How Dissolved Oxygen Analyzers Work
The oxygen content in water can fully show the degree of self-purification of water. For biological treatment plants using activated sludge, it is very important to know the oxygen content of aeration tanks and oxidation ditch. The increase of dissolved oxygen in sewage will promote biological activities other than anaerobic microorganisms, so it can remove volatile substances and facilitate The naturally oxidized ions purify the sewage.
There are three main methods for determining 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 by electrochemical method. Mai's plant has adopted HD369L dissolved oxygen sensor and MX30 dissolved oxygen transmitter.
Oxygen can dissolve in water, and the solubility depends on the temperature, the total pressure of 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.
Take the HD369L dissolved oxygen sensor as an example. The electrode is composed of a cathode (commonly made of gold and platinum), a counter electrode (silver) with current, and a reference electrode (silver) without current. The electrode is immersed in an electrolyte such as KCl, KOH, and the sensor is covered by a diaphragm. The electrodes and electrolyte are separated from the liquid being measured, thus protecting the sensor from the escape of the electrolyte and the intrusion of foreign substances that would cause contamination and poisoning.
A polarizing voltage is applied between the counter electrode and the cathode. If the measuring cell is immersed in water with dissolved oxygen, the oxygen diffuses through the diaphragm and the oxygen molecules present on the cathode (excess of electrons) are reduced to hydroxide ions:
An electrochemical equivalent of silver chloride is precipitated on the counter electrode (electron deficient): 4Ag+4Cl-? 4AgCl+4e-.
For each oxygen molecule, the cathode emits 4 electrons, and the counter electrode accepts electrons to form a current. The magnitude of the current is proportional to the oxygen partial pressure of the measured sewage. The signal is sent to the transformer together with the temperature signal measured by the thermal resistance on the sensor. The transmitter calculates the oxygen content in the water by using the relationship curve between the oxygen content stored in the sensor and the oxygen partial pressure and temperature, and then converts it into a standard signal output. The function of the reference electrode is to determine the cathodic potential.
The response time of the HDL369L dissolved oxygen sensor is: 90% of the final measured value after 3 minutes, and 99% of the final measured value after 9 minutes; the minimum flow rate requirement is 0.5cm/s.
