[Working principle]
 
1. Inject the water sample or pretreated water sample into the culture bottle, and at the same time put carbon dioxide absorbent (NaOH) into the absorption cup at the bottle mouth
 
2. Seal the culture bottle and place it in a special biochemical incubator
 
3. When the tested sample was cultured at 20 ± 1 ℃ for five days, the water sample in the bottle was subjected to biological oxidation reaction at a certain stirring speed
 
At this time, the organic matter is converted into oxides of nitrogen, carbon and sulfur, and carbon dioxide gas is generated, which is absorbed by sodium hydroxide. The oxygen in the upper space of the culture bottle continuously supplies the oxygen demand of microorganisms in the water sample, and the pressure difference is generated in the culture bottle. By detecting the height of mercury in the glass tube, the BOD value of the water sample in the specified determination period can be calculated.