By adding a diaphragm to this environmental sealed DF300E unit, the spring chamber is sealed completely and can effectively improve the performance of the regulator for diving under extreme conditions. Regardless whether the pressure of the tank is 500psi or 3000psi, this 1st stage regulator will always offer divers stabilized air. The intermediate pressure internal adjustments allow divers to choose any one of the 2nd stages to connect with Promate DF300E regulators in today’s market.  Recommended 2nd Stage: OC350, OC300, OC450, OC400  Advantages: reliable, ideal regulator for dirty water environment and all diving conditions. Environmentalized to prevent freeze-up.  Users: ice divers, tech divers, pro divers