Waterflow alarm valves (alarm check valves) are installed in either vertical or horizontal positions in wetpipe sprinkler systems to sound a fire alarm gong when a flow of water from the system equals or exceeds that of a single sprinkler. They are designed for use with hydraulic or electric gongs or both. There are two designs, the differential type and the pilot valve type. The differential type has a rubber-faced clapper with an annular groove in the seat; the pilot valve type has a metal-faced clapper with a pilot valve attached to the clapper arm. A retard chamber, which minimizes false alarms due to surges and fluctuations in water supply pressure, may be supplied with the alarm valve. If a retard chamber is supplied, only the listed chamber for a specific manufacturer and valve model shall be used. Alarms are tested by flowing water through the system test pipe. Pilot valves may be field-adjusted to assure alarm reliability. Unless otherwise noted in the listing, these valves have 175 psi (1205 kPa) rated working pressure.