Electrochemical sensors, both polarographic and galvanic, consist of an anode and a cathode that are confined in electrolyte solution by an oxygen permeable membrane. Oxygen molecules that are dissolved in the sample diffuse through the membrane to the sensor at a rate proportional to the pressure difference across it. The oxygen molecules are then reduced at the cathode producing an electrical signal that travels from the cathode to the anode and then to the instrument. Since oxygen is rapidly reduced or consumed at the cathode, it can be assumed that the oxygen pressure under the membrane is zero. Therefore, the amount of oxygen diffusing through the membrane is proportional to the partial pressure of oxygen outside the membrane.

A circuit completes the measurement, but, unlike the polarographic sensor, the galvanic sensor does not have or need a constant voltage applied to it. In the Galvanic sensor, the electrodes are dissimilar enough to self-polarize and reduce oxygen molecules without an applied voltage. It is similar in function to a battery.

A galvanic dissolved oxygen system uses a meter to read the electrical signal coming back from the probe and this signal is proportional to the amount of oxygen passing through the membrane. Oxygen passing through the membrane and being reduced at the cathode increases the electrical signal(current) read by the probe. As oxygen increases, the signal increases and as oxygen decreases, the signal decreases.