The first stage of a scuba regulator reduces the gradually decreasing high pressure of a scuba tank to supply a constant low pressure to the second stage regulator. This output pressure of the first stage to the second stage is known as the intermediate (aka inter-stage) pressure or IP. When diagnosing regulator problems, it's often very helpful to know if the IP is out-of-range. As the knife edge seat in most first stages wear from use, their IP tends to become higher and will cause the second stage to dribble gas or even free flow. Slightly out-of-range values typically indicate the need for a simple IP adjustment to compensate for wear. Significantly out-of-range values, particularly with "creep," often indicate a more serious impending high-pressure seat failure. In addition, slow recovery time can indicate a clogged inlet filter.
Most modern scuba regulators sold for use with breathing gas, should have an intermediate pressure of about 135 to 140 psig. The unique design of the Poseidon Xstream requires 123 psig and the Poseidon Cyklon is 166 psig. Specialized inflation only first stages and oxygen first stages used on rebreathers usually have IP values in the range of 80 to 110 psig depending on the application.