ISO 1431-1:2012 pdf download – Rubber, vulcanized or thermoplastic – Resistance to ozone cracking
5.3 Means of adjusting the ozone concentration
When an ultra-violet lamp is used, the ozone concentration can be controlled by adjusting either the voltage applied to the tube or the input-gas or diluent-air fow rate, or by shielding part of the tube from the UV light. When a silent-discharge tube is used, the ozone concentration can be controlled by adjusting the voltage applied to the generator, the dimensions of the electrodes, or the oxygen or diluent-air fow rate. Two-stage dilution of the ozonized air may also be used. The adjustments shall be such that they will maintain the concentration within the tolerances given in 9.1. In addition, after each occasion that the test chamber is opened for insertion or inspection of test pieces, the ozone concentration shall return to the test concentration within 30 min. The concentration of the ozone entering the chamber shall at no time exceed the concentration specifed for the test. Such adjustments may be manual or automatic.
5.5 Means of adjusting the gas fow
A mechanism shall be provided that is capable of adjusting the average velocity of the fow of ozonized air in the test chamber to a value of not less than 8 mm/s and preferably to a value between 12 mm/s and 16 mm/s, calculated from the measured gas fow rate in the chamber divided by the effective cross-sectional area of the chamber normal to the gas fow. In tests intended to be comparable, the velocity shall not vary by more than ±10 %. The gas fow rate is the volume throughput of ozonized air in unit time, and this shall be suffciently high to prevent the ozone concentration in the chamber being signifcantly reduced owing to ozone destruction by the test pieces. The rate of destruction will vary depending on the rubber being used, the test conditions and other details of the test. As a general guide, it is recommended that the ratio of the exposed surface area of the test pieces to the gas fow rate not exceed 12 s/m (see Note 1). However, the value of this ratio is not always low enough. In cases where there is doubt, the effects of destruction should be checked experimentally and, if necessary, the test piece area decreased. A diffusing screen or equivalent device shall be used to assist thorough mixing of incoming gas with that in the chamber. In order to adjust the ozone concentration in the chamber and to exclude the effect of volatile components that are produced by test pieces, air circulation apparatus that draws in fresh ambient air may be used.
The use of a mechanically rotating carrier mounted in the test chamber and upon which the clamps or frames holding the test pieces are mounted is recommended to equalize the effect of different ozone concentrations in different parts of the chamber. In one example of a suitable carrier, the test pieces move at a speed between 20 mm/s and 25 mm/s in a plane normal to the gas fow and each follows, consecutively, the same path in such a manner that the same position within the chamber is visited by the same test piece every 8 min to 12 min, and the area swept by the test pieces (shown shaded in Figure 2) is at least 40 % of the available cross-sectional area of the chamber.