When equipped with closed-loop Series PRS, air-loaded pressure regulator maintains downstream pressure of 5-125 psi with greater precision, regardless of variation in flow. Proportional controller installs on 1/2 in. line at point of desired control and interfaces with regulator via 1/4 in. pneumatic tubing. When line pressure surges past setpoint, it reduces control pressure to regulator; when line pressure dips, it increases control pressure. Device works with ultra-pure deionized water and highly corrosive chemicals, and is available in PVC, natural polypropylene, Kynar PVDF, and Teflon.