this invention provides a calibration unit for generating known concentrations of an oxidizing or a reducing gas in an inert carrier gas . the gas - generating electrode is sandwiched between an ion - exchange membrane and a porous diffusion membrane for minimizing the liquid layer thickness around the electrode and for regularizing the rate of diffusion of the gas generated . the present invention is especially useful for calibrating gas sensors which detect , measure , and monitor the concentration of a gas in the atmosphere , and for providing an accurate low concentration of gas for use wherever desired .