PPO2 maximum safe value: 1.4, 1.6

[Kazeone]

I can see the wisdom in what you are saying and there may come a day when I follow your advice.

I've done the planning for a bunch of imaginary deco dives with both back gas deco (air and ean) and accelerated deco. I have a spreadsheet somewhere where I calculated the max bottom time on a pair of HP-100s with air deco at various depths taken from the IANTD tables, based on the gas planning consequences of all the deco stops. Then I did the same thing with accelerated deco for the last two stops. This is my way of learning things. In the process, I have not been able to convince myself that there are any dives that require planned deco that I want to do badly enough to make it worth it. Least of all do I ever want to carry an O2 bottle with me to depth. I take your point however that incidents do occur where an unplanned deco obligation is incurred due to entanglement or some other problem. If it happens I'll keep my cool, watch my computer, do what it says, and try to make sure it doesn't happen again.

Don’t just follow your computer. Build some conservatism into your dive plan. Take a course. Dive safe.

 

[Dan_P]

Forget ppO2 and nitrogen levels for a while.
Look at the gas density.

If it's higher than 6g/L, that's the first problem.
When we've found a suitable gas density, we can look at how much N2 we can displace with O2 and still stay within ppO2 1.2 (or 1.4).
THEN comes the question of decompression/NDL.

 

[Storker]

Look at the gas density.

If it's higher than 6g/L, that's the first problem.

So, max 36m then. On EAN28 you'll stay below 1.3 bar pPO2 at that depth, and EAN30 gives you 1.4. If you want to stay below 1.2 bar on EAN28, the limit is 33m.

It seems to me that if you're backing off from the 1.4 bar pPO2 limit, gas density seldom is a real issue if you're on a sensible mix. I'm obviously disregarding slightrox (come with a tank of EAN, top off with air locally after the dive) here.

 

[Dan_P]

So, max 36m then. On EAN28 you'll stay below 1.3 bar pPO2 at that depth, and EAN30 gives you 1.4. If you want to stay below 1.2 bar on EAN28, the limit is 33m.

It seems to me that if you're backing off from the 1.4 bar pPO2 limit, gas density seldom is a real issue if you're on a sensible mix. I'm obviously disregarding slightrox (come with a tank of EAN, top off with air locally after the dive) here.

So long as the planned gas density is within reasonable limits, especially when in a class setting, I don’t have a problem.
I personally prefer to educate students on the ‘why’ of things, and I personally prefer standard gases.
But that’s just personal preference.

When it comes to taking uninformed students to as deep as 7,7g/L, my opinion is that if there’s an accident in that setting, the instructor and agency should be legally culpable.
That’s all.

I don’t think that’s a radical position or a strong way to phrase it.

 

[Storker]

So long as the planned gas density is within reasonable limits, especially when in a class setting, I don’t have a problem.
I personally prefer to educate students on the ‘why’ of things, and I personally prefer standard gases.
But that’s just personal preference.

When it comes to taking uninformed students to as deep as 7,7g/L, my opinion is that if there’s an accident in that setting, the instructor and agency should be legally culpable.
That’s all.

I don’t think that’s a radical position or a strong way to phrase it.

Being Scandinavian, you might know the tale "Good day, fellow!" "Axe handle!". For some reason, I was reminded of that.

 

[stuartv]

Being Scandinavian, you might know the tale "Good day, fellow!" "Axe handle!". For some reason, I was reminded of that.

When all you have is gas density, even an axe handle looks like a ppO2 value, I mean piece of firewood... Uhh, I mean hammer. Nail! Aarrghh!!

LOL

 

[fsardone]

When all you have is gas density, even an axe handle looks like a ppO2 value, I mean piece of firewood... Uhh, I mean hammer. Nail! Aarrghh!!

LOL

ROTFL

 

[NAM001]

I don't ever intend to do deco dives, trimix dives, dives with gas switches, or dives deeper than 130 feet. The interesting local dives are cold water dives most often in roughly 60-100 feet of water, sometimes deeper. When I have the training and experience to do these dives, I anticipate diving 100cf doubles. NDLs will be the governing factor in dive planning. I will be diving over a hard bottom that will rarely if ever be deeper than 130 feet. My dive computer calculates CNS O2 loading.

Given all that, there's simplicity to be gained by using EAN32 for all dives. Banked EAN32 is widely available here and doesn't require the lead time of a custom blend. I could permanently mark the MOD of each tank. I could refill tanks after each dive rather than waiting to decide the location and depth of the next dive. I could be flexible enough to change dive sites when the weather requires, a common problem here. There would be no reason to keep the tanks O2 clean since there would never be a reason to use partial pressure blending. Using the same gas all the time would reduce the risk of ending up with the wrong gas. I'd still get an analyzer and check every tank after it is filled, and again at the dive site.

The question I have is the handling of dives to 120-130 feet. With EAN32, PPO2 is between 1.4 and 1.6 at these depths. NDLs on EAN32 at these depths are 18-20 minutes, give or take a minute or two depending on which agency's table you use.

Is there anything more than a hypothetical risk from the higher PPO2 of 1.6 under these circumstances? Given that NDLs limit the dive duration, it's hard for me to believe that there is. Do agencies vary in their teachings on this subject? Is the added CNS safety margin of a custom EAN28 mix meaningful enough to warrant the risks inherent in keeping track of a tank inventory with multiple mixes? Have there been accidents or incidents involving tox hits at 1.6 for dive durations this short?

Hello air. Th root to your answer lies in the toxic curve . there is a formula to calculate it and if you were to build a spread sheet and plot the toxicity curve in .1 PPO2 steps you would find that at about 1.65 or so the curve changes from relatively linear to exponential and sky rockets north.

It looks something like this

on the curve it looks flat . When it gets to the horizontal point of about 6 is where 1.6 PPO2 is. My opionion it is safe to go to 1.6 but with the understanding you body may not react to this curve like this model suggests. the 1.4 area is about where horizontal 4 is. Yes by the way if you want to go to perhaps 2.0 then this rat graph may be accurately labeled. Play it safe and limit your risk to deco at 1.6 with minimal activity.

 

[Dan_P]

Being Scandinavian, you might know the tale "Good day, fellow!" "Axe handle!". For some reason, I was reminded of that.

I don’t really see what you’re getting at here.

I’m saying I don’t think it’s important how one lands on a lower gas density (for instanse backing down from ppO2 1.4), but that I think it makes sense to just educate students on “why”.

My point isn’t that ppO2 should be disregarded, obviously, but gas density prioritized.

Do you disagree with that?

 

[leadduck]

two remarks:

1) oxtox risk correlates also with high ppCO2 in your blood. Rebreather divers with failed scrubbers are at risk also at ppO2 1.3bar or lower. Exertion is an issue, maybe also natural CO2 retainers and skip breathers. Hence the idea that 1.6 is OK for quiet deco but not for the working part of a dive.

2) Not all deco is on 1.6bar. When you switch to EAN50 at 70ft/21m then yes it's 1.6bar but only for a minute or so, the next stop is already 1.4bar, then 1.25bar, ... so you don't spend much time on 1.6 with one deco stage. Only the 6m stop on a second O2 stage gives you a longer period of 1.6bar, and that's with air breaks. CCR divers usually use 1.3bar setpoint during deco. So I wouldn't say that 1.6bar is safe because all decompression divers stay long time on 1.6, as they don't.