Posted By Philip McAleenan Diane, 23.5%. Gas detectors calibrated for oxygen will alarm at this level, Philip

Posted By Diane Thomason Thanks Philip, and just out of interest what would you normally set oxygen depletion alarms at? 19%, 18% or "other"? (have heard a lot of arguments sbout this.) D

Posted By Diane Thomason Philip Aha! It's the 18-19% range I'm interested in. I have heard lots of discussion on this, with much opinion being that 19% does not warrant evacuation. Can you tell me where the 19.5% industry norm comes from? Is there a reference to a document somewhere?

Posted By Philip McAleenan Diane, If the oxygen content drops from 21% to 19.5% it indicates that it is being replaced by something not conducive to breathing, and that it is likely to continue to drop. AT 19.5% it is breathable and people in that environment are stiull capable of unimpaired judgement. However to wait any longer before evacuation may put them into a situation whereby their judgement is so unoimpaired that they are not capable of makinga decision to evacuate at all. In addition, with the drop in oxygen content, the unknown factors are 1) the source of the contamination is unlikely to be known, and it may be possible that the oxygen content between the worker and the exit may be much less that 19.5%. 2) the rate of decrease in oxygen content is unknown and may continue rapidly, causing the workers to be overcome if they delay. The alarm sounds at 19.5% giving the workers a small window of opportunity to don their SCBA and evacuate. This must be done immediately. If the margin was any less, they would have no time to evacuate. A statistic to note is that on average if anything goes wrong in a confined spaces there is a 1 in 10 likelihood that it will result in a fatality, and when that happens there is a 50:50 chance of multiple fatalities. Philip