The Dennerle CO2 Special Check Valve reliably protects CO2 units from backflowing water. CO2 is a special gas which has quite different physical properties to air. Dennerle has thus developed a special check valve for operation with CO2. The core element is a special permanently elastic CO2-proof membrane. It is fitted under tension in firm contact with sealing rims. 2 hose connectors with screw fittings ensure CO2-proof connection.