For continuous monitoring of dissolved oxygen, saturation, oxygen partial pressure, and temperature in solution, the oxygen content is an important indicator in the water treatment process. Real-time measurement can reflect the effect of water treatment in a timely manner. The online dissolved oxygen analyzer can realize the real-time measurement of dissolved oxygen in water, which brings a more direct feeling and fully replaces the previous experience. The dissolved oxygen meter is widely used in thermal power, chemical fertilizer, metallurgy, environmental protection, pharmacy, biochemistry, and other industries.

Principle of dissolved oxygen measurement

The cathode of polarographic determination of dissolved oxygen in water is a gold or platinum electrode, and the anode is a silver electrode (Ag/AgCl reference electrode). The electrolyte is KCl solution, and the DC power supply voltage is 0.65~0.85 V. When the anode material is changed to lead, the electrolyte is changed to KOH solution, which is called the Galvanic method. Compared with the polarographic method, the Galvanic method is more rapid in the oxidation reaction of anode lead, so the test reading is faster and does not require polarization. It is mostly used for general analysis in the laboratory. Pay attention to frequent maintenance during use. Polarography must be polarized before use, but it does not need frequent maintenance. It is especially suitable for the selection of an online dissolved oxygen meter in a continuous process.

Selection of online dissolved oxygen measuring instrument

Sensor part: the sensor is the key part for the selection of dissolved oxygen sensors. Because it is measured by the current method, the output signal is relatively small, and there are many requirements for the manufacturing process of dissolved oxygen. A slight deviation will lead to a large signal output error. Therefore, when selecting the dissolved oxygen sensor, special attention should be paid to the sensor with a good manufacturing process.

Transmitter part: with a good sensor, the transmitter part is mainly used to collect the signal of the sensor, and process and transmit the signal. The signal processing of the transmitter needs to be very strict in the process of signal amplification and transmission, otherwise, it will lead to large errors in the final measurement data.

Electrode maintenance of online dissolved oxygen measuring instrument

Whether the measured value of the instrument is correct or not is greatly related to the measuring electrode. Therefore, the maintenance of dissolved oxygen electrodes is a key point in the whole measuring system.

(1) If it is found that the response time of the whole measuring system is long, the membrane breaks, and the current in the oxygen-free medium increases, it is necessary to replace the membrane head and add electrolyte for maintenance. The maintenance work of replacing the membrane and adding electrolytes shall be carried out once every six months or so. After changing the membrane or adding an electrolyte each time, the electrode shall be repolarized and calibrated.

(2) Treatment of gold cathode After using the oxygen electrode for a period of time, if there is a small amount of brown on the surface of the gold cathode, the membrane holder must be removed, cleaned, and dried with distilled water, and then lightly rub the gold surface with metallographic sandpaper above No. 005 for polishing. After polishing, rinse the installation membrane holder with distilled water (if there is no distilled water, pure water can be used instead).

(3) Cleaning of electrode membrane surface: if the anti-pollution Teflon membrane is contaminated, it can be lightly wiped with gauze dipped with a small amount of dilute detergent, or a water jet cleaning device can be installed to automatically and regularly clean the surface of the dissolved oxygen measuring electrode membrane.

The industrial dissolved oxygen meter is a device for measuring the dissolved oxygen in the water. Its working principle is that the oxygen is reduced by the working electrode through the diaphragm to generate a diffusion current proportional to the oxygen concentration. By measuring this current, the concentration of dissolved oxygen in water can be obtained.

On what occasions are industrial dissolved oxygen meters most commonly used, such as sewage treatment, power generation, aquaculture, schools, pharmaceutical factories, hospitals, fermentation, chemicals, tap water, etc? In sewage treatment, there is a biodegradation tank, which can also be called a sedimentation tank. In this process, the existence of dissolved oxygen is very important. The role of the industrial dissolved oxygen meter here is to measure the concentration of dissolved oxygen, ensure the number of microorganisms in the biodegradation tank, and ensure the safety and efficiency of the entire process. The same is true for other occasions. The concentration of dissolved oxygen is a very important indicator in industrial, agricultural, and breeding industries.

Industrial use will certainly encounter problems, so when using polarographic electrodes, preheat them for at least 15-30 minutes before calibration or measurement. In order to ensure that there are no bubbles in the electrolyte of the membrane, the ASI membrane cap is designed to remove all the air in the liquid cavity when installing the membrane head. No bubbles can be left on the membrane surface, otherwise, it will take the bubbles as oxygen-saturated samples for reading. Even if an instrument with automatic temperature compensation is used, the electrode should be calibrated at a temperature close to the sample solution. The electrode shall be calibrated in air, with air as the standard point of 100% saturated dissolved oxygen. Due to the consumption of oxygen by the electrode, the oxygen concentration on the probe surface will decrease instantaneously, so it is important to stir the solution during measurement. If the membrane is damaged, replace it.

It may be said that there is another question about what is called a polarographic electrode. At present, the membrane method and fluorescence method are the most popular methods on the market. The membrane method is also called polarography or the constant pressure method. The measurement principle of the dissolved oxygen analyzer is that the solubility of oxygen 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 generates 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 current is linear with the oxygen concentration when the temperature is constant.