Deco PPO2

[Ainsleyberryman]

I use 1.6 for deco. However recent experience is making me reconsider this.

On very long stops over an extended week of deep diving I started to get an irritated throat. Sometimes i would cough and feel slightly light headed after the dive. I was using 80% for upto 45 mins deco all week. On the final dive I really felt my lungs suffering and experienced some sort of inability to breath properly for a few minutes. I put this down to too much rich mix, so I may alter my PPO2 for long dives.

Short deco hangs as I do here in the UK I will keep it at 1.6, perhaps only for long stuff i will shorten it.

Play with the PPO2 on your deco software and see what time difference it makes - not much for short deco obligations. But everyone is different in how they may react physically, so perhaps only you can tell.

Also remember its eaiser to maintain your buoyancy at 6m than at 3m, particulary if your tanks are fairly empty. So gas choice and PPO2 choice has many implications.

Getting out and feeling fit is far better than shaving a few mins off your deco.

Believe me, if you get out and have a pain in your joints you will feel very very stupid.

[Brian Donaghy]

Originally Posted by Juz

Personally, and I honestly feel deco is a very personal thing

Only if you're a solo diver, otherwise its a team thing.

Originally Posted by iainmsmith

If using O2, then the extra 0.2 bar would seem primarily useful for keeping inert gas in solution.

And controlling bubble size.

Originally Posted by NotDeadYet

I'm not even convinced the whole idea that the percentage concept even works, it has no experimental basis and was just thought up by IANTD when they were writing the courses.

Are those numbers not just a linear interpolation of Bill Hamilton's NOAA limits which were experimentally derived? Were they not intended to be used as a dosage?

Here's an ancient article by him for anyone interested.

Originally Posted by NotDeadYet

I just don't think it is as simple as trading off the various limits as in the %CNS idea.

Agreed.

[NotDeadYet]

The limits AFAIK were established by exposure at a set ppO2, I don't think Hamilton's research went as far as seeing if you could do x minutes at 1.4bar followed with y minutes at 1.6bar mix and match thing that IANTD came up with. The NOAA stuff didn't address the CNS clock idea, this was an IANTD thing. I remember when it was introduced and there was scepticism then.

[Brian Donaghy]

Originally Posted by NotDeadYet

The limits AFAIK were established by exposure at a set ppO2, I don't think Hamilton's research went as far as seeing if you could do x minutes at 1.4bar followed with y minutes at 1.6bar mix and match thing that IANTD came up with.

But was the intended usage not for measuring a dosage which implies that kind of approach even though the experiments were x minutes on O2 at z ATA.

I think IANTD just like pointless tables so that when the train monkeys to dive they feel they got something technical (you know, piles of numbers and stuff) out of it.

[Juz]

Originally Posted by Juz

Personally, and I honestly feel deco is a very personal thing

Originally Posted by Brian Donaghy

Only if you're a solo diver, otherwise its a team thing.

We'll have to agree to disagree!

2 reasons.

Firstly, people differ phsically, I'm 33, not very fit and very overweight, it would be foolish to think that if Chris Boardman and I followed the same schedule that we'll have the same amount of off gassing.

Secondly, people differ physiologically, 2 people of similar age, fitness and experience could do the same dive and same deco schedule and one could feel great (indicating getting out of the water clean) and one could feel a little low (indicating sub clinical issues).

For these reasons I still think Deco is a personal thing.

Juz

[Brian Donaghy]

Originally Posted by Juz

We'll have to agree to disagree!

No, you're not getting out that easily.

If you want to pursue what you consider a more conservative deco profile you must agree this with the rest of your team. It always has to be a team decision, even if everyone is diving according to the lowest common denominator, unless you're diving solo. The real rub is: what you may consider conservative for you may increase the risk for someone else.

[chrisch]

The BSAC SDP of 1.4 PO2 max is controversial and has lost them some members...

For myself, I tend to switch to 50 at 21m and sit there a couple of minues to clear some helium and make a bubble stop. Under BSAC this would be at 18m. To be honest I have not done enough of this sort of diving to say if there is likley to be a difference. My (IANTD) training argues that the switch is better made sooner, but no-one was really making a big deal of it.

The branch we used to dive with rarely made deep dives so it would not have been an issue. Our DO never mentioned PO2s and was not Nitrox qualified - I doubt the subject would have arrisen.

If your DO says no and you ignore them s/he could, in theory, kick you out or at least refuse to let you dive. So the real question is not PO2 (you know the answer to that one) but whether you care about staying in your branch and whether you have the decency to respect your DO's position even if their position is "wrong".

That, I can't answer for you!!

Chris

[nigelH]

Originally Posted by chrisch

If your DO says no and you ignore them s/he could, in theory, kick you out or at least refuse to let you dive.

*shrug*

My DOs have usually been good friends so I try to avoid breaking BSAC SDPs on club dives. High PPO2s normally are only an issue on long/deep dives which I don't do with the club anyway. I know that he wouldn't give me any flack but it's common courtesy when diving from 'his' boat....

nigelH

[divetheworld]

Originally Posted by JohnD

what PPO2 do people use, I am having an argument with my DO as he wants to stick to 1.4 and I like 1.6 on deco stops. gets me out of the water quicker and I am usually nowhere near my 80% CNS for an individual dive. comments? pros and cons, please.

I use 100% in the bags if my loading is anticipated to be less than 80%. I only start to sh1t my pants when CNS is expected to be a lot more than that.
Someone mentioned that the % of the gas is more important than the PPO for deco, I kind of agree. I would prefer full flush and use 100% at 6m, then ascend slowly to decrease the loading. At 6m the CNS loading is just over 2 (if my memory serves) and at 3m it is just over 0.5. If using 80% then you get the same loading at 6m as 100% is at 3m. It's the old gradient argument.
Oxygen is transported in the blood in two ways; dissolved in the blood (1.5%) and bound to hemoglobin (98.5%). The idea is to saturate the blood so the hemo binds to all 4 O2 molecules. Saturation increases with PO2 in a curvlinear fashion.

Now down to earth and decide how easy it is to get pure O2 at pressure in OC rigs and how restrictive it is when planning your dive (not many alternative plans available), MCCR it's a no brainer.

You might want to have another read of Anne Maries post, bit of wisdom in between those lines.

One more point to ponder, with all this talk earlier regarding the validity of CNS, I wonder how many Vestibular hits have been wrongly seen as CNS hits?

[Decodiver]

Originally Posted by JohnD

it gets me out of the water 8 minutes earlier

Hi John,

I was once on a boat with a pedantic twat, (who shall remain nameless), who bored us all to death talking about his custom dive tables, where he was shaving time off here, and time off there.

Every one else was on CCR on the boat, this one guy was on OC, and after we had all got out of the water he started giving it 'Using the table today I was out of the water 15 minutes earlier' and what one of my sharp tongued colleagues said was 'And what exactly did you do in that 15 minutes, develop a fucking cure for cancer?'.

For me the overriding issue is one of quality of decompression versus quantity.

To that end I am somewhat wary of the 'This gets me out 5 minutes quicker than the other one' school of thought.

Cheers

Dave Cooper