The oxygen content in the water can fully show the degree of self-purification of the water. For biological treatment plants that use activated sludge, it is very important to understand the oxygen content of the aeration tank. The increase of dissolved oxygen in the sewage will promote biological activities other than anaerobic microorganisms, thereby removing volatile substances and easily oxidized naturally. Ion, so that the sewage can be purified. There are three main methods for measuring oxygen content with a dissolved oxygen meter: automatic colorimetric analysis and chemical analysis measurement, paramagnetic measurement, electrochemical measurement, and fluorescence method. The amount of dissolved oxygen in water is generally measured by electrochemical methods.

Oxygen is soluble in water, and its solubility depends on the temperature, the total pressure of the water surface, the partial pressure and the dissolved salts in the water. The higher the atmospheric pressure, the greater the capacity of water to dissolve oxygen.

The oxygen measurement sensor consists of a cathode (commonly made of gold and platinum), a current-carrying counter electrode (silver), and a current-free reference electrode (silver). The electrode is immersed in an electrolyte such as KCl and KOH. The sensor is covered by a diaphragm. The coating separates the electrode and electrolyte from the liquid to be measured. Only the dissolved gas can penetrate the coating, thus protecting the sensor. It not only prevents the electrolyte from escaping, but also prevents foreign substances from invading people and causing pollution and poisoning.

A polarization voltage is applied between the counter electrode and the cathode. If the measuring element is immersed in water with dissolved oxygen, the oxygen will diffuse through the diaphragm, and the oxygen molecules appearing on the cathode (excess electrons) will be reduced to hydroxide ions IOH-]. The electrochemical equivalent of silver chloride is deposited on the counter electrode (lack of electrons). For each oxygen molecule, the cathode emits 4 electrons, and the counter electrode accepts electrons, forming a current: 4Ag+4Cl=4AgCI+4e.

The magnitude of the current is proportional to the partial pressure of oxygen in the measured sewage. This signal and the temperature signal measured by the thermal resistance on the sensor are sent to the transmitter, and the oxygen content stored in the sensor is used between the oxygen partial pressure and temperature. The relationship curve calculates the oxygen content in the water, and then converts it into a standard signal output. The function of the reference electrode is to determine the cathode potential.

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