Who's using 100% O2 for deco?

[Dive Bug Bit Me]

Seems like a 30% change in pressure added from the water's surface is significant change on bubble tension.

I'm not sure what difference it makes since I've never tried it, but I like keeping things standard.

This is proving to be an interesting thread. I am not totally sure I follow, so bear with me (I expect that I have lost the plot here).
I take it you are assuming deco stop at 20 ft. Ambient pressure = 1.6 ATA.

PPO2 of 100% mix = 1.60 ATA
PPO2 of 80% mix = approx 1.3 ATA
PPN of 80% mix = approx 0.3 ATA.

This being the difference of 30% you mention.

I think we agree that 100% mix is the ideal deco gas for this depth as it will result in reduced Nitrogen levels the fastest (with acceptable O2 risks).

I expect that bubble models (VPM is most likely what you are using) are able to adjust for using different gas mixes for deco profiles. Therefore, if we look at all the previous stops combined (at say 30 ft on 80% versus 30% on back gas) the impact on the bubble tension over the entire deco profile is adjusted for in the model (and in some scenarios 80% mix is modeled to get me out of the water faster). I guess what I am getting at is why are you so concerned about the bubble tension on the last stops, particularly when bubble models have controlling the bubble growth and size at depth as one of the key principals? I would have guessed that as 80% is getting you out of the water faster with VPM, VPM is saying that 80% is the more optimal deco strategy for that dive (taking into account bubble tension and all the other inputs that I don't understand)?

 

[Dive Bug Bit Me]

Just saw your previous post. Jax beat me to the question.

Agreed that change for the sake of change is bad.

Still ask why if I have decided that VPM is the model for me I would distrust changing to 80% at 30 versus changing to 100% at 20. Ultimately the same model is telling me this is OK. (and in some circumstances slightly optimal). I see the points that you list in your reply, but this is an optimization exercise. By definition, that means that by changing to 80% at 30 I will gain some benefits and lose on some other modeled parameters. Point is that the model (after adjusting for what you list) says that I am marginally better off.

 

[ucfdiver]

This is proving to be an interesting thread. I am not totally sure I follow, so bear with me (I expect that I have lost the plot here).
I take it you are assuming deco stop at 20 ft. Ambient pressure = 1.6 ATA.

PPO2 of 100% mix = 1.60 ATA
PPO2 of 80% mix = approx 1.3 ATA
PPN of 80% mix = approx 0.3 ATA.

This being the difference of 30% you mention.

I think we agree that 100% mix is the ideal deco gas for this depth as it will result in reduced Nitrogen levels the fastest (with acceptable O2 risks).

I expect that bubble models (VPM is most likely what you are using) are able to adjust for using different gas mixes for deco profiles. Therefore, if we look at all the previous stops combined (at say 30 ft on 80% versus 30% on back gas) the impact on the bubble tension over the entire deco profile is adjusted for in the model (and in some scenarios 80% mix is modeled to get me out of the water faster). I guess what I am getting at is why are you so concerned about the bubble tension on the last stops, particularly when bubble models have controlling the bubble growth and size at depth as one of the key principals? I would have guessed that as 80% is getting you out of the water faster with VPM, VPM is saying that 80% is the more optimal deco strategy for that dive (taking into account bubble tension and all the other inputs that I don't understand)?

Basically I was saying that 30ft = .9ATM of water pressure and 20ft is .6. I have no doubt the models can adjust, but if you did 20min of 30ft deco with 80% one dive and felt fine, then did 20min of 20ft deco with 100%, what conclusion would you draw? We can't control what bottom times and depths we have as those are often mission/site specific, but we can control deco gases on nearly every dive excluding remote dive sites that 99% of SB posters don't visit more than a week a year.

 

[Levigarrett]

I'd like to retract my retraction....I guess this is more controversial than the first few responses would have led me to believe. 11 pages of good stuff!

I'm going to stick with 100% until my instructor provides me with reason to do something different. Thus far, he's not hinted at using anything less than 100%. We'll see.

 

[Slamfire]

According to my analyzer, yesterday I decoed on 100.3% oxygen :shocked2:. Should I add a little squirt of air to make it an even 100%?


Liability Edit: The above text is an attempt at humor. There is no such thing as a >100% O2 mix. Please refer to your advance nitrox training if you have any doubts (or consider going back to high school chemistry if you missed school that day).

 

[ajduplessis]

According to my analyzer, yesterday I decoed on 100.3% oxygen :shocked2:. Should I add a little squirt of air to make it an even 100%?

No, just calibrate your analyzer :shocked2:!!

 

[Dive Bug Bit Me]

I have no doubt the models can adjust, but if you did 20min of 30ft deco with 80% one dive and felt fine, then did 20min of 20ft deco with 100%, what conclusion would you draw?

Assuming both dives were properly planned and executed I would expect to be fine, but after each dive I would monitor myself for possible niggles that might be symptomatic of something.

I think it was mentioned before that we do not fully understand decompression and the bends. We have a number of imperfect models that best attempt to explain it. Therefore at some point I guess deco becomes an art form rather than a precise science (which is why the 60 second saving in deco from 80% is meaningless in reality). I also know that the susceptibility of a diver to DCS varies from one day to the next, hence a need to be familiar with your body both before and after each dive. I guess the art element of decompression means we should stick to where our individual comfort (and training) is.

 

[ucfdiver]

Assuming both dives were properly planned and executed I would expect to be fine, but after each dive I would monitor myself for possible niggles that might be symptomatic of something.

I think it was mentioned before that we do not fully understand decompression and the bends. We have a number of imperfect models that best attempt to explain it. Therefore at some point I guess deco becomes an art form rather than a precise science (which is why the 60 second saving in deco from 80% is meaningless in reality). I also know that the susceptibility of a diver to DCS varies from one day to the next, hence a need to be familiar with your body after both before and after each dive. I guess the art element of decompression means we should stick to where our comfort (and training) is.

Remember there's people with scar tissue that get bent every dive following the model and have to adjust their deco based off of knowledge they gain working up to longer/deeper dives.

 

[iztok]

If you have both, why can't you transfill to make a partial pressure nitrox blend somewhere in the 21-99% range? That's what I do

Your air might not be 100% O2 compatible?

 

[w ripley]

Remember there's people with scar tissue that get bent every dive following the model and have to adjust their deco based off of knowledge they gain working up to longer/deeper dives.

Interesting. I have always thought that I was getting hits in the same places, and figured it was due to scar tissue. But during the last year I heard a doctor from DAN tell me, when I asked him that same question, that the odds really aren't in favor of getting a hit in the same place twice, and that there wasn't a correlation between location of scar tissue and location of the hit. Actually, he says that one of the first screening questions they ask at DAN, when you call in, is where the supposed hit is and had you ever had a hit there before. If you answered it's the same place as before they usually remove you from the suspect hit list.