The F-2804 Series Small Check Valves permit flow in one direction only. The operation of the check valve is based on the movement of a small disc. The disc shifts within the housing as the pressure differential changes from forward to reverse. A flat surface on one side of the disc seals off flow, while the other side allows flow to pass. Two models are available from the Standard Units: F-2804-401, 402, 403, and the High-Flow Unit: F-2804-404. The advantages of the check valve design is the low "cracking pressure" minimum differential required for forward flow, which is less than the 0.005 PSI differential in the standard units. Secondly, there is no residual pressure difference across the check valve once flow has ceased. Flow in the forward direction is relatively unrestricted, approximately equivalent to the restriction of a 0.040 inch orifice in the standard units.