I want to measure certain gases in a nitrogen stream, but I have to dilute with nitrogen flow, which is flowed at a much higher rate for a photo-ionizing detector. However, I am getting backflow into the low flow stream. I tried adding a various mixing chambers of increasing size as the PID pumps the air as well; I was hoping that would work. Are they any special valves to prevent it? I know SEM's have pressure limiting apertures, but those seem pretty sophisticated. I would think there is something standard as this is probably done a lot.