1. Insert the sampling tube into the bottom of the dissolved oxygen bottle to make water sample be filled in the dissolved oxygen bottle, and to overflow not less than 3 minutes.
2. Slowly insert the zinc reduction burett to the dissolved oxygen bottle, gently pull out the sampling tube, immediately calculate the amount according to the following formula, and the add  reduced form blue disulfonic acid sodium solution.
3. Gently pull out the burette and immediately plug the cork tightly, then mix the solution in the water, place the bottle for 2 minutes, to ensure the completly reacting.
4. Immediately compare the dissolved oxygen bottle under the natural light or fluorescent lamp with the standard colur in the background of white, the dissolved oxygen content are obtained. The amount of reduced indigo disulfonic acid sodium:
                 1.3C Max.×V
        D=                                         
                    1000×20
Where :
C Maximum - the dissolved oxygen content equivalent to maximum standard clour, 100 ug/L.
V -  volume of water sample, mL.
Note:
(1) After each measurement, the remaining solution of burette shall be put on the level higher than the zinc mercury layer, in the next time, the newly prepared solution shall be injected.
(2)If it is  found  that the surface of zinc amalgam becomes dark, it shall be treated, then it can be used.
(3) The ammoniacal indigo disulfonic acid sodium can not be placed for more than 8 hours, otherwise it shall be prepared again.