Nitrox - 1.40 or 1.60 PO2?

[Ben_Ayers]

It amazes me that I'm apparently the only one that plans working bottom gas at 1.2.

James,
I, too, have chosen 1.2.:shocked2: A few of the factors that went into my decision are: age; relative inexperience; meds; diving cold and wet; hx of smoking (quit to dive); and evaluating the risks vs rewards.
I feel that my inexperience could lead to a loss of buoyancy control if task loaded/stressed and/or an increased workload in certain circumstances due to poor decisions/technique. Further, I've seen many smoker that were co2 retainers.
Once I gain experience and confidence, I may reevaluate my choices.

Thanks,
Ben

 

[hobodiver]

Further, I've seen many smoker that were co2 retainers.

How can you tell that?

 

[dieter]

When everybody started to use Nitrox, everybody dove 1.6 ppo2. As you do not know how your body reacts on the higher oxygen level in the mix, you should be more conservative and dive 1.4 ppo2. Be a save diver!

 

[String]

Im quite possibly a CO2 retainer or at least i seem more susceptible to CO2 headaches than most even sometimes for no effort dive profiles plus the increased "dark nark" at depth (and not its not breathing technique). It varies massively from person to person.

(and FWIW i dont smoke and am a relatively fit long distance runner so i dont think physical health really has much influence. Although smokers statistically DO retain more as proven with above water dry tests)

 

[Scuba-Stu]

*edit*

 

[Ben_Ayers]

How can you tell that?

Well...hummm....Go to the mirror, and look for a ......... Couldn't do it!

Short answer = blood work/gases. It can probably be extrapolated from Pulmonary Function Tests also, not sure. Is there an MD in the house?

Most "COPD'ers" can be recognized by their breathing patterns, barrel chest, complexion, etc., however, not all of them are "retainers" - at least at 1atm.

BTW-IIRC nearly all Emphysema cases (97-98%) are related to smoking.

In my case, I'm just going to assume that 30 pack/years of smoking has done some damage to my lungs - at the very least my alveoli are probably less compliant than a twenty year olds!

Ben

 

[nereas]

When you dive high _ppO₂s you take a risk. There are several questions that then need to be answered, including what benefit are you getting from the higher _ppO₂?

In the old days we used to do Oxygen Tolerance Tests to 60 FSW and we used to dive pure oxygen to 30 feet on a routine basis. There were very few problems. Since then we have come to not trust Oxygen Tolerance Tests, and have cut back our maximum exposure from 2.0 ATA, to 1.8, to 1.6, to 1.4 and now (some) to 1.2.

Clearly using higher _ppO₂s have risks, and benefits. As they say, "you pays your money and takes your chances."

Thal's wealth of experience is always invaluable in understanding scuba issues, and a benefit to the rest of us, whenever he posts.

Thank you, Thal, again, for the great backgrounder that you just provided.

To everyone else, I think you should remember that just because you set your dive computer to 1.6, this does not mean you are going to be diving at a depth corresponding to a ppO2 of 1.6. It's just a limit that you dialed into your computer.

As to what the limit should be, based on the variety of replies, here, it is going to be different for each individual diver, apparently.

For NDL non-deco diving, either 1.4 or 1.6 is fine for most people. Take your pick. There is no technical reason why either of these limits would ever result in excessive exposure over the course of NDL dives with reasonable surface intervals. [Tech-deco on O/C or else CCR diving at a constant ppO2 is a completely different story, wherein ppO2 needs to be planned and limited.]

The training agencies have chosen 1.4 apparently because they prefer to be excessively conservative. Perhaps that is because of the wide range of different people who get certified for basic nitrox with them. Or perhaps it is merely a liability issue with them.

Your whole dive is not going to be spent at this level of ppO2. Just remember that in analyzing the situation.

To Thal, I have to ask: Why on Earth would you go to the extreme of setting your ppO2 LIMIT to 1.2? Is there something about old age and diving that I do not yet know about? or are you speaking strictly regarding the application to CCRs?

I have not heard or read of anyone toxing at anything less than about 2 or 3 atas ppO2. The most recent incident was reported in the DAN magazine about the wreck dive in Virginia where the techdiver switched to pure O2 (the wrong gas at the wrong depth) at 70 fsw giving a ppO2 of 3.1, toxing within 10 or so minutes.

And absent the threat of toxing, then the next most likely threat becomes DCS. And DCS is minimized with nitrox by maximizing ppO2. Thus a ppO2 limit of 1.2 is "less safe" from the risks of DCS than is a ppO2 limit of 1.6.

N'est ce pas?

 

[Thalassamania]

...

To Thal, I have to ask: Why on Earth would you go to the extreme of setting your ppO2 LIMIT to 1.2? Is there something about old age and diving that I do not yet know about? or are you speaking strictly regarding the application to CCRs?

I don't, I was just saying that some now dive 1.2 ... I dive typically at 1.4 or a little more.

 

[wve]

How can you tell that?

To hobodiver: The best way to know whether someone is a CO2 retainer is to draw an arterial blood gas (ABG), a nifty little test where we stick a needle into an artery (usually in the wrist) and actually measure the gases in your blood.

To String: CO2 retainers are typically in the later stages of emphysema and cannot perform the level of exercise that you engage in.

It is important for medical personnel to know if a person is a CO2 retainer because breathing is normally driven by CO2: when it builds up, you breathe to get rid of it. CO2 retainers, however, lose their CO2 drive, and they respond instead to oxygen. When it gets low, they breathe to get some more. This can be a problem because giving these people oxygen actually can cut off their drive to breathe!

To heelsfaninpa: To provide a longer-winded answer to your original question, I think of this by imagining a teeter-totter or balance scale with Oxygen Toxicity Risk on one end, and Nitrogen Toxicity Risk on the other. The goal is to keep the thing balanced as much as I can, trying not to tip either end into the toxicity range.

Breathing gases under super-atmospheric pressures is not normal, and doing it can cause problems.

The risk of Nitrogen Toxicity is increased by (a) diving deeper and (b) diving longer. How can one reduce the risk? Besides limiting the depth of dives and staying within NDL time limits, the risk can also be reduced by reducing the amount of nitrogen we breathe. While some mistakenly imagine that the goal of Nitrox is to increase the amount of oxygen we breathe, it is more accurate to say that the goal is to reduce the amount of nitrogen we breathe.

The problem, though, is that lowering the percentage of nitrogen in the gas is accomplished by increasing the percentage of oxygen. That, in turn, introduces a new problem, namely, Oxygen Toxicity Risk.

Curiously, like nitrogen toxicity risk, oxygen toxicity risk is increased by (a) diving deeper and (b) diving longer. But unlike nitrogen toxicity, which usually develops after the dive while on the surface, the most life-threatening oxygen toxicity -- the risk of a seizure -- develops during the dive while you are still under water!

Considering this, I agree with those who would rather get bent than to have a seizure and drown. On the other hand, I don't really want to experience either problem.

There are no hard and fast numbers above which one is guaranteed to have a problem or below which one is guaranteed NOT to have a problem. The guidelines are simply that: just guidelines.

Divers assume a number of risks when they strap on tanks, go under water, and breathe pressurized gases. Smart divers try to minimize those risks by staying within guidelines which are based upon experience (good and bad).

Do people get away with ignoring the guidelines? Sure. People survive running red lights, speeding, eating horribly, and smoking. But will your luck hold out? I don't intend to find out.

Knowing how conservative most agencies are when setting guidelines, and also having read quite a bit on this topic from a medical standpoint, I feel very comfortable with the standard recommendation to use the maximum (planned) PO2 of 1.4 and contingency (unplanned) PO2 of 1.6. My computer goes off at 1.2 just in case I have gotten distracted by something else, but I don't get nervous at that level. That said, most of my dives do not even break 1.0.

 

[Papa_Bear]

To each his own no matter how much they talk about it no one has ever suffered a hit at 1.6 or below and even with a number above it they had to be in that environment for 30 minutes or more and be working really heard! Lost I can find was a 30 minute exposure at 1.7 with heavy exercise! So with that in mind lets all drive 25mph! After all we kill 50 thousand people every year and if we had Zero tolerance we really would have a bad economy! Or lets keep all people off the Black Diamond slop, look at poor Sony! The benefit Vs risk is true of every dive and no one should ever dive beyond their ability! But we will see more and more orgs. and operators requiring, unnecessarily, enforcing some rule they read on the Board! Until I see hard core evidence that it is a risk I will set my computer to 1.6 hard bottom and dive that MOD profile! I think this is a good topic so that just maybe people diving VooDoo Gas will do their own home work and not just take someones word for it!