Introduction to the principle of portable dissolved oxygen meter
There are three main methods for measuring oxygen content: automatic colorimetric analysis and chemical analysis measurement, paramagnetic method measurement, and electrochemical method measurement. The amount of dissolved oxygen in water is generally measured using electrochemical methods. The plant uses COS4 dissolved oxygen sensor and COM252 dissolved oxygen transmitter. Oxygen is soluble in water, and its solubility depends on temperature, total pressure at the water surface, partial pressure, and 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 a gas is proportional to its partial pressure. Taking the COS4 oxygen measurement sensor as an example, the electrode consists of a cathode (commonly made of gold and platinum), a counter electrode with current (silver), and a reference electrode without current (silver). The electrode is immersed in an electrolyte such as KCl, In KOH, the sensor is covered by a diaphragm. The diaphragm separates the electrode and electrolyte from the liquid being measured, thus protecting the sensor, preventing the electrolyte from escaping and preventing the intrusion of foreign substances that may cause contamination and poisoning. On the contrary, a polarizing voltage is applied between the electrode and the cathode. If the measuring element is immersed in water with dissolved oxygen, oxygen will diffuse through the separator, and the oxygen molecules present on the cathode (excess electrons) will be reduced to hydroxide ions: O2 +2H2O+4e-® 4OH-. An electrochemical equivalent of silver chloride precipitates on the counter electrode (electron deficiency): 4Ag+4Cl-® 4AgCl+4e-. For each oxygen molecule, the cathode emits 4 electrons, and the counter electrode accepts the electrons to form a current. The size of the current is proportional to the oxygen partial pressure of the sewage being measured. This signal is sent into the sensor together with the temperature signal measured by the thermal resistor. The transmitter uses the relationship curve between the oxygen content, oxygen partial pressure, and temperature stored in the sensor to calculate 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. The response time of the COS4 dissolved oxygen sensor is: 90% of the final measurement value is reached after 3 minutes, and 99% of the final measurement value is reached after 9 minutes; the minimum flow rate requirement is 0.5cm/s.
