A Reduced Pressure Zone Device (RPZD) is a type of backflow prevention device which is used to protect water supplies from contamination. They may also be known as reduced pressure zone assemblies. An RPZD is suitable for high hazard situations where the outcome of backflow would cause significant harm. They are also considered suitable because they prevent both back flow and back siphonage mainly due to their design but also because they are testable to verify correct operation. They are often chosen to install instead of an “air gap” because the head loss across the RPZD is much smaller. The structure of the RPZD consists of two independent check valves which are plumbed in series, with a pressure monitor chamber between them. This chamber is maintained at a pressure that is lower than the water supply pressure, but high enough to be useful downstream. The reduced pressure is guaranteed by a different relief valve which automatically relieves excess pressure in the chamber by discharging to a drain.