They are used as fix set one-way flow regulator, it allows free flow in one way and works as flow regulator by means of a calibrated orifice on the opposite one. The sizes are from 1/4 BSPP (1/4 G) to 1BSPP (1 G). It is useful in small places or to compact the hydraulic installations. The coupling is made by high resistance steel and externally protected from oxidations by zinc treatment. Different opening pressure than the standard one (1 bar) are possible: 3-4.5-6 bar.