A check valve is a valve that allows the flow of gasses or liquids in one direction only. Thus, they are classified as one-way directional valves. Flow, in the desired direction with enough force, will open the valve allowing flow through the valve. If flow is directed through the valve in the reverse direction, the valve closes and there is no flow. Most check valves are designed similarly. They consist of a seat, a ball or poppet, and a spring. The spring rests behind the ball or poppet, forcing the ball or poppet to rest firmly against the seat. The ball or poppet is slightly larger than the seat which forms the seal. To open the check valve, a force (flow) greater than that of the spring must be applied against the ball or poppet to overcome the resistance of the spring. Once the force (flow) drops below the force applied by the spring , the valve closes. Reverse flow is not permitted since the ball or poppet cannot go through the seat.