The Self-Flushing QD Sample Kit has "concentric" flow paths. When not taking a sample, there is a flushing circuit open. Fuel inside the piping impacts the probe inlet at or near the center of the pipe, which produces a slightly higher pressure. This causes fuel to flow down the probe tube and into the QD body. Fuel flows down the center of the poppet and then outwards, around the outside of the poppet and back to the main pipe through passageways on the outside of the probe. This flow sweeps the QD clean whenever there is flow in the pipe. When you actuate the QD on the Self-Flushing QD Sample Kit, the poppet moves back and closes off the flushing circuit, so that only fuel from the center of the pipe, the probe tube inlet, will reach the QD.
