Ease of deco from He based gases...

[Dr Deco]

Dear fins wake:

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.

There is great debate about “tissues.” These were originally thought by JS Haldane to be actual parts of the body; Buhlmann thought this as well. Bubbles released from these would enter the blood stream, circulate (through arteries as well) until they stuck (embolized) somewhere. The equal distribution of the blood was the reason for the equal distribution of the DCS sites. With Doppler bubble detectors, it was clear that bubbles were often present in the veins without DCS and never present in arteries even with “the bends.”

Tissues were changed to compartments. These were composed of some lipid (fat) and some proteins. The solubility of the tissue was different from the blood and the nitrogen and helium portioned (= distributed) itself according to this solubility. Depending on the blood supply and the fat/water solubility ratio, the “speed” of a tissue for each gas would differ.

Solubility Ratios

A gas, e.g. nitrogen, which was very lipid soluble, would require a very long time to saturate where the solubility in blood was quite a bit less. The blood/tissue solubility of helium is more evenly matched and the halftimes are thus shorter. This is not only in theory but appears to be true in practice.

Nuclei

Nuclei in tissues are distributed in size and number. His is proposed because all fluids in nature have such a distribution. It is generated in part by fluid motion. Nitrogen and helium will diffuse into these nuclei upon decompression, and helium diffuses faster than nitrogen (because it is a lighter molecule = Graham’s Law of Diffusion).:mean:

The difference in decompression behavior of helium and nitrogen arises from the solubilities and diffusion speeds. It requires a larger volume of tissue to contain enough dissolved helium to make a given sized gas bubble. In this volume, capillaries will be found that can transport the helium away before it reaches the bubble. Thus, the “drop out depth” of helium is greater (about 40 FSW) than that of nitrogen (about 33 FSW).

Fast diffusion also means that helium can reach a bubble faster than nitrogenand “the bends” are said to appear quicker when decompressing from helium than nitrogen.

Dr Deco :doctor:

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

 

[BIGJC]

So if you were to compare both dives side by side (one on He the other on N2 based gasses) with a "standard" decompression strategy I should be able to off gas more helium than nitrogen....

this would mean that with proper care I would be more "clean" after a mix dive than an air one...

is this correct?

 

[Dr Deco]

Dear Readers:

Helium and Deep Diving

Around 1918, Elihu Thompson, then with the US Bureau of Mines, suggested helium as a diving gas because of its low solubility. The rational was that, since helium has a very low solubility, it would allow fast decompression. In practice, this was not found to be true. Helium did, however, obviate the nitrogen narcosis engendered from diving with air.

Thus, if you were to make a helium dive and an air dive, the decompression time would be about equal. I have personally never played with the decompression tables for this, and do not known, therefore, how close approximately equal will be. One of the readers with access to a decompression algorithm might want to calculate deco times with 79% He/ 21% oxygen and compare it to air.

I would think that the surface intervals would show a faster off gassing for helium.

Dr Deco :doctor:

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

 

[DivingDoc]

Dr. Deco wrote:

Solubility Ratios

A gas, e.g. nitrogen, which was very lipid soluble, would require a very long time to saturate where the solubility in blood was quite a bit less. The blood/tissue solubility of helium is more evenly matched and the halftimes are thus shorter. This is not only in theory but appears to be true in practice.

Maybe I am exhibiting my ignorance, but I am not really sure what you mean by this. Do you mean that the ratio: dissolved gas in blood/dissolved gas in tissues is lower for nitrogen than for helium and that the ratio for helium is closer to 1:1? I should think this would depend upon which tissue is in question. For example, which drugs get into the central nervous system depends upon the drug's lipophilicity.

Also in connection with another thread in which I recently participated, you said that studies in livaboard divers showed that venous bubbles did not carry over into the next day of diving. But surely, this does not mean that the tissues are free of dissolved nitrogen, which could still either come out of solution or at least add to the nitrogen tissue load of the next day? Especially, there could still be dissolved nitrogen in the "slow" tissues, right?

 

[Braunbehrens]

Dr Deco once bubbled...


Thus, if you were to make a helium dive and an air dive, the decompression time would be about equal. I have personally never played with the decompression tables for this, and do not known, therefore, how close approximately equal will be. One of the readers with access to a decompression algorithm might want to calculate deco times with 79% He/ 21% oxygen and compare it to air.

I would think that the surface intervals would show a faster off gassing for helium.

If you just run a dive in decoplanner, say 180 for 20mn with air and then with heliox, the deco time on heliox will be 10 times as long!!!! Obviously nonsense. Even with O2 deco, the deco for the heliox is more than twice as long.

Unfortunately most deco programs give horrendous deco times with Helium. In practice this is obviously not reflected in how much deco is needed, but I don't want to take responsibility for anyone's deco based on what I say.

I have also heard it say that if you do get bent on mix, the pain will go away quicker.

Trey has mentioned that Helium will have less negative impact on your body in other ways as well, but rather than parrot him here I suggest that interested people read his posts directly.

I personally dive Helium as if it wasn't there, and it has worked well for me. Your mileage may vary. I also do slow moves between stops and a very slow ascent from 20 ft and I deco out in a horizontal position which is also helpful.

I also assume that SI times are shortened with Helium.

P.S. Did you know that a steel tank will not hold helium? The helium molecules actually fit through the matrix of the steel molecules and escape. Of course it takes something on the order of a million years for all the helium to leak out through your tank... ;-)

 

[Dr Deco]

Dear DivingDoc:

Hydrophilic/Lipophilic

The ratio of solubility of helium in blood to solubility in tissue is approximately one. That means that the tissue will saturate rather quickly (for a given blood flow). If the tissue had an extremely high solubility for helium (infinitely soluble in the limit), the blood would carry helium until doomsday and the tissue would not saturate (or desaturated if going the other way).

Halftimes are a function of solubility. A poorly soluble substance will saturate quickly. An example of this is seen in anesthesiology where a poorly soluble gas such as nitrous oxide produces narcosis very quickly (though not necessarily a deep plane) and it reverses itself quickly. A very lipid soluble substance (e.g., ether or chloroform) requires a much longer time for induction and the reversal is also very slow. [The level is deeper, though, because the concentration will be greater in the end.]

Tissues and Gas Partial Pressures

Venous bubbles are rather fleeting. The dissolved nitrogen in the tissues will exit in relation to the “Residual Nitrogen Tables” and the length of the surface interval. In general, a 120-minute halftime is probably as long as a recreational SCUBA diver will encounter (according to the table algorithm). That is two hours and, with six times the halftime being considered desaturation, that would say that tissue have “cleared” and are at ambient nitrogen loads after twelve hours.

Thus, a dive that ended at 8 PM would be considered as a “new diver” at 8 AM as far as gas loads are concerned. Microbubbles are a different story. Laboratory studies and field data suggest that bubble loads do not increase from day to day. :scuba:

Dr Deco :doctor:

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

 

[fins wake]

Unfortunately most deco programs give horrendous deco times with Helium. In practice this is obviously not reflected in how much deco is needed,

Problem is, if we don't go by more-or-less accepted decompression program times or more-or-less tested decompression tables, any 'true reflection' of accurate times is as best a guesstimate ...

I have also heard it say that if you do get bent on mix, the pain will go away quicker.

Not true. And nor are we talking 'less serious' DCS either, but rather the reverse, a higher incidence of neurological DCS. I'm still waiting for the official US Navy data on this, but anecdotal evidence seems to bear this theory out rather than Trey's.

Trey has mentioned that Helium will have less negative impact on your body in other ways as well,

Yes, this is the origin of 'helium is a benign gas' hypothesis. As a diver I don't agree with Mr Irvine on this, and I know a lot of true hyperbaric specialists - which neither I nor George Irvine are -don't, either.

I personally dive Helium as if it wasn't there,

Dr Jerome Meynie, the great Anglo-French cave explorer has described his experiments doing just that in a record-breaking cave dive in the South of France recently (to -178 metres, no stroll in the park). At the time , he followed the same advice you're citing. Unfortunately, he also ended up with DCS. I gather from his posts (on another forum) he won't be doing it again.

I also assume that SI times are shortened with Helium.

This is consistent with accepted concensus (and with what Dr Deco has written below) and makes sense.

As for Dr Deco, I've been reading his posts very carefully, as they are very carefully written.

Thus, a dive that ended at 8 PM would be considered as a “new diver” at 8 AM as far as gas loads are concerned.

This is dependent on the following qualifying statement:

In general, a 120-minute halftime is probably as long as a recreational SCUBA diver will encounter (according to the table algorithm)

We are talking US recreational scuba diving, i.e. 42 metre (140 feet) depth limit and within NDL:s. What Braunbehrens and many others are talking about are of course deeper and longer 'technical dives' where the parametres in many cases will have changed. For Trey's advice this certainly holds true, and I certainly respect his and his group's accomplishments in the long cave pushes. Still doesn't mean I accept or trust his deco theories, though ...

Personally, for my recreational diving I trust the RDP implicitly, I think it's a great piece of research and thinking and Dr Powell and Dr Rogers are to be congratulated.

'Edited for typo in Jerome's surname'

 

[Doppler]

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.

SNIPPED


:deal:

SENSE CHECK

Using a 30/21 at 180 fsw will get you killed! This is not an appropriate mix for this depth. There is too much oxygen and too little helium. And the are no agencies teaching that it is the correct mix for 100 - 180 foot range.

Sorry to be picky but I have this real fear that someone is going to read something like this, somehow get a fill and go dive without knowing a ****ing thing about how to plan and manage a mix dive to 180 feet, except what they read on the internet.

 

[BIGJC]

He's right.

I just noticed I got the numbers backwords....


21/30 is the mix in question here....

good catch

:wacko:

 

[Dr Deco]

Dear Readers:

Long Halftimes and Desaturation

As indicated by “fins wake,” the desaturation times are different from recreational and commercial or military divers, or even technical divers.

It is my [I opinion], but shared by others, [/I] that “tissues” with halftimes of 400 to 700 minutes do not in reality exist. Rather, when such is needed to calculate a DCS-free decompression, what is really being done is to allow free gas to exit from a true tissue gas bubble. Val Hempleman (a British researcher) in the 1950s referred to this as a “tissue-bubble complex.”

This will definitely persist. I am aware of a diver from Ocean Systems (in the 1960s) who sustained a case of joint pain DCS during a chamber dive, was successfully treated, and then took an airplane flight three days later. The pain developed in the same place (probably Boyle’s Law expansion). This tissue gas phase can persist, and it is a part of the thorny question of flying after diving.

Dr Deco :doctor:

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