The principle of dissolved oxygen analyzer

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 and the oxidation ditch. The increase of dissolved oxygen in the sewage will promote biological activities other than anaerobic microorganisms, thus enabling the removal of volatile substances and ease of use. Naturally oxidized ions purify sewage.

There are three main methods for determining oxygen content: automatic colorimetric analysis and chemical analysis measurement, paramagnetic measurement, and electrochemical measurement. 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.

Take the COS4 oxygen measurement sensor as an example. The electrode consists of a cathode (usually made of gold and platinum), a current-carrying counter electrode (silver), and a current-free reference electrode (silver). The electrode is immersed in electrolytes such as KCl and KOH. The sensor is covered with a membrane and the membrane The electrodes and the electrolyte are separated from the liquid being measured, thus protecting the sensor, preventing both the escape of the electrolyte and the ingress of foreign substances, resulting in contamination and poisoning.

A polarizing 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 oxygen molecules that are present at the cathode (excess electrons) are reduced to hydroxide ions: Electrochemically equivalent silver chloride precipitates on the counter electrode (electron deficiency): 4Ag 4Cl-4AgCl 4e-.

For each oxygen molecule, the cathode emits 4 electrons. The counter electrode receives electrons and forms a current. The current is proportional to the oxygen partial pressure of the tested sewage. This signal is sent to the transformer along with the temperature signal measured by the thermal resistance of the sensor. The sender calculates the oxygen content in the water using the relationship curve between oxygen content, oxygen partial pressure, and temperature stored in the sensor, and then converts it into a standard signal output. The function of the reference electrode is to determine the cathode potential.

4 The response time of the DO sensor was: 90% of the final measurement after 3 minutes, 99% of the final measurement after 9 minutes; the minimum flow rate requirement was 0.5 cm/s.

Dissolved oxygen meters generally use immersed installation. It should be noted here that the original installation bracket must be used. The mounting bracket provided by the manufacturer is made of stainless steel and has a plastic chain. The length of the sensor can be changed by adjusting the length of the chain. The guide tube on the bracket ensures that the sensor is always in a vertical position. The bracket part is specially designed to transmit the fluctuation of the water surface to the dip tube, causing slight vibration of the dip tube, so that an additional cleaning effect is produced on the surface of the probe through the dip tube. In order to reduce the investment, some users make their own mounting brackets, which often leads to poor sealing between the dip tube and the sensor on the bracket, which causes the sewage to infiltrate. This makes the connection between the special cable and the sensor soaked in the sewage for a long time, which may easily damage the sensor. Some do not even install a bracket and directly put the sensor into the water, so that a large pulling force is generated between the sensor and the cable, and the sensor is more easily damaged.

Dissolved oxygen probes were gently washed with water every week and it was found that the membrane head damage should be promptly replaced, and the electrolyte should be promptly replaced when it is contaminated. When the sewage contains H2S, NH3, benzene or phenol, they are harmful to the membrane head. In this case, the membrane head must be replaced frequently. To judge whether the electrode in the probe is good or bad, just look at the color, the reference electrode should be dark gray, the cathode (gold electrode) should be yellow, and the counter electrode must be bright, otherwise it should be cleaned or regenerated.

With the increasing emphasis on the protection of water resources in China, the purification of sewage becomes more and more important, and the detection instruments necessary for the processing process are indispensable. The water quality analyzer is the most important instrument in the sewage treatment industry. Besides the correct selection and installation, regular maintenance and calibration are also very important, and it is the key to making the instrument work.