Working principle of dissolved oxygen meter

The oxygen content in water can fully show the degree of water self-purification. For biological treatment plants using activated sludge, it is very important to understand the oxygen content in the aeration tank. The increase in dissolved oxygen in sewage will promote biological activities other than anaerobic microorganisms, thereby removing volatile substances and ions that are easily naturally oxidized, thereby purifying sewage.

There are three main methods for determining oxygen content: automatic colorimetric analysis and chemical analysis, paramagnetic measurement, electrochemical measurement, and fluorescence measurement. The amount of dissolved oxygen in water is generally measured by electrochemical method.

Oxygen is soluble in water, and the solubility depends on the temperature, the total pressure on the water surface, the partial pressure, and the salts dissolved 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 a gas is proportional to its partial pressure.

The oxygen measurement sensor consists of a cathode (usually made of gold and platinum), a counter electrode (silver) with current, and a reference electrode (silver) without current. The electrodes are immersed in an electrolyte such as KCl, KOH, and the sensor is covered with a diaphragm. The membrane separates the electrode and the electrolyte from the liquid being measured. Only dissolved gases can penetrate the membrane, thus protecting the sensor, preventing the escape of electrolytes, and preventing the invasion of foreign substances that may cause contamination 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 will diffuse through the diaphragm, and the oxygen molecules appearing on the cathode (excess electrons) will be reduced to hydroxide ions [OH-]. The electrochemical equivalent of silver chloride is precipitated on the counter electrode (lack of electrons). For each oxygen molecule, the cathode releases 4 electrons, and the counter electrode accepts electrons, forming a current: 4Ag+4Cl-=4AgCl+4e-.

The magnitude of the current is proportional to the oxygen partial pressure of the sewage being measured. This signal, together with the temperature signal measured by the thermal resistor on the sensor, is sent to the transmitter. The oxygen content in the water is calculated using the relationship curve between the oxygen content, oxygen partial pressure and temperature stored in the sensor, and then converted into a standard signal output. The function of the reference electrode is to determine the cathode potential.