ing for oxygen service. While we have over 35 years of rea- sonably safe experience with this practice, prudent operators follow the more conservative NFPA guideline of 23.5% oxy- gen. OSHA : The Occupational Safety and Health Admin- ▪ istration sets the upper limit for breathable air in work- place atmospheres at 23.5% oxygen. Any atmosphere above that is considered oxygen-enriched and haz- ardous. NFPA : The National Fire Protection Association de- ▪ fines an oxygen-enriched atmosphere as one in which the oxygen concentration exceeds 23.5% by volume. Why 23.5%? The 23.5% oxygen threshold isn’t arbitrary. According to the NFPA-99 Hyperbaric Facilities Code – sup- ported by the Koch/Cook flame spread diagram in the ap- pendix – 23.5% marks the point where flame propagation in open air accelerates dramatically. As Glenn Butler, who was the sole source for design and installation of the nitrox sys- tem at NASA’s Neutral Buoyancy Lab, explains: “The 23.5% figure came from the NFPA-99 Hyperbaric Medicine code and the Koch’s diagram showing rapid accel- eration of flame spread. It was placed as the maximum al- lowable percentage of oxygen in free air. Air to mix with oxygen is an oil contamination issue.” Above that level, fires ignite faster, burn hotter, and requires less fuel to sustain combustion. Even a small hydrocarbon TRAINING continued
contaminant – like a trace of compressor oil – or oils from your own skin can cause ignition if exposed to high-pressure O ₂ (pressures above 200 psi ( ≈ 1,380 kPa / 13.8 bar). When adding air to oxygen, any oil contamination from the air source becomes a primary hazard, making filtration and air quality just as critical as oxygen handling. I service every regulator, cylinder, and valve as if it were to be used with pure oxygen. We don’t use silicone-based lubri- cants for diving gear – only oxygen-compatible lubricants. Keeping Your System Clean: Even with the best prac- tices, nitrox blending systems can accumulate contaminants over time. Routine cleaning is crucial for maintaining system integrity and does not have to be complicated. According to the NOAA Diving Manual, all equipment ex- posed to oxygen concentrations above 23.5% must be free from hydrocarbons, particulates, and other combustible con- taminants. Cleaning should be done using oxygen-compatible solutions, followed by a thorough rinse with deionized water. Finally, dry the equipment using filtered, oxygen-compatible compressed air or nitrogen. The NFPA 53, which outlines recommended practices for materials, equipment, and sys- tems used in oxygen-enriched atmospheres, supports this guidance. Removal must be verified through visual inspection and, where applicable, with white-light and ultraviolet inspec- tions. For larger fill systems, a high-pressure steam cleaner can effectively remove trace oils, moisture, and particulate matter
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