Dry Pipe Valve is essentially a differential check valve. The bronze clapper carries two rubber gaskets. The larger (air) gasket is rubber and seats against pure tin. The smaller (water) gasket is a specially designed rubber disc, which also seats on pure tin. These seats are proportioned so that 1 pound (.454 kg) of air pressure will hold back approximately 6 pounds (2.72kg) of water pressure.
When the air pressure on the surface of the priming water is relieved by the opening of a sprinkler, the upward pressure of the water underneath the water gasket causes the clapper to lift, the chamber instantly fills and sounds the alarm. The water pressure acting on the entire surface of the clapper pushes it over to the wide open position and thus leaves a passage for the water to the sprinkler system.