Nitrox table vs computer question

[Tyger]

There are other things to look at as well such as the NOAA 24h O2 exposure chart which if you are diving a high ppo2 for multiple dives will significantly come into play, also your workload can affect it as well, which is why on a hard working dive most will limit their ppo2 to 1.2ata or less

I don't know about other training, but PADI doesn't really teach oxygen exposure anymore beyond "Do what your computer says and here is what happens if you don't". When I took the nitrox class, I had to go out and find the tables myself, just to be satisfied that I had a personal level of understanding I was comfortable with.

 

[rhwestfall]

I was trying to get a clearer understanding of the physiological difference of 5 minutes at 1.48ppo2 then 30 minutes at 1.39 and lower, vs 35 minutes at 1.39 or lower.

sometimes - death.....

 

[beaverdivers]

I haven't done this dive.

Thunderbolt :This 188-foot military ship is the jewel of the Marathon wreck fleet. Sunk intentionally as a dive attraction in March 1986, she now sits perfectly upright in 115 feet of water offering 45 feet of relief. As you mentioned the sand is at 120'.

I have had friends dive it. They had a very strong current.

If it were me I'd dive 32%, knowing I might only be at 120' for a few minutes, if I went that deep. Most of the dive would be above 110'.

For those that dive this wreck, What mix do you use?

What is the depth of the top of the wreck?

Lizard Leg, sounds like a nice dive. If I were you I would plan as a shallower dive, 100' maybe 110'. I would remove the word about from the plan "max will probably be about 110'"

Good Luck & Good Diving!

 

[Lizard Leg]

sometimes - death.....

I guess that is where I am confused. Almost every agency it seems teaches max 1.4PPO2 for recreational diving - and I 100% understand a MOD is a MOD. (yes, it's a period).

*But* doesn't it stand to reason, with limits that are that tight, that if even a few minutes at +.08 PPO2 past 1.4 is possible death, it would seem that -.08 could be possible death asd well, so my EAN30 mix with a MOD of 121 and a PP02 of 1.39 could be just as dangerous.

 

[beaverdivers]

Remember the time element. You don't die if you hit the MOD. You have 150 minutes at 1.4

Are you going to stay at 120' for 150 minutes?

At 1.6 ppO2 you have 45 minutes.

It is a function of time.

Just like the nitrogen, you don't get bent if you hit 100'. You get bent if you stay too deep too long.

The same is true with O2. But the result can be more permanent.

 

[rhwestfall]

and, even if you follow it perfectly, it still can happen. It becomes a question of how close do you want to approach the line. And the line is not clearly defined (research "Undeserved DCS Hit"). Each of us can have a slightly different threshold, and even that can change...

Risk Management - take the class, understand the associated risks, and it is your call....

 

[Them]

I guess that is where I am confused. Almost every agency it seems teaches max 1.4PPO2 for recreational diving - and I 100% understand a MOD is a MOD. (yes, it's a period).

*But* doesn't it stand to reason....

There are two issues.

First treat it as though the risk starts at some low but non-zero value and goes up exponentially. So from .2 to 1.2 PPO the risk is fairly similar and fairly low (but not technically zero, just so close to zero as to be ignored), then 1.3 has more risk but not much more, then 1.4 has appreciably more - about as much risk as anyone is willing to recommend these days, and 1.5 is higher still, and 1.6 is even higher, and 1.7 is beyond any reasonable risk. Somewhere north of 2.0 it stops being risk and becomes certainty. So a bump from 1.0 to 1.08 is a much smaller increase in risk than a bump from 1.4 to 1.48 would be.

Second, the risk associated with those numbers changes depending on factors you may not be able to control or even be aware of. The amount of physical work you are doing, your body composition, etc. all change the risk for a given PPO. Now, I don't know anything about it but a question that leaps out at me is, What would, just to pick something at random, removing a leg do to those numbers? Would it mean you are working harder and for you a 1.3 is like a 1.4 to someone else? Or is it the other way around? Many of the studies that scuba practices are based on were done using young men who were fit to military standards.

It is a soft line, which is part of the danger.

 

[Tyger]

Remember the time element. You don't die if you hit the MOD. You have 150 minutes at 1.4

Are you going to stay at 120' for 150 minutes?

At 1.6 ppO2 you have 45 minutes.

It is a function of time.

Just like the nitrogen, you don't get bent if you hit 100'. You get bent if you stay too deep too long.

The same is true with O2. But the result can be more permanent.

The time element is an issue for pulmonary oxygen toxicity, but that won't kill you underwater. That is a function of time.

It's the CNS toxicity that kills you, due to causing convulsions, and that's purely a function of the PPO2, with no time element, and it can hit quite suddenly when it hits.

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A ScubaBoard Staff Message...

The statement above (highlighted in red by me) is FALSE. The two key elements to the risk of CNS Oxygen Toxicity are PO₂ and TIME. There are many other factors that can also also influence one's susceptibility to OXTOX, but let there be no doubt that TIME is hand-in-glove with PO₂ as one of the top two.
Your friendly Accident & Incident Mod.

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[beaverdivers]

The maximum single exposure limits recommended in the NOAA Diving Manual are 45 minutes at 1.6 ppO2, 120 minutes at 1.5 bar, 150 minutes at 1.4 bar, 180 minutes at 1.3 bar and 210 minutes at 1.2 bar.

Pulmonary oxygen toxicity results in damage to the lungs, causing pain and difficulty in breathing. Oxidative damage to the eye may lead to myopia or partial detachment of the retina. Pulmonary and ocular damage are most likely to occur when supplemental oxygen is administered as part of a treatment, particularly to newborn infants, but are also a concern during hyperbaric oxygen therapy.

 

[waterpirate]

Since we are getting all MOD has a period after it. I will tell you that the gas choice is dictated by your dive plans MOD. Now we come full circle back to basic skills like " peak bouyancy ". Can you trust yourself to dive a perfect mix for a planned MOD, even though the hard deck of 120fsw is too deep for the hotter shallow mix?
Eric