The RD-100 & RD-1900 Valves are pressure compensated adjustable flow control valves. By rotating the handle the flow out the "CF" or controlled flow part can be varied within the range given in the chart below. Any remaining flow is by-passed to the "EF" or excess flow port. This flow can be used to power another circuit or can be returned to the tank. Once the controlled flow is set it will remain nearly constant with variations in blocked, the valve will compensate in such a way as to shut off flow to the excess port. These valves can also be used as a restrictive flow control by plugging the excess flow port. The RDRS-100 & RDRS-1900 have a built-in adjustable pressure relief. For these models the excess flow must be connected to the tank.