2.19.2. When freezing a water pipe in a trench or in an obstructed space by using a liquified, physiologically inert and nonflammable gas, the following measures shall be taken:
(a) the trench or the obstructed space shall be ventilated by an exhaust ventilation system so that the concentration of oxygen in the air is at all times equal to or greater than 19.5% at the place occupied by the worker to carry out his work;
(b) a device to measure the concentration of oxygen in the air shall be available on the work site at all times.
For the purposes of this section, an obstructed space is a space marked off by a wall, a partition wall, a ceiling or any other physical obstacle where the circulation of air with the outside is insufficient to maintain a percentage of oxygen in the obstructed space equal to or greater than 19.5% in air volume.