Vacuum Pressure Measurements using a Magneto-Optical Trap
  The loading dynamics of an alkali-atom magneto-optical trap can be used as a reliable measure of vacuum pressure, with loading time T indicating a pressure less than or equal to [2x10^(-8) Torr s]/T. This relation is accurate to approximately a factor of two over wide variations in trap parameters, background gas composition, or trapped alkali species. The low-pressure limit of the method does depend on the trap parameters, but typically extends to the 10^(-10) Torr range. 
T. Arpornthip and C. A. Sackett and K. J. Hughes
physics.atom-ph