───────────────────────────────────┬─────────────────────────────────── Time required for decolorization at│ Relative stability. 20° C. │ ───────────────────────────────────┼─────────────────────────────────── _Days._ │ _Percentage._ 0.5│ 11 1.0│ 21 1.5│ 30 2.0│ 37 2.5│ 44 3.0│ 50 4.0│ 60 5.0│ 68 6.0│ 75 7.0│ 80 8.0│ 84 9.0│ 87 10.0│ 90 11.0│ 92 12.0│ 94 13.0│ 95 14.0│ 96 16.0│ 97 18.0│ 98 20.0│ 99 ───────────────────────────────────┴─────────────────────────────────── The theoretical relation is, S = 100 (1 − 0.794_t__{20}) The relation between the time of reduction at 20° C. and that at 37° C. is approximately two to one, but if an observer incubates at 37° C. he should work out his own comparative 37° C. table or factor. A relative stability of 75 signifies that the sample examined contains a supply of available oxygen equal to 75 per cent of the amount of oxygen which it requires in order to become perfectly stable. The available oxygen is approximately equivalent to the dissolved oxygen plus the available oxygen of nitrate and nitrite. Nitrite in sewage is usually so low as to be negligible. BIOCHEMICAL OXYGEN DEMAND OF SEWAGE AND EFFLUENTS.[60a][60c][60d] RELATIVE STABILITY METHOD. The relative stability method may be employed to obtain a measure of the putrescible material in sewages and effluents in terms of oxygen demand. _Procedure for effluents._—Divide the total available oxygen, including the oxygen of nitrite and nitrate, by the relative stability expressed as a decimal.