1. The principle of DPD colorimetric method: under the condition of PH6.2~6.5, ClO2 first reacts with DPD in one step to form a red compound, but the amount only reaches its total available chlorine content (equivalent to reducing ClO2 to chlorite ions) one-fifth. If the water sample is acidified in the presence of iodide, chlorite and chlorate also react, and when neutralized by the addition of bicarbonate, the resulting color corresponds to the total available chlorine content of ClO2 . The interference of free chlorine can be controlled by adding glycine, which is based on the fact that glycine can immediately convert free chlorine into chloroaminoacetic acid, but it has no effect on ClO2. Features: unattended operation, stable measurement value, user can choose the measurement cycle, need to replace the reagent. Application: Drinking water, sewage, waste water and industrial circulating cooling water, etc. 2. Film-covered electrode method Principle: The electrode is immersed in the electrolyte cavity, and the electrolyte cavity is in contact with water through a porous hydrophilic membrane. The hypochlorous acid diffuses into the electrolyte cavity through the porous hydrophilic membrane, and forms a current on the surface of the electrode. The magnitude of the current depends on the speed at which the hypochlorous acid diffuses into the electrolyte cavity, and the diffusion rate is proportional to the concentration of residual chlorine in the solution. The measured current The size can determine the concentration of residual chlorine in the solution. Features: No reagent required. There will be drift when used in the place containing interface active agent, and the membrane pores will be blocked by lipids, and the diaphragm and electrolyte need to be cleaned and replaced regularly. DPD indicator must be used for calibration. The measurement environment is synchronized with the pH of the water, and the combination of chlorine interferes with the bactericidal ability of the water, and the actual bactericidal strength of the water may be low. Application: drinking water, swimming pool water, industrial water, etc., measuring range 0-20mg/L 3. Constant voltage electrode method (membraneless electrode method) The measured components will produce different current intensities under this potential. It consists of two platinum electrodes and a reference electrode to form a micro-current measurement system. In the measuring electrode, chlorine molecules or hypochlorite will be consumed, and the resulting current intensity is related to the concentration of residual chlorine in the water. Features: In the range of pH 6-8, the measured signal decreases with the increase of pH value, so it is recommended to use this method for residual chlorine under the condition of pH ≤ 8. If the pH of the water is constant, then the pH effect can be compensated by field calibration. Real-time online detection, fast response, no need for regular replacement of reagents, electrolytes or membranes, simple maintenance, and low purchase cost. Application: tap water, swimming pool water, sewage wastewater, etc., measuring range 0-20mg/L.