Abstract: An electrochemical membrane sensor 20 for immersion in a test fluid, such as wastewater, is used to sense desired chemical species, such as dissolved oxygen, within the test fluid. The sensor 20 has an open housing that defines a chamber 24. The chamber 24 is sealed from the test fluid by a membrane 28, so that the chamber 24 can be filled with an electrolytic medium 30. The membrane 28 is selectively permeable so as to allow only the desired chemical species from the test fluid to permeate into the electrolytic medium. Spaced apart within the chamber 24 is a cathode 29 and an anode 29a which are electrically connected to a measurement transducer that responds to current between the cathode 29 and anode 29a. The current is proportional to the concentration of the desired chemical species within the test fluid.