random by him.
(1) The oxygen absorption rating test shall be made in a tank.
The tank shall be 1.05 m 3 feet 6 inches long by 475 mm one foot 7
inches wide, holding approximately 473 L 125 gallons. The tank
shall be filled with tap water, at 22.2 degrees C 72 degrees F, to
a depth of 900 mm 3 feet 0 inches, covering the top of the
diffuser tube. Sodium sulfite, to the amount of 1200 ppm, and 66
Baume scale sulfuric acid, to the amount of 55 ppm, shall be
dissolved in the water. The sulfite shall be put into solution by
removing some of the water from the tank, dissolving the sulfite
in this water and then pouring it back into the tank while air is
being diffused into the solution in the tank as to insure a
thorough mixing.
(2) Each diffuser tube to be rated for oxygen absorption shall be
submerged in tap water for one hour or more just prior to the
rating test. The tube shall then be clamped to the tube holder,
lowered into the tank to just below the solution and inspected
with the air diffusing. Any leaks along the gasket shall be
eliminated by additional tightening of the clamps. The container
with tube attached shall then be lowered to the tank bottom for
the rating test. With air diffusing form the tube at 0.59 L/s
1.25 cfm measured as air at 99.3 kPa 14.4 psi absolute pressure and
15.5 degrees C 60 degrees F, samples of the solution shall be
taken every 10 minutes for 40 minutes. These samples shall be
immediately tested for oxygen absorption. The oxygen absorption
rate for a 40-minute period, in which the indicated rate in any
10-minute interval does not vary more than 10 percent from the
average, shall be the oxygen absorption rating of the tube.
(3) No oxygen absorption rating tests shall be made when the
uncovered sodium sulfite concentration in the tank is lower than
300 ppm.
(4) The oxygen absorption rating shall be determined by measuring
the rate of conversion of sodium sulfite to sodium sulfate due to
the oxygen absorption taking place in the solution. This is done
by taking 50 cubic centimeter (cc) samples of the solution at
intervals and adding to each sample such a fixed amount of
acidified iodine solution that there is always iodine in excess of
the amount required to neutralize the sulfite. The change in
sulfite concentration is determined by measuring the change in the
excess iodine for each interval. This is done by titrating the
excess iodine with N/40 sodium thiosulfate. For a 50 cc sample of
the solution, a titration difference of 1.0 cc of sodium
thiosulfate is equivalent to 4.0 ppm of oxygen absorption.
(5) The results obtained by the foregoing testing procedures
shall be multiplied by a correction factor determined as follows:
Air shall be diffused into 14 L 0.5 cubic foot or less of
distilled water at 75 mm 3 inches depth for a period of two hours
in which the barometric pressure and water temperature remain
constant. D.O. determinations shall then be made on samples from
the water by the Azide Modification of the Iodometric Method, as
described in AWWA EWW, using sodium thiosulfate solution, the same
as used in the tube rating test. The D.O. thus obtained in ppm,
divided into the solubility of oxygen in ppm in water, for the
barometric pressure and water temperature which shall be obtained
SECTION 11375N
Page 86
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