A question for a friend

[Barracuda2]

Because my friend knows I'm a diver, he has a technical question that I don't have the math skills to give him his answer because the question goes beyond normal diving modes or tables. He has this very complicated idea dealing with the use of hyperbaric chambers and health, much too complicated to try to explain here; I don't even know if I really understand it.

Here's the question: At 2 atmospheres, breathing 50% o2 for 24 hrs., I would assume we are replacing n2 with the o2 would there be any decompression time coming back to 1 atmosphere? If so, what would that be?

The best I could tell him is that the NDL for 33ft on air is 4 hrs. 20 min. I could go into the nitrox mixes, but that really wouldn't answer his question concerning the 24 hour stay.

Thanks for your replies.

 

[wedivebc]

Because my friend knows I'm a diver, he has a technical question that I don't have the math skills to give him his answer because the question goes beyond normal diving modes or tables. He has this very complicated idea dealing with the use of hyperbaric chambers and health, much too complicated to try to explain here; I don't even know if I really understand it.

Here's the question: At 2 atmospheres, breathing 50% o2 for 24 hrs., I would assume we are replacing n2 with the o2 would there be any decompression time coming back to 1 atmosphere? If so, what would that be?

The best I could tell him is that the NDL for 33ft on air is 4 hrs. 20 min. I could go into the nitrox mixes, but that really wouldn't answer his question concerning the 24 hour stay.

Thanks for your replies.

The equivalent air depth would be less than breathing air at 1 ata so he would be OK from a deco point of veiw. I don't have a chart handy but I think 24hrs is about the limit for whole body toxicity exposure.

 

[Rec Diver]

I beleve that the O2 CNS limit for 50% oxygen at 33 feet is 5 hours.

 

[wedivebc]

I beleve that the O2 CNS limit for 50% oxygen at 33 feet is 5 hours.

You are thinking about CNS limits which are greatly increased in chamber operation where the consequence of seizure is rarely serious.

Whole body limits are rated in OTUs with 24hr max exposure at 850 OTUs
so if we apply this formula OTU=t(PO2 - 0.5 divided by 0.5) to the 0.83 power to the 24 hour exposure we get ~230 OTUs which are well within daily limits for single exposure

As to the second part of your question, decompressing would actually be more like a decent in terms of nitrogen gradient.

 

[Barracuda2]

You are thinking about CNS limits which are greatly increased in chamber operation where the consequence of seizure is rarely serious.

Whole body limits are rated in OTUs with 24hr max exposure at 850 OTUs
so if we apply this formula OTU=t(PO2 - 0.5 divided by 0.5) to the 0.83 power to the 24 hour exposure we get ~230 OTUs which are well within daily limits for single exposure

As to the second part of your question, decompressing would actually be more like a decent in terms of nitrogen gradient.

OK--so the bottom line answer is that a person could stay at 2 ata breathing 50% o2 for 24 hrs. and be ok (no o2 toxcisity) and come back to 1 ata without any decompression? Thanks for the replies to one NOT in the know.

 

[Barracuda2]

You are thinking about CNS limits which are greatly increased in chamber operation where the consequence of seizure is rarely serious.

Whole body limits are rated in OTUs with 24hr max exposure at 850 OTUs
so if we apply this formula OTU=t(PO2 - 0.5 divided by 0.5) to the 0.83 power to the 24 hour exposure we get ~230 OTUs which are well within daily limits for single exposure

As to the second part of your question, decompressing would actually be more like a decent in terms of nitrogen gradient.

OK--so the bottom line answer is that a person could stay at 2 ata breathing 50% o2 for 24 hrs. and be ok (no o2 toxcisity) and come back to 1 ata without any decompression? Thanks for the replies to one NOT in the know.

 

[vertaqua]

I think that you would have a signifigent deco obligation @2ata for 24hrs on 50% O2. The NDL @35FSW is 310 min so you could double or even triple that number to account for the extra O2 in your breathing mix and you'd still be entering into deco. Now there is no table for this scenario considering that in a chamber when your shallower than 60fsw your either breathing 100% O2 or air. In water the highest mix I could find tables for is 40%, but to guesstimate the ammount of deco I'm going to use the USN standard air deco table 40FSW. Now the longest bottom time they have listed is 720min, 24hrs is 1440 min so double the ammount of deco and then cut it in half again because your breathing 50% O2. So you arrive at the original deco time for 40 FSW for 720min which means you'll have a 69min stop at 10 FSW. I'd say that would take you somewhere close to the ammounty of deco required for such a chamber dive.

 

[wedivebc]

I think that you would have a signifigent deco obligation @2ata for 24hrs on 50% O2. The NDL @35FSW is 310 min so you could double or even triple that number to account for the extra O2 in your breathing mix and you'd still be entering into deco. Now there is no table for this scenario considering that in a chamber when your shallower than 60fsw your either breathing 100% O2 or air. In water the highest mix I could find tables for is 40%, but to guesstimate the ammount of deco I'm going to use the USN standard air deco table 40FSW. Now the longest bottom time they have listed is 720min, 24hrs is 1440 min so double the ammount of deco and then cut it in half again because your breathing 50% O2. So you arrive at the original deco time for 40 FSW for 720min which means you'll have a 69min stop at 10 FSW. I'd say that would take you somewhere close to the ammounty of deco required for such a chamber dive.

since the EAD for this mix is less than 10ft it would be the same as staying at 10ft for 24hrs. The reason tables don't go that low is that no amount of time will ever allow any tissue to reach m-value.

 

[Thalassamania]

The equivalent nitrogen depth of Nitrox-50 being breathed at 2ATA is slightly below 8 FSW. ((50%/80% * 2ata)-1ATA)*(33FSW/1ATA)

Since the limit for saturation with a direct ascent to the surface is on the order of 28 FSW, 24hrs at 33 FSW would not pose any problem.