Air Break Myth...

[techintime]

Is that 60 ft on 100% O2? Love to hear what the incidence of tox is at that PPO2 and how much the air break reduces it.

What is the regimen (how often and how long) for the air breaks at that PPO2? Is there a similar table on how often to apply air breaks for 20ft 100% O2 and 70ft 50% O2 deco hangs?

 

[PfcAJ]

I haven't seen an air break 'table' for divers (but that doesn't mean its not out there somewhere). Personally, I go with the "12 on 6 off" routine when the required 20' time gets to be around 30mins. If the time is 20ish or less I don't bother.

Before I do a gas switch, I spend about half the stop time on back gas. So if my 30' time was 10mins, I might do 5mins on 50% and 5mins on backgas. Or I might do the entire thing on backgas (especially if its a short stop). I always move from deco gas to backgas to deco gas.

The longest time I've spent on 50% was an hour (following avg220' for about an hour) , and I didn't feel the need to add any additional gas breaks except at 80, 30, and the 12/6 at 20'. Deco Planner shows the CNS% as 287% for that dive, but that doesn't consider gas breaks or the decent into the cave (10mins or so on 120 gas).

 

[rjack321]

Is that 60 ft on 100% O2? Love to hear what the incidence of tox is at that PPO2 and how much the air break reduces it.

This is a treatment Table 6. So dry chamber exposures which really don't relate to immersion very well. A treatment table 6 has 20mins on O2 and 5mins off onto air. There are 3 O2 breathing sessions at 60ft before beginning the ascent to 30ft.

 

[boulderjohn]

This is a treatment Table 6. So dry chamber exposures which really don't relate to immersion very well. A treatment table 6 has 20mins on O2 and 5mins off onto air. There are 3 O2 breathing sessions at 60ft before beginning the ascent to 30ft.

Just to expand the explanation slightly...

It is my understanding that for some reason oxygen toxicity occurs at significantly different depths when under water than when in a chamber.

 

[Duke Dive Medicine]

John, immersion is a risk factor for CNS O2 toxicity, possibly due to CO2 retention but the actual mechanism is still under investigation. Our deepest O2 treatment is at 68 FSW, which is just over 3.0 ATA. Our standard patient treatment is 33 FSW/2 ATA for 2 hours, without an air break. Our incidence of O2 toxicity at that partial pressure is extremely low.

 

[Thalassamania]

Is that 60 ft on 100% O2? Love to hear what the incidence of tox is at that PPO2 and how much the air break reduces it.
...

60 feet on 100% for 30 min (as best I recall) used to be the standard for an oxygen tolerance test, a ppO2 equivalent to 410 feet on air.

 

[Duke Dive Medicine]

The oxygen tolerance test has gone the way of the white buffalo. Too much individual variance to be valid.

 

[Thalassamania]

Yes, note the: "used to be the standard."