Nitrox diving and your computer settings

[scubadiver888]

Not to hijack the thread, but I always remember being struck by a comment that John Chatterton said during a seminar. He was saying "in the old days" people used to dive air to 230 feet and deeper, pushing the ppO2 of air above 1.6 ATA, but he could never recall anyone ever toxing on air.

Hardly a scientific survey, but I remembered wondering at the time (given how little we know about CNS toxicity) if there was something in that - it was not purely a feature of partial pressure, there was some kind of accelerated risk curve for richer mixes.

The value of 1.6 ATA is not a hard number. I've seen reports where people have suffered oxygen toxicity at 1.2 ATA. Everyone can dive any gas with partial pressure of oxygen beyond 1.6 bar and not suffer an O2 hit. With a small enough sample I'm sure it is very likely you never see anyone take an O2 hit at 230 feet and deeper.

However, if you talk to people who dive beyond 220 feet on a regular basis they will tell you they dive hypoxic blends (i.e. less than 21% oxygen).

There is nothing magical about air that makes it different from EANx.

 

[NJDiver07866]

Nitrox FAQ

Take a read, Haldane did some wonky experiments, people have done chamber dives as high as 7P02.

But everyone is different. Some rules are more like fuzzy logic, but For me? Don't cheat the chart. It is a decision you have to make as an individual. You can follow all the rules and still get bent, toxed or a mess of other perils that can happen...so don't think too much about it

 

[ianr33]

Not to hijack the thread, but I always remember being struck by a comment that John Chatterton said during a seminar. He was saying "in the old days" people used to dive air to 230 feet and deeper, pushing the ppO2 of air above 1.6 ATA, but he could never recall anyone ever toxing on air.

Hardly a scientific survey, but I remembered wondering at the time (given how little we know about CNS toxicity) if there was something in that - it was not purely a feature of partial pressure, there was some kind of accelerated risk curve for richer mixes.

I *think* its pretty much accepted that toxing on air is less likely than toxing on 100% O2 for the same pO2. Believe its to do with the sedating effect of the nitrogen offsetting the stimulating effect of the oxygen. Most toxing events certainly seem to be on rich deco mixes or trimix (where the sedating effect of nitrogen is lessened)

Of course,its not politically acceptable to suggest that diving beyond 1.6,even on air is acceptable

Would be interesting to know the shallowest depth at which anyone has suffered oxygen toxicity on air.

 

[scubapeople59]

I read an article recent which talked about why you should not set your computer to Air when you were diving with an enriched air nitrogen mixture.

Apparently, some people advocate setting the computer to Air or 21% Oxygen when diving with say EAN32. The advantage is that the nitrogen levels will be calculated assuming ~79% nitrogen. Because the nitrogen levels are actually much lower (~68%), it will be more conservative in calculating nitrogen loading.

The article pointed out that you run the risk of oxygen toxicity by doing this. The benefits of reduced nitrogen loading do not justify the increased risk of oxygen toxicity. Maybe you are mistakenly thinking about that article.

I found the article that was driving me nuts and what started this thread. You are right, it was talking about diving with your computer set on air while diving with nitrox. It was a letter posted in Dive Training magazine. Thanks to all that posted their thoughts.

 

[HowardE]

Wonder why Suunto dont program their computers to do the O2 calculations in the background when set to air?

The Maximum PPO2 a recreational diver could hit would be 1.04 PPO2. This is at 130 feet. The allowable dive time at that PPO2 in a 24 hour period is more than 4 hours. Therefore - as a recreational diver - diving air, it would be theoretically impossible to reach 100% ever.

The value of 1.6 ATA is not a hard number. I've seen reports where people have suffered oxygen toxicity at 1.2 ATA.

Where? Can you post them? How long were they at 1.2 ATA's? 3.5 Hours? Sure - if you stay at 1.2 ATA's for more than 3.5 Hours, you run the risk of ox tox.

 

[HowardE]

Not to continue on with your hijack, but...

Not to hijack the thread, but I always remember being struck by a comment that John Chatterton said during a seminar. He was saying "in the old days" people used to dive air to 230 feet and deeper, pushing the ppO2 of air above 1.6 ATA, but he could never recall anyone ever toxing on air.

Hardly a scientific survey, but I remembered wondering at the time (given how little we know about CNS toxicity) if there was something in that - it was not purely a feature of partial pressure, there was some kind of accelerated risk curve for richer mixes.

Yes. However.

1) John Chatterton's dives to deeper than 230 on air were short. Less than 20 minutes on average.

2) JC - when he was training as a commercial diver - was obligated to spend 30 minutes at 2.0 PPO2 without toxing, otherwise "you couldn't be a commercial diver"

3) "In the Old days, we didn't sling bottles. Everything was on our backs. The limiting factor in deep air diving was time.", says John.
15 minutes at a PO2 of 1.8 or even 2.0 wouldn't cause most people to tox.

 

[ianr33]

The Maximum PPO2 a recreational diver could hit would be 1.04 PPO2. This is at 130 feet. The allowable dive time at that PPO2 in a 24 hour period is more than 4 hours. Therefore - as a recreational diver - diving air, it would be theoretically impossible to reach 100% ever.

I understand that.

My point was that if you set the computer to "Air" you can not do a second dive on nitrox where the O2 logging could be useful.(Computer will not change)
Set the computer to 21% O2 and it will function fine for a second dive on nitrox.

If the computer was programmed to do O2 calculations in the background when set to air then it would be possible to do that second dive on nitrox.

Just seems like an unnecessary limitation.

 

[scubadiver888]

Where? Can you post them? How long were they at 1.2 ATA's? 3.5 Hours? Sure - if you stay at 1.2 ATA's for more than 3.5 Hours, you run the risk of ox tox.

I cannot find the original article at the moment. I received an email from DAN when I joined and it had a link to articles. One of the articles was about Nitrox being introduced to recreational divers.

I found a link on Rubicon but it is just an abstract. The whole article is not there: Rubicon Research Repository: Item 123456789/956. The abstract does mention 1.2 on Nitrox so I'm pretty sure this is the article.

I distinctly remember being on the DAN site. There was a list of articles and a link to Rubicon for older articles. The list was sorted by year.

 

[HowardE]

Sure... Here's a simple scenario where a diver toxes on 1.2 PPO2.

Dive 1) Diver uses 30% of O2 Clock
Dive 2) Diver uses another 30% of O2 clock
Dive 3) diver uses another 30% of O2 Clock

Dive 4... you get the picture.