Gas toxicity and narcotic pressures

[tlj3071]

While doing some research on the RBGM deco model, I came across a guy that had problems while doing 100'+ technical dives. This got me to thinking about what sort of mixture you would use for a 1000' dive, and this ran me down the road toward several other questions. Here's my train of thought followed by the questions.

For a 290m (30 bar) dive, in order to avoid O2 toxicity (using a ppO2 limit of 1.6 bar), the pp02 at the surface would have to be 1.6/30 = 0.053 bar, or ~5% of the mix. Since N2 becomes narcotic around 3 bar, we would want to have the mix contain no more than about 10% N2. This leaves about 85% of the mix open for He.

Now the questions that came about due to this mental exercise:
1) The mix is obviously hypo-oxygenated, but how little oxygen can still be tolerated? The FAA requires supplemental O2 for passengers above 15000' (FAR 91.211a(3) "[In general, no person may operate an aircraft...] At cabin pressure altitudes above 15,000 feet (MSL) unless each occupant of the aircraft is provided with supplemental oxygen." This is a ppO2 of ~0.12 bar. (flight crews require O2 above 14,000' or over 12,500' for more than 30 min). Acclimated climbers are able to survive on much less O2 - as low as ~0.09 bar [or even a little less] while doing pretty hard work! So, what is the lower limit below which a diver would not want to breath the mix?

2) We all know that N2 can become narcotic at ~3 bar partial pressure, but can N2 become toxic? If so, at what level? Also, "acclimated" divers can apparently tolerate greater levels of N2 prior to exhibiting narcosis. What is the upper level of acclimation?

3) I assume He can also become narcotic at some level - what level is that? And, can He also become toxic? At what level?

Tom

 

[Saturation]

You did a good job in analyzing the base information. The questions you ask are typical data provided during a full trimix technical course including how to manage hypoxic gas mixes.

Generally, one doesn't breath a mix with P02 of 0.16 on the surface. Any P02 < 0.21 is called a hypoxic. Although unconsciousness is likely to happen at 0.12 and less, the difference is the safety buffer. Also 0.12 is the resting rate, and one could lose consciousness during exertion at higher P02.

N2 can be toxic but its balanced by acclimatization. The cutoff for safe use is ~ that of diving at 100' with air.

He doesn't cause narcosis, but its use at deep depth causes high pressure nervous syndrome if not controlled for by spiking the gas with N2.

 

[muddiver]

Another point is that to do a dive to that depth it requires a number of gas switches during travel from the surface to the target depth. One would not start the dive on a 5% mix, but rather a normal mix that is good to say 100', like regular air. At 100' one would switch to a second travel gas good from 100' to the next point of optimal O2/He mix, and so forth. Ascent would be the opposite with a hiperoxic mix above say 70' to 20' and 100% O2 stop at 20' for decompresion. This is all highly simplified and not a true mixed gas dive plan.

 

[tlj3071]

Thanks to both of you...this is just what I was looking for.

Tom