Eliminating the Helium Penalty

[uwxplorer]

A new post on Shearwater's website, signed by Michael Menduno, discusses recent conclusions of NEDU (2015) that Heliox should be the gas of choice for tech divers and that deco algorithms should remove He penalties.
From the article: "Their findings: A bounce diver’s decompression requirements depend solely upon the time, depth and level of oxygen (PO2) over the course of the dive regardless of the fraction of helium and or nitrogen used in the breathing mix."
The article concludes: "Of course, the more pressing issue is, “What algorithm will tekkies use to make their heliox decompression calculations?” The Navy’s current Mk-16 MOD 1 tables, which are published in the U.S. Navy diving manual, are one possibility, but as noted above the current version contains a helium penalty. So tekkies will have to wait for NEDU or others like Duke University to publish a corrected algorithm, and then wait for dive computer manufacturers to implement them.
Unfortunately, that won’t happen overnight. “Because of funding cycles, it will likely take two to three years, for this work to hit the public domain,” Doolette explained."

 

[Tim1]

So we can all enter EAN 10 into our computers and duck the helium penalty?

 

[nickb]

So we can all enter EAN 10 into our computers and duck the helium penalty?

That's what a lot of people have been doing for years

 

[Michael Thornton]

When I saw David Doolete speak about this at TEKDiveUSA.2014 he was saying current algorithms are working because for the most part they are keeping people from getting bent. They may be working for the wrong reason. Just because we don't need extended Deco for a HE penalty does not mean we don't need it for other reasons. It is hard to compare someone diving at trimix depths using helium vs someone diving at trimix depths on nitrox because there are not a lot of trimix level dives being done on nitrox.
When he explained that a helium penalty does exist in saturation dives that in bounce dives like we do it is nearly non existent. I asked him if he still considered a 12hour long tech dive a "bounce dive" and he said yes.

 

[uwxplorer]

When I saw David Doolete speak about this at TEKDiveUSA.2014 he was saying current algorithms are working because for the most part they are keeping people from getting bent. They may be working for the wrong reason. Just because we don't need extended Deco for a HE penalty does not mean we don't need it for other reasons.

Which is why I found strange to see this posted on Shearwater's website without any context or comments.
For a 150 ft dive with 30 min BT (the kind of dive I do), air or 21/35 gives about the same deco time (Buhlman GF 30/80) but heliox would require 25 min more. I guess what this is saying is that heliox should not result in such a large delta compared to air (in this case) - in fact no delta at all if I understand correctly.
I wonder what kind of deep/tech-like dive the Navy might be interested in testing that would eventually help optimize table and algorithms. 2 to 3 years sounds like a short period of time to update algorithms covering the vast range of tech dives currently taking place.

 

[Tim1]

So yes, people have been 'fibbing' to their computers about their mixes. However anecotal evidence suggests that that a very conservative GF may be necessary. Whilst I am generally happy to run with a 50/80 for many dive sets on a (eg) 10/70 mix, this may not be useful when running with a 10/90...

 

[Jeff Pack]

as it was explained to me, you can change your comp from 14/50 > 14/25, thus removing some of the He penalty

 

[uwxplorer]

Reading the NEDU report (linked to in the article posted on SW's website), it turns out that the conclusion is: there is no advantage in using trimix over heliox for Navy rebreather dives. However trimix schedules were used, which used the linear-exponential model apparently favored by the Navy. In other words, it would be a leap of faith to use this result to:
- draw the same conclusion for other profiles;
- conclude that one can use a low He trimix profile with a higher He mix;
These extrapolations seem to go in the sense of the report's conclusion, but there is no way of quantifying how close or far they could be valid.
As an illustration, here is the profile used in the study:

Depth Time
200 40
70 4
60 2
50 2
40 6
30 16
20 89

The ascent rate is 30 ft/mn, so there is an extra ~4 min to add to get to 70 ft. The mix used were either 12/88 or 12/44/44.

This is much shorter than what Buhlman GF 30/80 would require, and unsurprisingly, they got a number of DCS cases in the trimix dives (not the heliox ones). This, by the way, was by design.
Buhlman would require more than 45 additional min for the heliox dive.
By comparison, the Navy heliox schedule is (was) only 15 min longer than the trimix one they used in this study.

Unless you want to be your own guinea pig, I would not experiment with heliox using Buhlman trimix schedules... or reduce arbitrarily the He content for planning. I'd be curious to know what the rule 15/50 -> 15/25 is based on.

 

[rjack]

You are thinking about this backwards. The Navy is already using heliox and are not going to be switching to trimix (based on this study) because its not efficacious to do so.

 

[uwxplorer]

You are thinking about this backwards. The Navy is already using heliox and are not going to be switching to trimix (based on this study) because its not efficacious to do so.

I understand that.
I am just saying that this single data point (telling us that heliox is safer than trimix for a single given profile - optimized for trimix) is a tenuous basis for tech divers to extrapolate to arbitrary trimix profiles (which is what Menduno's article concludes as well and what Doolette also says).
But apparently people have been experimenting following the following line of thought:
heliox (which contains less N2 than trimix) is safer than trimix when diving a trimix profile => a profile computed for a larger N2 content is somewhat too conservative when diving with a gas with less N2 => trimix XO2/YHe2 contains less N2 than trimix XO2/0.5xYHe2 so diving a profile calculated for XO2/0.5xYHe2 should be conservative enough.
I am not arguing with whoever does that, but I suppose there has been some experimentation with this logic and if people survived it, I am just curious to hear about that. The thing is I have been bent on an air profile using 21/35 so I tend to err on the side of caution.

(I note that in the NEDU experiment, the He content of heliox was twice that of the trimix chosen, but I don't recall whether the choice of trimix was a standard Navy choice or due to some other reason).

 

[Jneves]

There is something I don’t get here.
The name of the game for us civilians is usually DON’T GET BENT. But we know that accidents do happen, no matter how careful one may be. There are so many unknowns, so many risk factors involved that one can be sure of only one thing: there is no guaranty that you will be able to complete your deco successfully (i.e. no bend issues).
However there is another very important consideration. If you do get bent, how bad do you want it to be?
I believe no one will dispute that US NAVY protocols call for onsite recompression chamber onboard the vessel or diving platform. Their name of the game is more like INCOMING FIRE. GET OUT OF THE WATER FAST. If a deco problem arises get in the pot, or stay there longer, with the vessel already underway to a safer place. Simple.
I also believe no one disputes that bends with He content in the mix tend to be more neurological hits and bends with no He content mix tend to be more joint/skin/muscle/bone hits.
As civilians we can’t afford the luxury of a recompression chamber onsite. So we have to weight the possible outcomes and make a balanced decision. In case something goes wrong and I blow my deco, do I want a classical bend or a neurological hit? If it is a classical bends I may be recompressed and may recover totally. If I get paralysed from the waist down I may not recover fully and be confined to a wheelchair for the rest of my life.
Do I go for a Heliox mix or a trimix? The higher the He content in the mix the bigger the probability that a hit will be neurological…
Further more, the longer I stay underwater the higher the risk something goes south with deco, due to no related deco procedures but rather equipment, gas usage, sea conditions, animal life, etc., factors that are not always under my control. Shallow deco stops are the longer portion of the deco, so in therms of time spent underwater these are the statistically most critical for problems to occur. Taking this into account, should there be a diluent switch to nom helium based mix as early as reasonable in the deco protocol so to off-gas He before reaching the longest deco stops and avoid a neurological hit in case we have to miss part of the shallow deco obligation?
I think it is really a question of weighing the pro and cons.
Navy protocols will always be different from what the civilian protocols should be. After all, military will always accept a theoretical casualty number in any operation. That is the nature of warfare.

Just my 2 cents.

JNeves

 

[Randy Thornton]

Taking this into account, should there be a diluent switch to nom helium based mix as early as reasonable in the deco protocol so to off-gas He before reaching the longest deco stops and avoid a neurological hit in case we have to miss part of the shallow deco obligation?
JNeves

Hi Jneves,

I would be interested to read any supporting evidence for this theory. I have never seen anything which validates this approach. In fact, just the opposite. Most CCR divers I know do not make diluent gas switches, but remain on the same dil from bottom to surface.

Kind regards,
Randy

 

[PaulD]

Dil switches used to be fairly common with some of the deep RB divers here in Oz, it fell out of favour due to the added risks of mixing up gas swaps. People were even using switching blocks for their dil.

 

[Jneves]

Hi Jneves,

I would be interested to read any supporting evidence for this theory. I have never seen anything which validates this approach. In fact, just the opposite. Most CCR divers I know do not make diluent gas switches, but remain on the same dil from bottom to surface.

Kind regards,
Randy

Hi Randy,

It is not a question of published evidence in favor of gas switching or against, it is a question of common sense. Do you challenge any of the points I made? Is there a flaw in the rationing? If there is would like to ear it.
For many years, back when in OC and now with CC as well, people do use a lot of gas switching without any deco issues, including myself and many others. Then came the theory that claimed the enormous risk of bends by isobaric counter-diffusion just to sell a computer algorithm, when there was no, and still there isn’t, any true evidence of such being an issue in most bounce dives in a non-commercial diving context.
People just did what was most convenient for them, not having to carry an extra tank for diluent switching and deal with the increased task load of actually do the switch . It was much more practical to keep He all the way to the surface.
I am not saying either that deco will be worse quality if you keep on He, although it is debatable, if you DO GET TO DO your deco entirely. I am just saying that the grave consequences of missing a deco obligation while still on He fare outweigh the supposedly less efficient shallow deco stops on non He mix.
But, hey, if people just feel they are doing the informed right thing then let them be. I just wanted to raise some sanity thoughts and make sure they are aware that there is more to it than meets the eye.
As fare as I am concerned I will keep doing those switches. I might get bent one day, but at least I will have the peace of mind that I did everything in my power to avoid a neurological issue. I want to be able to urinate (and do other stuff) using my own organ, not from a rubber tube sticking out of my belly.

Safe diving

JNeves

 

[diverklondike]

Hopefully there will be the normal mix of great speakers that can speak to some of the latest research on these topics at TekUSA18.... There was definitely a foreshadowing to this by several of the speakers around this last year....

 

[rjack]

I also believe no one disputes that bends with He content in the mix tend to be more neurological hits and bends with no He content mix tend to be more joint/skin/muscle/bone hits.

I have not seen any actual data to support this assumption.

It is not a question of published evidence in favor of gas switching or against, it is a question of common sense. Do you challenge any of the points I made? Is there a flaw in the rationing? If there is would like to ear it.

The assumption is that people are getting bent from the helium fraction and that is not necessarily true. There's at least one older study in Rubicon that analyzed the content of bubbles in explosively decompressed rabbits - it was disproportionately N2 not helium as you might expect.

Doing dil switches to try and accelerate the offgassing of helium seemed to be popular about 8-10 years ago. Its fallen out of favor lately as excessively complicated - plus there's no data to support that it's doing what you think its doing.

 

[Jneves]

I have not seen any actual data to support this assumption.

Look better! Ho, come on, are going to say also there is no climate change due to human activity? Who did you vote for?

assumption is that people are getting bent from the helium fraction and that is not necessarily true. There's at least one older study in Rubicon that analyzed the content of bubbles in explosively decompressed rabbits - it was disproportionately N2 not helium as you might expect.

I am truly astonished with what you are saying... So if I don't have N2 in the mix I will never get bent? Why didn't you guys say it earlier? How stupid of me worrying all these years when the solution was so easy!!!! The hell with deco! Unlimited diving from now on, no more freezing my ass on long deco stops! Hurra the rabbits!!!!!

dil switches to try and accelerate the offgassing of helium seemed to be popular about 8-10 years ago. Its fallen out of favor lately as excessively complicated - plus there's no data to support that it's doing what you think its doing.

Have you ever ear about off gassing gradients?
Gee, one reply latter and I had forgotten all about the rabbits! Sorry folks!
Lets just forget all that came in the past, none of it is true! No knowledge from all the studies made by COMEX are true, what the hyperbaric medicine doctors see and say is untrue, the accidents with people that I know never happened and they are not paraplegic. Long live the rabbits! (hoops did they survive? - I forgot to ask!)

I give up! No point in continuing. See you when I see you!

Stay safe

JNeves

 

[sunnyboy]

Look better! Ho, come on, are going to say also there is no climate change due to human activity? Who did you vote for?



I am truly astonished with what you are saying... So if I don't have N2 in the mix I will never get bent? Why didn't you guys say it earlier? How stupid of me worrying all these years when the solution was so easy!!!! The hell with deco! Unlimited diving from now on, no more freezing my ass on long deco stops! Hurra the rabbits!!!!!


Have you ever ear about off gassing gradients?
Gee, one reply latter and I had forgotten all about the rabbits! Sorry folks!
Lets just forget all that came in the past, none of it is true! No knowledge from all the studies made by COMEX are true, what the hyperbaric medicine doctors see and say is untrue, the accidents with people that I know never happened and they are not paraplegic. Long live the rabbits! (hoops did they survive? - I forgot to ask!)

I give up! No point in continuing. See you when I see you!

Stay safe

JNeves

So with your back against the wall making unsupported statements and getting challenged, you resort to borderline name calling and innuendo. That sure swayed my opinion and convinced me you're the correct one.

 

[Bobjr]

Look better! Ho, come on, are going to say also there is no climate change due to human activity? Who did you vote for?

He was politely calling you out but probably should've said "There is no data to support your ideas". I suspect you should look harder

 

[hbm]

"]ets just forget all that came in the past, none of it is true! No knowledge from all the studies made by COMEX are true, what the hyperbaric medicine doctors see and say is untrue, the accidents with people that I know never happened and they are not paraplegic. Long live the rabbits! (hoops did they survive? - I forgot to ask!)"


You actually know people who are paraplegics from HE accidents??? I've been doing this diving thing awhile and I know of nobody who is a paraplegic from a high HE dive. Or any other dive for that matter. Actually I know some very high HE CCR divers who are doing some serious stuff without so much as a skin bend. Who are these guys breathing low N2 mixes and winding up in wheel chairs. My info is only anecdotal but I'll be open minded about the new studies coming out. I'm a believer however in getting the N2 out of the mix