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Hi everyone, So helium decompression, (as I understood) was that since Helium saturates and desaturates faster, Trimix divers will need an earlier stop (deep-stop) if you may use that term. This stop would happen at a depth where your tissues will still be deep enough to take in nitrogen and you will have to compensate for this stop by extending that shallower portion of the dive, aka helium penalty.
If I am to understand more modern opinion on this then that stop is no longer necessary as for some reason unknown to decompression science, most tissues desaturate with helium the same way as they desaturate nitrogen? In other words a deep air diver surfacing from a 180 ft dive can follow the same decompression schedule as a Trimix diver coming up from the same depth and they would be in exactly the same risk bracket?
If someone could elaborate more on this subject, Id be grateful.
Deep stops have gone out of favour now, lots of debate ensued.
As you said, Helium leaves the system faster than Nitrogen (2.5 ish times faster). So nothing special as you're ascending (assuming not crazy depths - 180 feet = 60m isn't that deep). On OC, you'll switch from a rich trimix (18/45?) to a rich Nitrox (50%) around 21m and again to a richer gas (80%?) at 9m. If you're doing much deeper dives on Open Circuit, you need to be aware of ICD (Isobaric Counter Diffusion) at deeper gas switches -- where helium's rushing out and nitrogen's rushing in. This will be an issue if switching from a very rich helium mix to a weaker one at depth.
Deep diving on open circuit's very expensive with increasing helium costs, so most deep diving's now done with rebreathers which don't have radical gas changes, just the gradual increase in O2 partial pressures.
The planning works. Most people now use Buhllman with gradient factors. Some debate over which GFs people use, but 50:80 is quite common, bringing you up to a reasonable first stop and not dragging out the last. Recommend you play with some planning software such as MultiDeco (available on phones, tablets and desktops). Other deco programs are available.
The OP, point re deep air diver ascending at the same profile as a trimix diver is being expressed the wrong way round. If your saying can we ignore the helium penalty then you need to express this as the trimix diver ascending with the same profile as the deep air diver. The air profile will be shorter due to no helium penalty.
I believe it was recently explained by prof. Simon Mitchell , whose research I recommend you look up , who said with reference to this issue , we are doing the correct amount of decompression but for the wrong reason. So I would suggest to ignore the helium penalty written into current deco algorithms could be foolish.
We wanted to preface this article with a statement from Dr. Simon Mitchell about the message of this post. We feel it summarizes the important takeaways from this article and captures where Shearwater stands on this issue:
“The obvious question is whether this means that we can all tell our computers that we are diving air diluent when we are really diving trimix. The study does not answer that question unfortunately. Although it suggests that there is no evidence for treating helium and nitrogen differently (as reported by Doolette in animals) it is conceivable that the long held belief that helium needs more decompression has compensated for underestimation of required decompression (to achieve acceptable levels of risk) by the decompression algorithms we all use, and that we are doing the right amount of deco but probably for the wrong reason."
- Dr. Simon Mitchell
I took this to mean that as far as available science goes, air and trimix should be treated the same way. Since Trimix has a more conservative dive profile, instead of taking this to mean that trimix divers should mimic air decompression, it would make more sense for air divers to mimic trimix decompression.
Seems like you’ve read the research what exactly are you looking for here ?
Who’s sock puppet are you? 🤪
Originally Posted by Sinbad the Diver
Sent from my iPhone using Tapatalk Pro
I was not confident in my own understanding of the concept so I wanted to have it double checked and discussed. That is all. Thanks so much everyone.
In MultiDeco - the desktop version, you can put this 'helium penalty' theory to the test. There is a control screen where you can set the "new" rates and half times and co-efficient's, for both ZHL or VPM-B. All the expert ideas of late are preset under various buttons. Here you can also create your own brand new dissolved model, or modify the existing ones.
Go ahead, try your favorite dive plan - see how much difference helium penalty theory really makes.
On MultiDeco main screen, press the Tau(T) button. Enjoy.
Hindsight is a wonderful thing
Back in 2000's we were using VR3s for trimix decompression dives and those who felt they were "in the know" about the Helium penalty myth were shaving helium from the VR3 to speed up deco. So id do a dive on say 13/65 and tell the VR3 I was on 13/40. End result shorter deco which we kidded ourselves was "efficient" and based on our deeper understanding of the overcompensation errors in Trimix planning software
This was of course rubbish
The truth was the VR3 far from being the Bendy box people alleged, was in fact a pretty conservative computer. Something we started to realize after the first SHeerwater came out and we were leaping out of the water on 30/90 profiles MUCH faster than the boys still on the old VR3s
Frist bad bend using shaved Helium numbers I remember was Dr Jerome Meer a very deep cave diver who end up with a big bend after telling his VR3 pork pies.
There's a simple rule in deco diving if environmental concerns are not an issue, the more deco you do the safer you will be. At least in terms of DCI.
After decades of looking for the ultimate deco (IE quickest out of the water with acceptable levels of risk) It comes down to this
Reducing deco on dives of 2 hours or under is a fool's game
If you want more bottom time less deco, you must accept the risks of aggressive deco
IMHO there's a time (run time) and a place (depth / bottom time) for aggressive deco where the risks of being in the water too long far outweigh the risk of sub clinical damage or possible known DCI risk
Its like doing 100MPH on a moterway. OK, you got away with it his time but what about next time.
Denying the risks and coming out with crap like your journey was more efficient due to the reduced journey time and safer due to your reduced exposure to the risks of being on the motorway, are more obviously seen as bullshit when used in the context of speeding. So when divers tell you about faster deco being more efficient and safer, treat with the same very large pinch of salt.
Last edited by Mark Chase; 22-11-2020 at 10:03 AM.