Our check valves with plug-in connection let media flow in one direction only. The screw connections indicate the flow direction either with a switching symbol or with the printed-on text "IN" or "OUT". "IN" marks check valves that permit unhindered flow from the thread to the hose while preventing flow from the hose to the thread. The print "OUT" marks check valves where the flow is unhindered from the hose to the thread but blocked from the thread to the hose.