Check valves allow the flow of refrigerant in only one direction by automatically closing when fluid attempts to flow in the opposite direction. The moving element may be assisted by gravity so that the valve closes unless a slight pressure drop in the permitted flow direction opens the valve, as in Fig. 11.3a. This swing type of check valve is not as reliable as the spring-actuated valve of Fig. 11.3b, but this check valve imposes a pressure drop because of the spring pressure. A more sophisticated check valve is the gas-powered type in which the valve is normally open and held open by spring force. When the downstream pressure rises above the entering pressure to the valve, vapor from a high-pressure source closes the valve, which remains closed as long as the downsteam pressure exceeds the upstream pressure.