The oxygen meter and its electrode function according to the Clark principle with silver as cathode and lead as anode in an electrolyte cell. Oxygen gas present in the electrolyte is reduced to OH ions at the cathode. The resulting current is diffusion limited and therefore proportional to the oxygen concentration in the sample solution. This current is amplified, corrected, and displayed in mg/l, ppm or % dissolved oxygen.
All substances which can diffuse through the membrane and for which 800 mV potential suffices for polarographic reduction, will be reduced in the electrode. This will give a corresponding current contribution, if they are present. Interference can be caused by ions entering the electrode through porous or mechanically damaged membranes and by diffusion of other reactive gases apart from oxygen, e.g. CO2, Cl2, SO2, and H2S. These substances react in undesired manner with the electrode. Acidic or basic gases change the pH value of the electrolyte solution and thus disturb the reading, particularly when measuring small oxygen concentrations. High salt concentrations in the sample solution can falsify readings too.