The dissolution of molecular oxygen in the air in water is called dissolved oxygen. The content of dissolved oxygen in water is closely related to the partial pressure of oxygen in the air and the temperature of water.
In natural conditions, the oxygen content in the air does not change much, so water temperature is the main factor. The lower the water temperature, the higher the content of dissolved oxygen in the water. The molecular oxygen dissolved in water is called dissolved oxygen, usually referred to as DO, expressed in milligrams of oxygen per liter of water.
The amount of dissolved oxygen in water is related to two aspects: the influence of light intensity and air pressure. This also includes environmental factors such as temperature, oxygen partial pressure, and salinity.
1. Dissolved oxygen affects light: Oxygen in water mainly comes from the aquatic photosynthetic conversion function, followed by dissolved oxygen in air. Sudden weather changes often lead to sudden changes in temperature, light, and air pressure. My friend upstairs, the main reason for the sudden change in water temperature is not the change in dissolved oxygen in the water, which is a better relatively constant temperature. However, light can seriously affect the aquatic photosynthetic conversion process, leading to a decrease in oxygen production.
2. Dissolved oxygen affects pressure: a decrease in pressure reduces the water solubility of oxygen, leading to oxygen deficiency in water. Under certain conditions, the air pressure is very low, and pollutants from the bottom of the flushing water are often visible. This is known as the "universal pond" phenomenon (which also indicates the pressure from one side to the water). The suppression of the bottom of the Pan pond is a result of the opportunity for anaerobic aerobic bacteria to obtain oxygen and rapidly consume dissolved oxygen from the water. The low environmental pressure and ability to raise animals with dissolved oxygen also have a negative impact, leading to blood loss and oxygen consumption. Therefore, animals need to breathe more to increase their oxygen intake.
3. Water temperature: Oxygen partial pressure and salt content are constants, and the content of dissolved oxygen saturation is related to the increase and decrease of temperature.
4. Salt: Water temperature and oxygen partial pressure are constant, and the higher the mineralization degree of water, the smaller the dissolved oxygen content in the saturated solution.
5. Oxygen partial pressure: In saline water, the saturation of dissolved oxygen increases with the increase of surface oxygen partial pressure of the liquid.
Monitoring of dissolved oxygen in water
Dissolved oxygen analyzer is an instrument used to measure the dissolved oxygen content in water. According to the measurement principle, there are two types: electrochemical and photochemical. Currently, most dissolved oxygen analyzers use electrochemical sensors. Although the accuracy and usability of electrochemical sensors are widely recognized, their drawbacks become apparent when used in wastewater treatment, and electrochemical sensors have obvious problems and limitations. Including complex and frequent daily maintenance, which often requires calibration; Moreover, the breathable film of the electrode is prone to aging; And it requires the redox reaction of the electrode itself to determine the concentration of oxygen, which requires the consumption of dissolved oxygen in the tested sample during the measurement process; And it also limits the measurement flow rate. In order to achieve equilibrium of diffusion controlled electrochemical reactions within the electrochemical cell, a minimum flow rate of 200mL/min needs to be maintained on the membrane surface. Below this flow rate, the measurement results are greatly affected by the water flow. In addition, excessive flow rate can also affect the thickness of the membrane and cathode electrolyte layer, thereby affecting the measurement results.
Therefore, optical dissolved oxygen sensors based on fluorescence technology have emerged, and the fluorescence method for DO determination of dissolved oxygen in water is simple and fast. Compared to the first two methods, the fluorescence LDO method for measuring dissolved oxygen in water does not require calibration, has a fast response time, stable measurement results, no flow requirements, no interference, reduces cleaning frequency, and low maintenance.
The fluorescence dissolved oxygen analyzer has four major advantages over the electrochemical dissolved oxygen analyzer:
1. No need for preheating, no need for electrolyte, more convenient to use;
2. Free from maintenance and frequent calibration, with a longer service life;
3. No flow rate limit, response within 10 seconds, higher detection efficiency;
4. High precision, anti-interference, better stability, and higher durability.
