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Always know your PO2 (oxygen partial pressure). However it is simply not enough to ‘always know your PO2’, of greater importance is that you understand your PO2 and the potential implications should you embark upon a particular course of action. When you use a rebreather you breathe from an artificial environment and during an emergency, if the electro / mechanical PO2 management or monitoring systems fail, for a short period at least, you may require to safely manage this artificial breathing environment.
When moving from open to closed circuit SCUBA, you must alter your thought processing from a fixed oxygen percentage to a fixed oxygen partial pressure if incorrect decisions are to be avoided. Understanding a fixed oxygen partial pressure and its potential implications must become intuitive, simply knowing your PO2 is insufficient. It has beem witnessed on numerous occasions rebreather divers making an incorrect decision when they knew their PO2 in numerical terms but failed to understand the reality and implications of the numbers (PO2) being displayed. For example, during a rebreather instructor training course, an oxygen addition valve (solenoid) failure occurred and the PO2 was allowed to drop to a level that eventually triggered a dangerously low PO2 alarm. The initial emergency response was correct, an immediate and effective diluent flush elevating PO2 to a temporary safe level. However the subsequent action was to spend too long trying to diagnose the problem and then to ascend directly to the surface breathing from the ‘loop’. At this point the instructor candidate was prompted to go to open circuit before the ascent was made. When questioned post-dive, the diver thought that it would be safe to ascend on the ‘loop’ as we were only in 15m of water and at a safe ascent rate it would only take a minute to reach the surface/safety. Had he done so he would have likely lost consciousness before he made it to the surface as his PO2 was down to 0.25 bar before the intended ascent was due to commence. This demonstrated a fundamental lack of understanding of the effects of a fixed / constant PO2
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