The U.S. Gauge 1590 sprinkler system gauge is designed for use on wet or dry sprinkler systems. These 3-1/2" gauges are approved by the Factory Mutual System and are listed by Underwriters' Laboratories. The R-1590 retarded air gauge is used on the sprinkler head side of the control valve in a dry system to accurately monitor line air pressure, and also has the ability to accept water pressure up to 250 psi upon system activation. The P-1590 is used on the water supply side of the system to indicate available source pressure, and is used alone on wet systems where water is available to the sprinkler head at all times. The P-1590 also comes in an air/water version.