Current best practice for deco gas: 75% vs 100%

[kensuf]

Are you OK with the oxygen window theory and the use of the S-curve on decompression profiles?

I believe there's a value in spiking the PO2 to 1.6 for brief periods during OC deco, so yeah on the oxygen window kind-of. No on most of the rest of the deco stuff. The s-curve, etc started coming out around the time I dropped out of the WKPP and then ratio deco got seriously bastardized by AG.

 

[cerich]

and then ratio deco got seriously bastardized by AG.

that is British level of understatement

 

[boulderjohn]

I believe there's a value in spiking the PO2 to 1.6 for brief periods during OC deco, so yeah on the oxygen window kind-of. No on most of the rest of the deco stuff. The s-curve, etc started coming out around the time I dropped out of the WKPP and then ratio deco got seriously bastardized by AG.

Do you have any research to support the O2 spike idea? The article on which it was based essentially said that it created an "oxygen vacancy" that left more room for the nitrogen to come out. Mark Powell said in Deco for Divers that this theory violates Dalton's Law, and everyone I know agrees with that. As I have noted in the past, that theory is expressed in one paragraph in the original article, and I was unable to find any supporting evidence for that conclusion in any of the data in the article.

In a personal echange I had with Jarrod Jablonski about 7 years ago (and I still have it), he said he knew that the idea was suspect and GUE would no longer support it. They were still doing the S-curve that was based on it then, but they have since dropped it.

 

[tbone1004]

@rjack321 thanks, guess that interviewer was mistaken

 

[JohnnyC]

I've always treated the spiking of O2 to 1.6 to be a consequence of getting onto the gas as quickly as practical, not out of a desire to run my PO2 that high. If there is a benefit other than that, great, if not, I'm happy that it still gets me onto an inert-gas free gas ASAP.

 

[rjack321]

I've always treated the spiking of O2 to 1.6 to be a consequence of getting onto the gas as quickly as practical, not out of a desire to run my PO2 that high. If there is a benefit other than that, great, if not, I'm happy that it still gets me onto an inert-gas free gas ASAP.

I always treated the lingering at 1.6 concept as more of a "give the switch some time to actually work"
In the ocean with a modest BT switching to EAN50 you might only have a 1 or 2 min stop (which is highly dependent on GFs and/or settings in VPM) but I would stay for 3 to 5 mins then follow the schedule after that. Back when I was diving OC I never had a "big" required EAN50 70ft stop (10+mins). If I did, I wouldn't be lengthening that because 10+mins is already plenty of time for blood to circulate a few rounds.
Moving all the time around into an S ala UTD of 2013 era because AG knows better than buhlmann is not something I would support. GUE stopped that quite a few years ago and supposedly UTD has stopped this practice as well.

 

[boulderjohn]

supposedly UTD has stopped this practice as well.

If so, only recently. I believe it was still part of their version of RD during the recently concluded Spisni study. They revised RD after those bad results for RD, and I don't know about those changes.

When I was with UTD, I argued vigorously against it. I pointed out that the oxygen vacancy statement in the study on which it was based came out of nowhere, with no supporting evidence in the study itself. I talked about Dalton's Law. Then, just before I left the organization, I participated in a RD class conducted by AG himself. In it, he admitted that the oxygen vacancy theory on which the S-curve was based was possibly incorrect, but he came up with two other reasons to keep the S-curve. I don't remember either one because I thought they were both bogus. I was out of the organization a few weeks later, so I don't know how things followed up from there.

 

[rjack321]

UTD quit doing S curves around 2 or 3 years ago (2016ish)

 

[EireDiver606]

Hi I’m confused by this as I’ve yet to a technical course. I have a few questions if you don’t mind answering:

What is the difference in efficiency between O2 and EAN 50 for deco?

Why do not many people like 80%?

Is there a massive difference between 80% and 75%, why have people used 80% but 75%?

Thanks

 

[rjack321]

What is the difference in efficiency between O2 and EAN 50 for deco?

Depends on the deco you have to actually do.
A 20 min deco obligation coming up from 150ft? Easy to do on 50% and keeps you from having to do stops on backgas at 60,50,40,30
A 40min deco obligation after a long dive at 90-100ft? on 32%? Best to bring O2 for that as the offgassing gradients are way better (68% nitrogen to 50% nitrogen is not a big gradient) and rock bottom isn't as much of an issue.

Why do not many people like 80%?

Because it has nitrogen in it, exactly what you are trying to get rid of. Having N2 in your deco gas is counterproductive.

Carrying EAN50 is useful (despite the 50% N2) because you can often switch right about the depth of your first required stop. So its a mediocre deco gas, but has the co-existing benefits of being able to start using it sooner and of minimizing your rock bottom. 80% doesn't reduce the rock bottom nearly as much because you can't use it until 30ft

Is there a massive difference between 80% and 75%, why have people used 80% but 75%?

Nobody uses 75% for anything. It was an erroneous question.​