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An absolute vacuum gauge exploiting molecular drag between coaxial discs, a configuration suggested by Langmuir, is being further investigated. The device operates in a range similar to that of the spinning rotor gauge. It consists of a silicon disc 92 mm in diameter suspended on a calibrated torsion fiber a few mm above a metal disc of the same size which rotates at speeds up to 50 000 rpm. Molecules leaving this rotor communicate a torque to the suspended disc by molecular drag. The angular deflection thus produced is sensed optically and is about 2° at 10−6 mbar. The critical factors relevant to absolute measurement, such as tangential momentum accommodation, geometry, and edge effects have been investigated and experimental comparisons with a spinning rotor gauge demonstrate that absolute pressure measurements with uncertainty better than ±5% are possible at pressures ∼ 10−6 mbar. Measurements of similar uncertainty will be possible at 10−7 mbar.

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Vakuurn in der Praxis (1994) Nr 1 S 42-47

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