Spinning Rotor Pressure Gauge

A magnetically suspended rotor pressure gauge is described for measuring pressures below 10−4 torr. The data are in good agreement with those obtained with a calibrated Alpert ionization gauge over the range 10−4 to 5×10−8 torr. When the gas in the vacuum system was frozen out with liquid helium, the rotor gauge recorded a residual pressure of about 5×10−10 torr. A brief discussion is given of a number of effects which unless eliminated may introduce errors into the measurements at the lowest pressures.