Dissolved oxygen meter works

Dissolved oxygen meter works:

The oxygen content in the water can fully show the degree of water self-purification. For a biological treatment plant using activated sludge, it is very important to know the oxygen content of the aeration tank. The increase of dissolved oxygen in the sewage will promote biological activities other than anaerobic microorganisms, and thus can remove volatile substances and easily oxidize naturally. Ion, so that the sewage is purified.

There are three main methods for determining oxygen content: automatic colorimetric analysis and chemical analysis measurement, paramagnetic measurement, electrochemical measurement, and fluorescence method. Dissolved oxygen in water is generally measured electrochemically.

Oxygen is soluble in water and the solubility depends on the temperature, the total pressure on the surface of the water, the partial pressure, and the dissolved salts in the water. The higher the atmospheric pressure, the greater the ability of water to dissolve oxygen. The relationship is determined by Henry's law and Dalton's law. Henry's law states that the solubility of gas is proportional to its partial pressure.

The oxygen measurement sensor consists of a cathode (usually made of gold and platinum), a counter electrode with current (silver), and a currentless reference electrode (silver). The electrodes are immersed in electrolytes such as KCl and KOH, and the sensor is covered with a diaphragm. The membrane separates the electrode and the electrolyte from the liquid being measured. Only the dissolved gas can penetrate the membrane, thus protecting the sensor. It not only prevents the electrolyte from escaping, but also prevents foreign substances from invading 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, oxygen diffuses through the membrane and the oxygen molecules present at the cathode (excess electrons) are reduced to hydroxide ions. [OH-]. Electrochemically equivalent silver chloride precipitates on the counterelectrode (electron deficiency). For each oxygen molecule, the cathode emits 4 electrons and the counter electrode accepts electrons, forming a current: 4Ag+4Cl-=4AgCl+4e-.

The magnitude of the current is proportional to the partial pressure of oxygen in the measured effluent. This signal is sent to the transmitter together with the temperature signal measured by the thermal resistance of the sensor. The oxygen content and oxygen partial pressure stored in the sensor are used to vary the 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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