Polarographic Dissolved Oxygen Electrode (Polarographic method)
A polarographic sensor consists of a silver anode and a ring-shaped gold cathode at the bottom. A thin semi-permeable membrane is spread over the sensor, isolating the electrodes from the external environment while allowing gas to enter. During operation, the bottom of the sensor is filled with an electrolyte containing a small amount of surfactant to improve wetting. When a polarizing voltage is applied to the electrodes of the polarographic sensor, oxygen penetrates the membrane and reacts at the cathode, generating a current. The current flowing through the electrodes is proportional to the oxygen concentration, exhibiting a linear relationship with oxygen concentration at a constant temperature. The main principle is to apply a constant voltage between the two electrodes, promoting electron flow from the cathode to the anode, thereby generating a certain amount of diffusion current.

Fluorescence Dissolved Oxygen Electrode (Fluorescence method)
The fluorescence-based dissolved oxygen meter is based on the principle of quenching of active fluorescence by specific substances in physics. Modulated green light is directed onto a fluorescent material, exciting it and causing it to emit red light. Since oxygen molecules can carry away energy (quenching effect), the duration and intensity of the emitted red light are inversely proportional to the concentration of oxygen molecules. A red light source synchronized with the green light is used as a reference. By measuring the phase difference between the excited red light and the reference light, and comparing it with internal calibration values, the oxygen concentration is calculated and output after temperature compensation. Using the quenching principle, this method requires low maintenance, does not consume oxygen, has no flow or agitation requirements, is not interfered with by chemicals such as sulfides, and offers high resolution and measurement accuracy.