CY-III BOD5 tester is to put the amplifier in the incubator. The temperature of the incubator is controlled at 20 + 1 DEG C and the water sample is heated at five days. Adequate dissolved oxygen must be guaranteed in culture flasks. In the samples of organic matter through biological oxidation into nitrogen, carbon and sulfur oxides, in this process, by sodium hydroxide from only gas ran out of water (or potassium hydroxide) absorption of carbon dioxide. Therefore, reduce the air pressure in the bottle, the equivalent of the dissolved oxygen content, microbial consumption of this sample, the BOD5 values decreased with the air pressure in the bottle is proportional to the degree of change, the air pressure can be obtained by measuring the BOD5 value. Increasing or decreasing the amount of samples taken can increase or decrease the pressure reduction. In this way, the operator can accurately measure a wide range of BOD5 values without complicated dilution steps. The variation of the air pressure in a cylinder is detected by a semiconductor pressure sensor, and the BOD value is displayed by signal amplification and microprocessor processing.