The 1/4" Watts Series SD-3 Dual Check Valves with Atmospheric Port & Strainer for Carbonated Beverage Machines prevent the reverse flow of polluted water, such as carbon dioxide gas and carbonated water, from entering into the potable water supply due to backpressure backflow. It consists of stainless steel body construction with internal rubber components and is designed for continuous or intermittent pressure applications. Series SD-3 has an atmospheric port that provides visual indication of failure of the second check and a wye pattern strainer. Commonly found on carbonated beverage machines and coffee makers it has the ASSE 1022 approval required for such devices.