In buildings where it is impractical to keep temperatures above freezing, automatic sprinkler systems can be equipped with an Approved dry pipe valve. The sprinkler piping contains air under pressure, instead of water. When a sprinkler operates, air escapes, the dry pipe valve trips, and water is admitted automatically to the system. Dry pipe valves are designed so that a moderate amount of air pressure will hold back a much greater water pressure. In the differential type, the air pressure holds the water clapper closed either by means of a difference in areas on which the water and air pressures act, or by a combination of this principal with a lever action. In the mechanical type, a clapper on the water seat is held closed by a system of levers which are controlled by a relatively small air clapper. A suitable alarm is connected to the dry pipe valve so that a warning is given when the valve operates. This may be a mechanical alarm that operates by a hydraulic motor, or a hydraulic pressure switch that closes an electric circuit to operate a bell, horn, or remote signaling system. Unless otherwise noted in the listing, these dry pipe valves have 175 psi (1205 kPa) rated working pressure.