CY-5 LABORATORY BOD5 ANALYZER Pour water sample into a culture bottle with sodium hydroxide in seal cup on bottle mouth, then seal the bottle and put in the incubator, keep the temperature at 20℃±1℃ and Continuously stirring for 5 days. Organic matter in the sample will be converted to the oxides of nitrogen, carbon and sulfur because of the biological oxidation. During this process, the only gas (CO2) overflow from the sample then is absorbed by sodium hydroxide. At the same time, oxygen consumption of microorganism results in air pressure difference. The BOD5 value can be calculated through measuring the height of the mercury.