Biological Oxygen Demand is the amount of oxygen used by micro-organisms when they bio-degrade organic material in a water sample. It is used as an important measure of the degree of contamination in a supply. The conventional BOD system comprises 6 bottles and 6 mercury capillary tubes with a constant scale of up to 1000mg/L. This allows exact and error free readings of the BOD value to be read directly in mg/L or to be calculated by single multiplication.