  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.
