Dissolved Oxygen refers to the oxygen dissolved in the molecular state of water, that is, the oxygen in water, expressed in DO. Dissolved oxygen is an indispensable condition for the survival of aquatic organisms. One source of dissolved oxygen is that when dissolved oxygen in water is not saturated, oxygen in the atmosphere seeps into the water; Another source is the oxygen released by aquatic plants through photosynthesis. Dissolved oxygen changes with temperature, air pressure, and salinity.
Generally speaking, the higher the temperature is, the greater the dissolved salinity is, and the lower the dissolved oxygen in water is; The higher the air pressure, the higher the dissolved oxygen in the water. Dissolved oxygen is not only consumed by sulfide, nitrite, ferrous ion, and other reducing substances in water, but also by the respiration of microorganisms in water and the oxidative decomposition of organic substances in water by aerobic microorganisms. Therefore, dissolved oxygen is the capital of water bodies and the expression of the self-purification capacity of water bodies. The dissolved oxygen in natural water is close to the saturation value (9ppm), and the content of dissolved oxygen decreases when the algae are thriving.
The pollution of the water by organic substances and reducing substances can reduce the dissolved oxygen. For aquaculture, the dissolved oxygen in the water body has a vital impact on the survival of aquatic organisms, such as fish. Fish will die of suffocation when the dissolved oxygen is lower than 4mg/L. For humans, the dissolved oxygen content in healthy drinking water should not be less than 6mg/L. When the consumption rate of dissolved oxygen (DO) is greater than the rate at which oxygen dissolves into the water, the content of dissolved oxygen can approach zero. At this time, anaerobic bacteria can reproduce and worsen the water body. Therefore, the size of dissolved oxygen can reflect the pollution of the water body, especially the degree of organic pollution. It is an important indicator of water pollution and a comprehensive indicator of water quality. Therefore, the measurement of dissolved oxygen content in water is of great significance for environmental monitoring and aquaculture development.
Measuring principle of membrane dissolved oxygen analyzer: oxygen solubility in water depends on temperature, pressure, and salt dissolved in water. The sensing part of the dissolved oxygen analyzer is composed of the gold electrode (cathode), silver electrode (anode), and potassium chloride or potassium hydroxide electrolyte. Oxygen diffuses into the electrolyte through the membrane, forming a measuring circuit with gold electrode and silver electrode. When a polarization voltage of 0.6~0.8V is applied to the electrode of the dissolved oxygen analyzer, the oxygen diffuses through the film, the cathode releases electrons, the anode accepts electrons, and it generates a current. The whole reaction process is anode Ag+Cl → AgCl+2e cathode O2+2H2O+4e → 4OH -. According to Faraday's law, the current flowing through the electrode of the dissolved oxygen analyzer is proportional to the oxygen partial pressure, and the relationship between the current and the oxygen concentration is linear when the temperature is constant.
The fluorescent probe is equipped with a built-in light source, which emits blue light to irradiate the fluorescent layer, and the fluorescent material is excited to emit red light. Since oxygen molecules can take away energy (quenching effect), the time and intensity of the exciting red light are inversely proportional to the concentration of oxygen molecules. The concentration of oxygen molecules can be calculated by measuring the phase difference between the excitation red light and the reference light and comparing it with the internal calibration value. The measurement does not consume oxygen, the data is stable, the performance is reliable, and there is no interference.
For a dissolved oxygen analyzer, as long as it is properly selected, set, and maintained, it can generally meet the measurement requirements of the process. The main problems of the poor use of dissolved oxygen analyzers are: incorrect use and maintenance; The internal leakage of the electrode causes abnormal temperature compensation; The input impedance of the electrode decreases.
(1) The electrode shall be cleaned once every 1-2 weeks. If there are pollutants on the membrane, measurement errors will be caused. Be careful not to damage the diaphragm during cleaning. Rinse the electrode in clean water. If the dirt cannot be removed, carefully scrub it with a soft cloth or cotton cloth.
(2) From February to March, the zero point and range shall be calibrated again.
(3) The electrode shall be regenerated about once a year. When the measurement range cannot be adjusted, it is necessary to regenerate the dissolved oxygen electrode. Electrode regeneration includes the replacement of internal electrolytes, replacement of the diaphragm, and cleaning of the silver electrode. If the silver electrode is observed to be oxidized, it can be polished with fine sandpaper.
(4) If the electrode leaks during use, the electrolyte must be replaced.
SUP-DY2900 online dissolved oxygen meter is a highly intelligent online continuous monitor produced by Hangzhou Meacon Company. It can be equipped with a fluorescence electrode to automatically realize a wide range of measurements from ppb level to ppm level. It is a special instrument for detecting oxygen content in liquid in aeration tanks, boiler feed water, condensate, environmental sewage, and other industries. It has the characteristics of fast response, stability, reliability, and low use cost, and is suitable for large-scale use in thermal power plants.
Equipped with automatic temperature compensation and pressure deviation correction, DY2900 and DM2800 dissolved oxygen meters both provide high-precision mg/L, ppm, or saturated air% dissolved oxygen values. The rugged primary battery probe requires only minimal maintenance and is very durable. You can also customize the display of dissolved oxygen values in mg/L, ppm or% saturated air. The galvanic cell probe is integrated with the automatic temperature compensation sensor - completely waterproof, and the submerged type work is very safe.
Meacon is a leading online dissolved oxygen meters supplier, which has 23 offices and 3 factories in China, a branch office in Singapore, offices abroad, and cooperation with 127 countries around the world. If you have needs for automatic control instruments such as temperature sensors, pressure transmitters, level meters, display meters, and control instruments, please consult us immediately, and professional technical engineers will answer your questions.