Laboratory Faucet Backflow Preventers are designed to protect the potable water supply from pollutants or contaminants entering the system due to back siphonage or back pressure. They are designed for installation on laboratory faucets on the discharge side of the last shutoff valve. They are not for use under constant pressure conditions. They consist of two independently acting check valves, biased to a normally closed position, and between the check valves a means for automatically venting to atmosphere, biased to normally open position.