Designed by pressure-difference method, simulating the degradation of natural organic matter, carbon dioxide (CO2) produced in the process of organic matter degradation is absorbed by sodium hydroxide coming from sealed cap, pressure sensor monitor constantly the pressure change of sample bottle. There is a correlation between Biochemical Oxygen Demand (BOD) (correspond to the consumption of oxygen in the test bottle) and gas pressure, such relevance is dealt with by this instrument, direct display of BOD value on the screen.