Abstract: An improved second stage regulator employs a pneumatically-activated anti-set poppet and inhalation resistance adjustment control knob accessible externally of the regulator. The anti-set poppet utilizes a pressure-activated compression spring to seal the air inlet during exhalation. When the regulator is stored, the relaxed spring permits the seal to withdraw from the sharp edge orifice of the air inlet thereby avoiding seal wear which would otherwise diminish the performance of the regulator. The adjustment control knob permits the diver to modify the required cracking effort by changing the amount of spring compression when the regulator is pressurized.