"The photograph illustrates a common, but incorrect, placement of a sprinkler system pressure gauge on a main drain outlet. There are two pressure gauges required on wet pipe sprinkler systems having an alarm check valve: one on the supply side of the check valve (lower), and one on the system side (upper). In addition to measuring the normal incoming water pressure, the lower gauge is used for documenting water supplies during periodic main drain tests."
Coffee Break Training - Fire Protection Series No. FP-2006-36
United States Fire Administration: http://www.usfa.fema.gov/