FED-STD-228A
4.2 Determination of ozone concentration. The ozone sampling bottle containing 100 ml. of the 1-
percent potassium iodine solution acidulated with a few drops of acetic acid shall be connected to the gas
burette and the two-way stopcock from the test chamber as shown on figure 4111. The two-way
stopcock on the burette shall be opened to the air and the burette filled with water to the mark by lifting
the aspirator bottle. The stopcock shall be closed to the air and opened to the sampling bottle, and the
sampling cock on the test chamber opened. The aspirator bottle shall be lowered until the burette is
emptied. At this point, 500 ml. of the gas will have bubbled through the potassium iodide solution. The
stopcock shall then be closed and the bottle removed, a few drops of starch indicator added and the
solution titrated with standard sodium thiosulfate solution. Since 1 mg. of iodine is equivalent to 0.1 ml. of
ozone at room temperature and pressure (within the accuracy of this method of analysis at room
temperature and pressure) the ozone may be calculated as follows: O= E x 0.1, where O is the number
of ml. of ozone at room temperature and pressure equivalent to 1 ml. of sodium thiosulfate solution used,
and E is the iodine equivalent of sodium thiosulfate expressed as mg. of iodine per ml. of thiosulfate. The
percent of ozone = S x O x100
M
where S is the number of ml. of thiosulfate used to titrate the solution and M is the number of ml. of the
sample collected.
4.3 Testing of rubber specimen.
4.3.1 Unless otherwise specified in the detail specification, the test shall be carried out using an
exposure period of 3 hours at a temperature of 25° ± 2°C (77° ± 4°F) and an ozone concentration of not
less than 0.015 percent by volume.
4.3.2 Air shall be passed through the test apparatus at a constant rate of flow for at least 15 minutes prior
to inserting the specimen. The flow of gas shall be adjusted to between 10 and 20 cubic feet per hour as
measured on the flow meter. The manometer shall indicate a slight pressure of approximately ½ inch of
water above atmospheric pressure in the test chamber. The pressure may be controlled by the degree of
closure of the discharge stopcock. The voltage of the ozone generator shall be regulated so as to give a
concentration of ozone as required. The ozone concentration shall be checked as described in 4.2. The
temperature of the air in the test chamber shall be regulated as required. When constant conditions are
obtained, the specimen free from mechanical damage shall be bent, inserted in the test chamber, and
exposed for the required period of time. At the end of the exposure period, the specimen shall be
examined for cracking.
4.3.3 Flat twin cable shall be bent on the minor axis of its cross-section only.
5. RESULTS
5.1 Unless otherwise specified in the detail specification, two specimens from each inspection unit shall
be tested.
5.2 The ozone resistance of the insulation or sheath of the inspection unit shall be the results obtained
from the specimens tested.
5.3 Any cracking or other damage of the insulation or sheath shall be recorded.
5.4 The ozone concentration temperature and time of exposure shall be recorded.
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