The working principle of the water quality analyzer mainly has the following two aspects: 1. Working principle of pH meter The pH value of water depends on the amount of dissolved substances, so the pH value can sensitively indicate changes in water quality. The change of pH value has a great influence on the reproduction and survival of organisms, and also seriously affects the biochemical function of activated sludge, that is, the treatment effect. The pH value of sewage is generally controlled between 6.5-7. Water is chemically neutral, and some water molecules spontaneously decompose according to the following formula: H2O = H OH-, which is decomposed into hydrogen ions and hydroxide ions. In a neutral solution, the concentration of hydrogen ion H and hydroxide ion OH- is 10-7mol / l, and the pH value is the negative number of the logarithm of the hydrogen ion concentration to the base 10: pH = -log, therefore neutral The pH of the solution is equal to 7. If there is an excess of hydrogen ions, the pH value is less than 7, and the solution is acidic; otherwise, if the hydroxide ions are excessive, the solution is alkaline. The pH value is usually measured by the potentiometric method. A constant potential reference electrode and measuring electrode are usually used to form a galvanic cell. The electromotive force of the galvanic cell depends on the concentration of hydrogen ions and the pH of the solution. The plant uses CPS11 pH sensor and CPM151 pH transmitter. The specific structure is shown in Figure 1. The measuring electrode has a special glass probe sensitive to pH. It is made of special glass that can conduct electricity and penetrate hydrogen ions. It has the characteristics of high measurement accuracy and good anti-interference. When the glass probe comes into contact with hydrogen ions, a potential is generated. The potential was measured by a silver wire suspended in a silver chloride solution against a reference electrode. The pH value is different, and the corresponding potential is also different. It is converted into a standard 4-20mA output by the transmitter. Second, the working principle of dissolved oxygen analyzer The oxygen content in the water can fully show the degree of water self-purification. For biological treatment plants that use activated sludge, it is very important to understand 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 easily Naturally oxidized ions clean 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. The plant used COS4 dissolved oxygen sensor and COM252 dissolved oxygen transmitter. Oxygen can dissolve in water, the solubility depends on the temperature, the total pressure of the water surface, partial pressure and dissolved salts in 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 believes that the solubility of a gas is proportional to its partial pressure. Take the COS4 oxygen measurement sensor as an example. The electrode is composed of a cathode (commonly made of gold and platinum) and a counter electrode (silver) with current and a reference electrode (silver) without current. The electrode is immersed in an electrolyte such as KCl or KOH. The electrode and electrolyte are separated from the liquid being measured, thus protecting the sensor, which can not only prevent the electrolyte from escaping, but also prevent the intrusion of foreign substances and cause pollution and poisoning. Apply a polarization voltage between the counter electrode and the cathode. If the measuring element is immersed in water with dissolved oxygen, the oxygen will diffuse through the membrane, and the oxygen molecules that appear on the cathode (excess electrons) will be reduced to hydroxide ions: O2 2H2O 4e-®4OH-. Electrochemically equivalent silver chloride precipitates on the counter electrode (electron deficiency): 4Ag 4Cl-®4AgCl 4e-. For each oxygen molecule, the cathode emits 4 electrons, and the counter electrode accepts the electrons to form a current. The magnitude of the current is measured. Figure 1 The structure of the pH measuring electrode (left) and the reference electrode (right). The oxygen partial pressure of the sewage is proportional to the structure diagram. This signal is sent to the transmitter together with the temperature signal measured by the thermal resistance of the sensor. The relationship between the oxygen content stored in the sensor and the oxygen partial pressure and temperature is calculated. The oxygen content in the water is then converted into a standard signal output. The function of the reference electrode is to determine the cathode potential. The response time of the COS4 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 is 0.5cm / s.
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