O2 facts & figures
The chart illustrating this article should be familiar to all nitrox divers. If you were not shown it when you took your course, your instructor did a shoddy job.
Scientists and navy divers were using nitrox for decades before sport divers started using it. The science behind nitrox diving, including this chart, came to us via a former US National Oceanic & Atmospheric Administration (NOAA) Diving Officer, Dick Rutkowski, who started teaching it to sport divers in 1985.
Nitrox 32 was originally known as NOAA 1; nitrox 36 was NOAA 2.
The NOAA oxygen exposure chart.
The chart details the level of oxygen exposure at which a diver may be at risk of an oxygen-toxicity convulsion. NOAA deliberately set conservative limits. It didn’t want to lose its scientists, after all.
As you can see, the limits do not simply derive from the PO2; they derive also from the time a diver spends exposed to that PO2. Therefore, spending 150 minutes at a PO2 of 1.4 carries the same risk as spending 45 minutes at 1.6.
But a PO2 of 1.6 is the absolute maximum. Beyond this, the permitted exposure time drops quickly to only a few minutes. So divers were advised always to choose a lower “target” PO2 of 1.5 or 1.4, to allow for defective gauges, defective analysers and diver inattention.
In the beginning, conventional wisdom among the established sport-diving hierarchy was that teaching nitrox to sport divers was dangerous. It would lead to mayhem, with divers suffering oxygen hits and drowning all over the place.
This didn’t happen. It still doesn’t happen today, with millions of nitrox dives taking place each year. Experience has shown that sport divers diving on open-circuit nitrox do not come to harm when diving within no-decompression limits and NOAA oxygen-exposure limits.
When the mainstream training agencies saw this (and also found that they were losing market share to the nitrox agencies), they started running nitrox diver courses too.
Those that came late to the party, however, could not completely shake off their anxiety. This has resulted in 1.4 being considered today as the maximum PO2 for a nitrox diver. Computer makers now preset 1.4 as the PO2 alarm, though on most units this is user-changeable.
The length of exposure factor seems to have been erased from the equation, to
a certain extent, leading to the sort of misguided response we saw among the group in the Philippines.
Nitrox 32 was actually the perfect mix for that wreck-dive. The water was warm, visibility excellent. With no current, the divers had to expend only minimal effort. There was no reason for extraordinary caution concerning PO2 levels.
Nitrox 32 gave the divers significantly more no-decompression bottom time than air. They did not have to go beyond 34m and a PO2 of 1.4 and, even with the extended no-decompression time, the time spent at 1.4 was a tiny percentage of the 150 minutes NOAA exposure limit.
Even when lying flat on the sand, a diver would have had a PO2 of 1.44, hardly something to panic about. As you see from the chart, the NOAA-permitted exposure at 1.5 is 120 minutes.
Sadly, it was unnecessary for the divers to abort. They missed a great dive.