Chemical oxygen demand refers to the amount of oxidant consumed to oxidize reducing substances in one liter of water sample under certain conditions, expressed in mg/L of oxygen. Reducing substances in water include organic substances and inorganic substances such as nitrite, sulfide, and feerous salts. chemical oxygen demand reflects the degree of contamination of water by reducing substances. Since it is a very common phenomenon for water bodies to be polluted by organic matter, this indicator is also used as one of the comprehensive indicators of the relative content of organic matter, but it can only reflect the organic matter oxidized by oxidants. Usually we use a strong oxidant to oxidize the reducing substances in the water sample, and then calculate the oxygen consumption from the amount of remaining oxidant. This method is time-consuming and cannot achieve real-time monitoring. Therefore, in current sewage plants and industrial processes, we will use online COD analyzers. The COD351 online digital COD Sensor developed and produced by Daruifuno uses two light sources, one ultraviolet light is used to measure the COD content in the water, and the other reference light is used to measure the turbidity of the water body. The light path attenuation is compensated through a specific algorithm, which can achieve a certain degree of to eliminate the interference of particular suspended matter, thereby achieving more stable and reliable measurement.