Ultra-Low-Friction, Zero-Power Magnetic Suspension System of the Spinning Rotor Vacuum Gauge

Magnetic suspension systems have been developed at the University of Virginia Holmes and Beams 1937) for the special purpose of spinning small bodies up to ultimate speed. The frictional torque acting on a rotating body when being suspended in a magnetic field with a highly symmetric flux distribution around the vertical axis was found to be many orders of magnitude lower than the torque exerted by gas friction at atmospheric pressure. Consequently, that type of suspension system was proposed and used 1ateron for pressure determination in rarefied gases. Drag torques down to 10-17 Nm have been resolved under optimum laboratory conditions. The present paper describes the magnetic suspension system of the "Spinning Rotor Vacuum Gauge" (SRG) which as a commercial unit resolves about 10-14 Nm under regular laboratory conditions. This figure corresponds to a pressure resolution of about 10-7 mbar, nitrogen equivalent, of the gauge. The SRG was introduced as a transfer standard in 1979 by BIPM and is commercially available since 1981. The gauge head functions will be discussed with special emphasis on minimization of the residual drag.