An altitude pilot valve that is spring and diaphragm operated; it controls the water level in a reservoir by sensing the hydrostatic head. When the hydrostatic head equals the spring force, the pilot connects port “X” (connection to main valve inlet ) to port “K” (connection to main valve bonnet). The main valve closes. When the hydrostatic pressure decreases slightly, the port “X to K” connection is closed by the inner valve. When the hydrostatic pressure is reduced even further, the pilot connects port “K” (main valve bonnet) to “EX” (exhaust to atmosphere). Then the main valve opens.
Model 301-4 pilot is used as the standard pilot on all 106 and 206 series altitude valves.