What ppO2 do you use and why?

[dfx]

Why's there two identical threads going?

http://www.scubaboard.com/forums/advanced-scuba-discussions/461959-what-ppo2-do-you-use-why.html
http://www.scubaboard.com/forums/basic-scuba-discussions/462066-what-ppo2-do-you-use-why.html

 

[Mr Carcharodon]

It seems that most everyone selected a limit based on the advice of an authority (myself included) and most often without a clear understanding of how the limits were arrived at. MudDiver has been the exception in that he mentioned statistical data presented by ANDI.

It would be interesting to see if there is a statistical difference in oxtox rates between European and North American divers. In my very small sample I found all the Europeans working to a MOD that corresponded to a ppO2 of 1.6 ATA, while the North American norm seems to be 1.4 ATA or lower.

I wonder if the limits are largely the result of training methods. It is easy to train to not go below your MOD ever. Training to not exceed your CNS clock and allow for short term spikes in ppO2 is more complicated and begs the question how to plan for it. It may be we do what we do because it fits within a bite sized class rather than having any more substantial reason.

 

[uncfnp]

I run 1.0 for the dive until I get to my 20 foot stop. Then I will crank it up to 1.6. I have been bent twice and had issues with skin bends on both OC & CCR. By dropping my PO2, especially on dives over 3 hours, the skin bend issues went away. Even on a short 2 hour dive at Ginnie (100 foot max) I would sometimes get skin bends with the higher PO2's. For each hour at depth it only adds a couple of minutes deco obligation & I feel much better after the dive with the lower PO2.

Bobby. I'm obviously missing something with your post. Wouldn't a tendency to bend make you want a higher ppo2 to decrease the percent nitrogen for less nitrogen loading?

 

[Keith.M]

I run 1.0 for the dive until I get to my 20 foot stop. Then I will crank it up to 1.6. I have been bent twice and had issues with skin bends on both OC & CCR. By dropping my PO2, especially on dives over 3 hours, the skin bend issues went away. Even on a short 2 hour dive at Ginnie (100 foot max) I would sometimes get skin bends with the higher PO2's. For each hour at depth it only adds a couple of minutes deco obligation & I feel much better after the dive with the lower PO2.

OK, I am confused here and maybe it is because of your reference to a CCR and myself having no exposure to one.

It is my understanding that your PPO2 is determined by your oxygen % (FO2) and your depth.

Figuring out your MOD is a simple equation of:

33 * ((PPO2/FO2) - 1)

so on EAN 32 using PPO2 of 1.4, your max depth before exceeding 1.4 would be 111' (111.375) Using a PPO2 of 1.6 would allow you to get to 132' before exceeding the PPO2 of 1.6

So if that is correct, how are you spending your dive at 1.0 until you get to 20' and then you "crank it up" to 1.6?

Is that a function of the CCR?

 

[ajduplessis]

We have some very "clever" instructors here.

 

[gcarter]

OK, I am confused here and maybe it is because of your reference to a CCR and myself having no exposure to one.

It is my understanding that your PPO2 is determined by your oxygen % (FO2) and your depth.

Figuring out your MOD is a simple equation of:

33 * ((PPO2/FO2) - 1)

so on EAN 32 using PPO2 of 1.4, your max depth before exceeding 1.4 would be 111' (111.375) Using a PPO2 of 1.6 would allow you to get to 132' before exceeding the PPO2 of 1.6

So if that is correct, how are you spending your dive at 1.0 until you get to 20' and then you "crank it up" to 1.6?

Is that a function of the CCR?

Yes. CCR divers control and adjust their oxygen to maintain a desired PPO2 setting - as I understand it.

 

[Bobby]

Bobby. I'm obviously missing something with your post. Wouldn't a tendency to bend make you want a higher ppo2 to decrease the percent nitrogen for less nitrogen loading?

That was the line of thinking that I had followed for a long time as well. However after talking with a good friend of mine, Dick, educated me regarding oxygen and nitrogen loading. From my simple understanding what I got was that the lungs become less and less efficient on longer exposures of high PO2. When the PO2 exposure is held down then the lungs can more easily manage off gassing of excess nitrogen. Holding the PO2 down on the working part of the dive allows higher PO2 on the deco portion and more efficient off gassing.

When I incorporated this into my planning I found that it worked and have not had skin bend issues since.

 

[Phil_C]

Altering the O2 percentage in the breathing loop when using a CCR affects PPO2 in the same way as diving with NITROX because PPO2 relates to percent O2 in your breathing gas adjusted for depth and is increased as you descend to depth, so if you dive NITROX with a higher percent O2 than 21% then you reach higher PPO2s at a lower depth. Increase the O2 % and you decrease the maximum depth before reaching PPO2 limits. Likewise switch to a mix with less O2 you can go deeper before you reach the recommended safe PPO2 limits, BUT if you don't want to avoid the increased risk nitrogen narcosis you have to introduce a third 'biologically inert' gas and you end up with TRIMIX- P

 

[Bobby]

OK, I am confused here and maybe it is because of your reference to a CCR and myself having no exposure to one.

It is my understanding that your PPO2 is determined by your oxygen % (FO2) and your depth.

Figuring out your MOD is a simple equation of:

33 * ((PPO2/FO2) - 1)

so on EAN 32 using PPO2 of 1.4, your max depth before exceeding 1.4 would be 111' (111.375) Using a PPO2 of 1.6 would allow you to get to 132' before exceeding the PPO2 of 1.6

So if that is correct, how are you spending your dive at 1.0 until you get to 20' and then you "crank it up" to 1.6?

Is that a function of the CCR?

Keith,
On CCR we change our FO2 to match our desired P02 for the dive. A CCR allows us to mix the gas on the fly to maintain a constant PO2. Of course this adds complexity and requires more training and experience. With OC diving I also calculate my PO2's to "average" 1.0 PO2. The reason that I specify average is that I dive more caves right now than I do wrecks. This translates to different profiles that work better with average depth compared to max depth. I still hold max PO2 to 1.4 when I am running averaging.

 

[uncfnp]

That was the line of thinking that I had followed for a long time as well. However after talking with a good friend of mine, Dick, educated me regarding oxygen and nitrogen loading. From my simple understanding what I got was that the lungs become less and less efficient on longer exposures of high PO2. When the PO2 exposure is held down then the lungs can more easily manage off gassing of excess nitrogen. Holding the PO2 down on the working part of the dive allows higher PO2 on the deco portion and more efficient off gassing.

When I incorporated this into my planning I found that it worked and have not had skin bend issues since.

I think I follow your logic. With your prolong dive times compared to oc recreational (me), you are looking at the pulmonary toxicity effects as opposed to CNS and theorizing that this can interfere in off gassing nitrogen, hence, increasing your risk of bends. Interesting.