Two independently acting check valves together with a hydraulically operating, mechanically independent pressure differential relief valve located between the check valves and below the first check valve. These units are located between two tightly closed resilient seated shutoff valves, as an assembly, and are equipped with properly located resilient seated test cocks.
(Image courtesy of Chicago Backflow Inc.)
Double Detector Check
This assembly consists of two internally loaded check valves, either spring or internally loaded, weighted, installed as a unit between two tightly closing resilient-seated shutoff valves as an assembly, and fittings with properly located resilient seated test ports or cocks.