Pressure equalization system for membrane type amperometric sensors

A membrane type amperometric gas sensor is provided with a pressure equalization passage between the exterior and interior of the cell in which the electrodes are located. Cell fluids move into and out of the passage as necessary to maintain the pressure inside the cell substantially equal to the pressure outside the cell over a wide range of pressures and temperatures. A movable fluid plug or seal located in the passage protects the fluids inside of the cell from contact with the gas being sensed and from the atmosphere for all permissible values of pressure and temperature.