This galvanic cell sensor that measures oxygen concentration has been in use since the mid-1960s to detect (and alert) oxygen deficiency throughout all industries.
The measurement principle involves an electrolyte-soluble metal anode and an insoluble metal cathode that are immersed in the electrolyte. As the metal of the anode dissolves, it emits electrons that reach the cathode.
In the cathode, the oxygen penetrating the thin membrane film absorbs the electrons emitted by the anode. The flow (current) of this electron is proportional to the oxygen concentration penetrating the membrane film, and the oxygen concentration can be measured with a metre. Since the reaction occurs spontaneously, this sensor type requires no power source.