How a differentially-pumped rotary seal works

Sometimes it becomes necessary to introduce rotating motion into a ultra-high vacuum (UHV) vessel while simultaneously providing access for electrical, cooling and auxiliary motions. The graphic below shows a cross section of a typical, differentially-pumped rotating seal.

Cross section

Fluorocarbon seals will easily withstand a pressure differential of 1 X 10-7 torr when properly fitted. Differential pumping can extend the range of a single seal significantly to maintain true UHV pressures. For example, in the graphic above, providing rough pumping (~10-3 torr) in the Outer pumpout volume and moderate pumping (small ion or turbo pump) on the Inner pumpout volume, results in a pressure differential between UHV and the Inner volume of 10-5 torr but virtually no "real" pressure differential to drive gas molecules across the seal. The result is a leak-tight, yet rotating, seal.

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Cross-over Manifold

When using both differentially-pumped volumes, it is necessary to rough the high vacuum volume before starting the pump. The cross-over manifold allows this.

Cross section

Connect the Inner volume port to the high vacuum pump (usually an ion pump) and connect the Outer volume port to the roughing pump. Open the all metal, bellows-sealed cross-over valve to rough both sections. Then close the valve and start the high vacuum (ion) pump. Piece o' cake.

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