Ease of deco from He based gases...

[BIGJC]

Recently a couple of agencies have been teaching normoxic trimix or triox (30/21 mix) as the proper gas for use on dives in the 100-180ft range. While I understand the advantages as far as narcosis goes I have been a little intrigued by the fact that some have said that it is easier to decompress from than air is..... what EXACTLY is meant by this? Are they saying that you are less likely to be bent when your "coming off" of trimix? this seems a little counter intuitive but I am always open to good ideas.... enlighten me doctor deco!
:deal:

 

[MonkSeal]

If mix you breath contains less nitrogen then air does, then less nitrogen will be disolved in your tissues. I haven't finished triox course but I checked deco times with GAP and got longer times with triox then air. Tissue graph showed less saturation for quick tissues for triox then air. Can anybody make a comment on this ?

 

[fins wake]

This is a potentially inflammable subject, which I've already delved into, so here goes again: Think about it! Why should helium, which is a truly inert gas, be any more 'benign' than an 'almost inert' gas such as nitrogen?

The reason one uses helium mixes (trimix, heliox, heliair, even hydreliox at stupid depths, whatever) is because of its great non-narcotic properties! That's great. But that's all.

I remain highly unconvinced by this helium-can-cure-all--deco-ills voodoo. Indeed, theoretically, going off a helium bottom mix one should really go over to a nitrogen mix and replace one inert gas with the other (at shallow depths) until one can go over to pure oxygen. What is the point of loading more fast helium into the slow tissues when decompressing? Please explain, whoever can. I can't. It makes no sense whatsoever.

However, I do believe trimix is the gas of choice for dives in the 180 ft range. Narcosis is a killer. Helium is a superb gas for true technical diving.

 

[BigJetDriver]

BIGJC once bubbled...
Recently a couple of agencies have been teaching normoxic trimix or triox (30/21 mix) as the proper gas for use on dives in the 100-180ft range. While I understand the advantages as far as narcosis goes I have been a little intrigued by the fact that some have said that it is easier to decompress from than air is..... what EXACTLY is meant by this? Are they saying that you are less likely to be bent when your "coming off" of trimix? this seems a little counter intuitive but I am always open to good ideas.... enlighten me doctor deco!
:deal:

BigJC, MonkSeal, and Fins Wake,

In the momentary absence of Dr. Deco (it is just now a few minutes after quitting time for him), I'd like to offer a few thoughts that may help out.

(1) Re: Fins Wake question about "benign" gasses---Helium is just one of several inert gasses that have been experimented with for diving purposes---Hydrogen, Helium, Nitrogen, Argon, and Neon are ones that a fair amount of work has been done with. "Benign" is a descriptive used in comparisons of the effect procuced in certain situations. NO inert gas is totally benign in hyperbaric situations. It is just that some are better than others for certain things.

(2) Let me state quite plainly to be sure we are on the same page: No inert gas is 'more' or 'less' likely to bend you. Regardless of the gas, it is ALWAYS a question of dosage. Stated simply: How long were you exposed and what was the maximum partial pressure of the gas or gasses? How long have we taken to go from that exposure back to 1 ATM?

(3) Re: BigJC's having heard that helium is "easier" to decompress from---This remark is only true in the sense that helium goes into the tissues faster, and comes out faster.---This fact must be dealt with in specific ways so that we do not encounter other problems.

(4) Re: MonkSeal's comment about time comparisons on deco programs---Some of the "older" (please note quote marks) deco programs are not set up to deal with helium as part of the original model. Operating the program takes some experience at setting the parameters to effectively deal with it.

(5) Fins Wake is absolutely right! Helium is NOT a cure-all to deco problems, NOR is any other inert gas. Anyone who tells you that is truly giving you "voodoo-hoodoo"!

Having said all that, and I hope that was helpful, I would like to point you to a reference that will truly be helpful. "Technical Diving in Depth" by B. R. Wienke, PhD, Los Alamos Laboratory, published by Best Publishing. If you are not a mathematician, and I for one am not, read around the formulas. There is sufficient verbiage to really enlighten you.

 

[BIGJC]

I remain highly unconvinced by this helium-can-cure-all--deco-ills voodoo. Indeed, theoretically, going off a helium bottom mix one should really go over to a nitrogen mix and replace one inert gas with the other (at shallow depths) until one can go over to pure oxygen. What is the point of loading more fast helium into the slow tissues when decompressing? Please explain, whoever can. I can't. It makes no sense whatsoever

So...... in deco with triox does it do you any good to switch to an intermediate gas (say 50/50) or should you stay on mix until you hit the O2? It sounds like it is a good idea to use both mixes. I do not mind a longer deco as long as it is a safer one, and it sounds like the jury is still out on that. I am considering experimenting with this as some of the wrecks offshore here in NC are in the right depth range, but I don't want to get hurt!!

:arrow:

 

[Dr Deco]

Dear Readers:

Gases

Certainly many gases have been tried for diving. Only nitrogen and helium have been of real benefit in a lasting way. Argon is too narcotic and fat soluble (can lead to numerous venous bubbles and arterialization). Neon is a good dive gas but is expensive. Neon was proposed to replace helium in out-of-the-way places, but helium is obtainable virtually worldwide.

Solubility

The very best diving gas would be one that is completely insoluble. If nothing dissolves and enters the tissues, then nothing could exsolve (a real word) and form gas bubbles. The less soluble a gas, the greater volume of solution must be needed for the “watershed,” the solvent volume from which the free gas in the bubbles can come. [A gas bubble of a certain size will contain the same number of gas molecules = Avogadro’s principle.] Complete insolubility (like the “universal solvent”) does not exist.

Gas Switching

Use has been made of the fact that tissues can be thought of as dissolving more gas than blood can. A less soluble gas can be switched for a more soluble one (e.g., helium to nitrogen). This difference in solubility is reflected in the partition coefficients (and the halftimes). Gas switching is common in commercial operations. Eventually one can switch to oxygen that is metabolized.

Dr Deco :doctor:

Please note the next class in Decompression Physiology :grad:
http://wrigley.usc.edu/hyperbaric/advdeco.htm

 

[BigJetDriver]

BIGJC once bubbled...


So...... in deco with triox does it do you any good to switch to an intermediate gas (say 50/50) or should you stay on mix until you hit the O2? It sounds like it is a good idea to use both mixes. I do not mind a longer deco as long as it is a safer one, and it sounds like the jury is still out on that. I am considering experimenting with this as some of the wrecks offshore here in NC are in the right depth range, but I don't want to get hurt!!

:arrow:

To add to Doc's excellent message, I can only advise the following when it comes to procedures: Use not one, but several of the currently available deco programs. Run the dive to your predicted depth using gas switches and without. Compare the results, and choose a schedule from those choices. Only you can decide what level of conservatism to use, for instance, and that is only one of the parameters that will affect your run time.

 

[fins wake]

(2) Let me state quite plainly to be sure we are on the same page: No inert gas is 'more' or 'less' likely to bend you. Regardless of the gas, it is ALWAYS a question of dosage. Stated simply: How long were you exposed and what was the maximum partial pressure of the gas or gasses? How long have we taken to go from that exposure back to 1 ATM?

Agree entirely.

(3) Re: BigJC's having heard that helium is "easier" to decompress from---This remark is only true in the sense that helium goes into the tissues faster, and comes out faster.---This fact must be dealt with in specific ways so that we do not encounter other problems.

Agree entirely.

A less soluble gas can be switched for a more soluble one (e.g., helium to nitrogen). This difference in solubility is reflected in the partition coefficients (and the halftimes). Gas switching is common in commercial operations.

I'm happy to read this.

I am considering experimenting with this as some of the wrecks offshore here in NC are in the right depth range, but I don't want to get hurt!!

Be very very careful! I think BigJetDriver's advice is excellent:

I can only advise the following when it comes to procedures: Use not one, but several of the currently available deco programs. Run the dive to your predicted depth using gas switches and without. Compare the results, and choose a schedule from those choices. Only you can decide what level of conservatism to use, for instance, and that is only one of the parameters that will affect your run time.

Myself I use VPM-B and Decoplanner, cross-referencing with some really tested, conservative tables (e.g. DCIEM, MN90) andI use whichever schedule is the more conservative! That's right, I'm a deco weenie and proud of it. Also, I always test out new programs and my own kitchen-sink theories on real easy dives to shallow depths first. And get good decompression diving training, the Net will only take you so far.

So be careful, BigJC, before you test out any new procedures or programs on tough off-shore North Carolina wreck dives. Don't get bent because of the personal opinions of people like me. :goofy:

Safe diving t'ya all!

 

[BIGJC]

In the "International TextBook of Mixed Gas Diving" Lettnin says that Helium is not absorbed as easily as nitrogen into the slower tissues and that is what makes it easier to decompress from... It really makes a lot of sense to use the mix.... just expensive! Plus I will have to dive self generated table instead of my computer or maybe get a VR3... my wife may not speak to me again....

I will be very careful.... I am planning on taking GUE's triox class in August and try it on a couple of inshore dives first..

 

[fins wake]

My understanding is that helium is absorbed faster than nitrogen into (and off-gassed faster out of) all tissues! Solubility is the key, but think about it! What is a 'slower tissue' (we're talking hypothetical tissue arrangements here, most lately re-arranged by Dr Bühlmann) and why would helium 'behave differently' from nitrogen when entering and exiting'? Dr Deco does this so much better than any of us, so ... :doctor:

am planning on taking GUE's triox class in August and try it on a couple of inshore dives first..

Sounds great. Don't mistake my scepticism of certain of Irvine's hypotheses for a general dislike of GUE ... I know some of these guys and they are very pleasant fellows and superb divers! Oh, and they don't really like computers, but personally I'd like a VR3 myself! It's a superb instrument! And as the great Exley said, what works works! Dive safe!

K.