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Why does the pumping speed of an oil-sealed rotary pump decrease at very low pressures?
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vacuum technology
Diagram of a McLeod gauge.
In vacuum technology, an oil-sealed rotary pump maintains its full pumping speed from atmospheric pressure down to approximately one torr. Below this threshold, the pumping speed gradually decreases, eventually reaching zero at the pump's ultimate pressure.1
This decline in performance occurs because the ultimate pressure is limited by internal leakage between the high-pressure and low-pressure sides of the pump. This leakage is primarily caused by two factors:
- Gas and Vapor Carryover: Gases and vapors that have dissolved into the sealing oil are released (or "flash off") when they are exposed to the low-pressure inlet side of the pump.1
- Oil Decomposition: The oil may undergo decomposition when exposed to hot spots generated by friction within the pump mechanism.1
As the inlet pressure approaches the pump's ultimate limit, these internal sources of gas effectively counteract the pumping action, causing the net pumping speed to drop.1