Accelerated deco: low PO2 vs high PO2?

[Rage]

During a recent conversation with a tech instructor, he said that he believed that it was safer to deco on a lower FO2 mix e.g 70% than 100% (at same depths, IE 6m).

By safer he meant, a lower PO2 and, in his experience, less incidents of DCS.

From a DCS point of view, is there any data to suggest that;

accellerated deco on a lower PO2 with a longer hang time, is safer than a shorter hang time on a higher PO2 (at the same depths)?

Thanks,
S.C.

 

[O-ring]

IMHO, that makes zero sense from a DCS standpoint since lowering the FO2 in this case is increasing the percentage of inert gas present in the deco mix.

 

[DepartureDiver]

You have to define safer. Sure you're at less ppO2, but you also increase your underwater time (if you switch at 20') which could prevent you from getting out if there was some sort of an emergency. Also, if you want to discuss half-time compartments, "slower" compartments could still be ongassing (but this would be negligible). But 70% would also allow you to get on it at 40' (ppO2 1.55) or 30' (ppO2 1.34) and not wait to 20'. The issue then becomes which is more efficient for your profile. But if you wanted to stay "DIR", then it would be 100%.

 

[100days-a-year]

Pub-Med findings on the NIH website indicate greater damage is done to the body by acidosis and necrosis caused by extended elevated p02s.There are studies now going on where a hi percentage of commercial divers who were using elevated po2s have Permanent bone damage,it's clear that either choice has drawbacks.DCS is walk in the park compared to necrotic bone damage.

 

[Saturation]

Continuous high pp02 exposure is toxic, that's what the OTU is all about and why one needs to be cautious about oxtox.

Deco is another issue, 100% is better than less only because it maximizes the 02 window at the 20' stop. However, using a lower mix 02% but starting the deco deeper provides a similar benefit but not equivalent because at some point, you'll also be ongassing the inert gas fraction used for deco. Using 70 or 80% can give one a buffer against rough seas doing the 20' stop but its not that critical.

There is data that shows heliox is more efficient in deco than just nitrox alone, because of the addition of countercurrent enhanced N2 elimination, just as Ellyatt did in his dive.

 

[MechDiver]

accellerated deco on a lower PO2 with a longer hang time, is safer than a shorter hang time on a higher PO2 (at the same depths)?

Thanks,
S.C.

Agree totally with O-ring. Also, how can you have "accelerated deco" without a high PO2?

The hang time is shorter due to the higher O2 content, not just because you decide to do it for less time.

Methinks your "tech" instructor needs more training.

MD

 

[DA Aquamaster]

I don't have the source in front of me (I am on the road this week) but I have seen a interesting comparison of the deco requirements for dives in the neighborhood of 150 feet for 40 minutes using various Nitrox mixes and also pure oxygen for a deco gas.

If you are using a single gas for deco dives in this range, Nitrox 80 may be about optimum as you get the benefit of starting the deco gas deeper and can actually get out of the water slightly faster with a lot smaller hit on the CNS clock.

Nitrox 70 and 50 also compared very well with deco times only slightly longer and again with a lot less of a CNS hit than 100% O2.

 

[O-ring]

I won't argue that 70% is a lower ppo2 than 100% and we all know (or can argue) the merits and drawbacks of lengthy exposures to high pp02s. The part of the post I was taking exception to was the DCS mentioned in the first post, which doesn't make sense given the increased n2 in a 70% mix. I am willing to buy that 70% in this example is safer because the original poster operationally defined safer as "lower po2".

 

[Rage]

Thanks for the replies so far. I've heard a few interesting things I'll look into. I am particularly interested in theories such as what Saturation mentioned regarding Heliox, though 100days comments led to some interesting reading too.

To rephrase the question: Is there any evidence to substantiate the theory that decoing on lower a PO2 (e.g 1.12) for a longer period, is safer (in terms of less likelyhood of suffering DCS), than decoing on a PO2 of 1.6 for a shorter time?

Yes, I know it doesn't make much sense if you apply what is generally taught. I am not saying that the above is accurate - neither is the instuctor, his observations may be attributable to other factors, but last I looked - decompression is still an inexact science, therefore, without trying to start an egg sucking contest, I am looking for data / information suggesting the *possibilility* of this being true. IE similar to the Heliox thing. BTW, got any links for this?

Finally, the instructor is a well respected individual in terms of his teaching ability and diving accomplishments over decades? He doesn't deserve to be slated for my shortened, and possibly inaccurate, second hand account of a conversation of his beliefs and observations.

Thanks for reading,
S.C.

 

[SDAnderson]

To rephrase the question: Is there any evidence to substantiate the theory that decoing on lower a PO2 (e.g 1.12) for a longer period, is safer (in terms of less likelyhood of suffering DCS), than decoing on a PO2 of 1.6 for a shorter time?

Not that I'm aware of any evidence that indicates decompressing on lower ppO2 mixes reduces the likelihood of DCS. If the instructor has some information, perhaps you can get him to share so that we can all learn.

There are lots of hazards to high O2 mixes (blindness, bone necrosis, lung damage, O2 tox, etc., etc.) and more are being investigated but that's not your question.

Steven