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I was recently involved in a discussion about the subject and within the group there was no definitive answer achieved. The issue is theoretical and relates to the partial pressure of gasses.
I'll try and summarise the key issues and assumptions.
1. For this hypothetical, disregard lung over expansion injuries, they are understood and agreed to be an issue that applies in reality.
2. The accepted mod of 21% Air is between 56.7m (ppO2=1.4) and 66.2m (ppO2=1.6ata).
3. A free-diver does not experience the negative effects of partial pressure of O2 because the air is breathed at 1 ata and at that point there are no negative effects.
A SCUBA diver using 21% (normal) air descended to 56m, took a breath of air and then holding his breath (contravening the basic rules, yes) descended to 100m and then ascended back to 56m without breathing from the tank below 56m.
- Would this diver be subject to the negative effects of the increased partial pressure of O2?
- Would this diver have had a deco requirement in addition to the 56m ?
This is a theoretical question and does not refer to the hero who bounce dives to 100m with a 50/50 risk of success.
Pressure is pressure. Free divers do technically expieience the same pressure effects. Some pearl divers have in fact gotten themselves bent free diving deep enough, long enough and frequently enough over the course of several hours.
Specifically with regard to Po2, the difference is in the actual O2 percentage as by the time the free diver gets to depth the fraction of O2 in his or her lungs is lower than when they started at the surface.
The same would apply to a diver at depth. A diver could hold his or her breath and the Fo2 would decrease due to normal metabolioc use of the O2 so technically they could go deeper and not exceed a PO2 of 1.4 for whatever the mix is in their lungs at a given time. If you installed an O2 sensor in a lung you could tell that accurately.
Of course when breathe holding, your CO2 levels also rise and elevated CO2 can preceipitate an oxtox event as well as increase the effects of narcosis. So it is a losing proposition.
And as a diver with a slower descent rate, more bottom time and much more nitrogen, a rapid ascent from where ever you breath held dove back to what ever depth you started at has bad written all over it.
Also, as pointed out in the previous post, PO2's of 1.6 and 1.4 are longer term exposure limits - the higher the PO2 the shorter the allowable exposure time. The US Navy developed very short extreme exposure limits that are much higher in terms of PO2 - but are not designed to be stacked on top of another PO2 limit.
That said if I am totally out of bottom gas at depth and have no buddy in sight, I'd opt to ascend on my lowest percetage bottom mix relying (hoping) on the time versus PO2 relationship to get to the MOD before I toxed. That is also theoretical as you have to be really stupid to get in that situtation in the first place.
I would say that perspective is needed here. O2 at that pressure is toxic. Like all "toxins" it is the dosage that gets you. The free diver is low dosing while a diver is accumulating the toxin constantly. Does that make any sense?