What ppO2 do you use and why?

[Mr Carcharodon]

On a recent live aboard to the Cocos I found that everyone excepting myself was using a ppO2 of 1.6 ATA as a hard limit. Most of the divers onboard were from Europe or the Near East. In practice there was little prolonged diving below a ppO2 of 1.4 ATA. My training on this was definitely in the camp of staying below 1.4 ATA always, and with GUE staying below 1.2 ATA during the working portion of dives.

What do you use as your limit? Does it make a difference to you if your peak ppO2 is significantly above your average. Do you treat ppO2 as a hard limit, or as a dose limit where both pressure and time come into play? Is using 1.4 ATA as a hard limit alarmist?

 

[ajduplessis]

1.4 in warm water with low workload. 1.2 in warm water with high workloads. 1.2 in cold water with low workloads and 1.6 for all deco.

PO2 limit is a guide and each agency have their own "hard" limits. Pushing PO2 is looking for trouble but like anything else in life, make you own risk assessment and live with the results.

IMO 1.4-1.2 is pretty safe.

 

[flylow47]

Good observation about the other divers.

They're probably not the ones I would want to buddy with.

This link is a good reference.

http://www.scubaboard.com/forums/archive/index.php/t-440726.html

 

[SailNaked]

You are taught safety, so be safe, or choose to walk the line and see what happens, watch some video of what happens to navy flyers with about 3.0pp, they start to lose motor function, and that would be bad underwater, 1.4 or 1.6 are what the navy considers limits for working as you said, well as rec divers we never work. so you can likely exceed these, no one will suggest that because if you have a problem they do not want to be responsible, and I am not suggesting it either, only stating the obvious. the navy also does deco at 2.0 that is resting and they always have a team. I am fairly certain you will not instantly combust if you go to 1.8, but you might start having some motor function issues or forget your training. it effects people differently like narcosis you might be the one that cant handle 1.6 or you might be able to function fine at 2.5


Edit: for the record " you will die if you exceed 1.4!"

 

[Wookie]

2.0 on deco
1.8 during the dive
1.6 if I'm working (hydraulic drilling, etc.)

I only dive warm water, so cold water limits would be moot.

Who teaches 1.4? I'm a PADI and SDI nitrox instrustor, and I teach 1.6.

 

[DivemasterDennis]

Safety is the key, and in recreational diving there is no reason to flirt with "borderline" limits. It just isn't necessary. So I am a 1.4 guy. I agree with ajduplessis.
DivemasterDennis

 

[LowDrag]

I understand that PPO2 means Partial Pressure of O2 but what I am having a hard time grasping is the relationship between PPO2 and 1.4 ATA and why do we set our computers at 1.4 ATA? This one is stumping me and I would be willing to bet there are others as well.

 

[SeaHawk12]

1.6 for all of my recreational diving, and this is because you never even get close to the CNS or OTU limits in that type of diving. Of course, you do need to plan to the situation. If you're doing 5 dives a day on a live aboard for over a week, then dropping down to 1.4 would be a good idea.

For tech I use 1.4 for the bottom portion of the dive and 1.6 for deco. This is due to the fact that you DO start to approach the limits in this type of diving, so safety first!

 

[SailNaked]

I understand that PPO2 means Partial Pressure of O2 but what I am having a hard time grasping is the relationship between PPO2 and 1.4 ATA and why do we set our computers at 1.4 ATA? This one is stumping me and I would be willing to bet there are others as well.

1.4 is the PPO2, not ATA.

 

[SeaHawk12]

I understand that PPO2 means Partial Pressure of O2 but what I am having a hard time grasping is the relationship between PPO2 and 1.4 ATA and why do we set our computers at 1.4 ATA? This one is stumping me and I would be willing to bet there are others as well.

1.4 is the PPO2, not ATA.

Actually, PPO2 is measured in Bar or Ata, so that is correct.

I understand that PPO2 means Partial Pressure of O2 but what I am having a hard time grasping is the relationship between PPO2 and 1.4 ATA and why do we set our computers at 1.4 ATA? This one is stumping me and I would be willing to bet there are others as well.

This all comes from Dalton's law,which basically states that the total pressure of a gas is equal to the pressures exerted by its individual components. So, at the surface, the total pressure of ambient breathing air is 1 bar/ata (they are essentially equivalent for our purposes). Since 21% of that gas (air) is composed of Oxygen, 21% of 1bar is the partial pressure of O2 at the surface, or .21(1)= .21bar.

Now lets dive down to 10m. At this depth we are experiencing absolute pressure of 2bar, of which 21% is still excreted by O2. This gives us .21(2)= .42 bar.

Now the importance of this comes from the fact that the human body runs on partial pressure of gasses, NOT percentages. Now oxygen is a very reactive substance, and actually screws up quite a few biological processes when in excess. At PpO2 >.5 some of these effects start to show up and we begin to have problems with CNS toxicity and Pulmonary Oxygen toxicity. 1.4bar ppo2 has been found to be a relatively safe operating partial pressure mostly via trial and error (like most things in diving).