The BOD test requires significant dilution in order to get the test in range. This makes the quality of the dilution water absolutely vital. The standard method does not permit the subtraction of the blank contribution for good reason. Since the blank tends to be very low in microorganisms, one can not draw a one to one correlation between the blank value and what that dilution water would have for an oxygen demand in the presence of seed. The blank is determined by taking the measured dissolved oxygen value before incubation and subtracting the measured dissolved oxygen after a five day incubation at 20 degrees C.