, and communicates with the suction chamber of the first stage via a small hole. If greater volumes of moisture accumulate and diaphragm pumps without gas ballast are being used, suitable separators or cooling traps must be connected upstream to prevent significant condensate formation in the pump. However the ultimate pressure will nevertheless increase.

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