Time to total saturation

[canuckton]

Well, Dr Paul, you have me...a bit

I am aware that breathing diluent gas (A) for a time, and then switching to (B) gas will allow (A) gas to off-gas at its tissue half-time, whilst gas (B) starts to on-gas tissue compartments at a different rate, determined by many factors, including solubility.

BUT...does gas (B) have an effect on gas A that might *change* the off gassing properties of gas (A)?

My question was not "if I want to flush out the helium, will breathing nitrogen instead of helium help?"
Rather it was "does breathing helium after nitrogen change the rate at which nitrogen leaves my tissue, as compared to breathing pure oxygen?"

It strikes me, that on a given trimix or heliox dive, a decompression stop is done (usually around 130 feet) to let helium off gas. The tissue compartment times are much shorter than nitrogen, due to the solubility of helium. However, that tissue compartment is quoted the same no matter what gas mix you breathe while decompressing, as long as it doesn't include helium. SOOooooo....would that helium tissue half-time change if I switched to, say....argox? Would the solubility characteristics of a different gas be relevant?

I'm talking theory here, not practicality. I just want to understand a little more about any possible interaction between non-metabolised gases.

I dunno, when I called DAN, they were stumped.

Whaddya think, am I just missing something obvious?

 

[Walter]

Paul,

I enjoy your posts, but I've read all the Sherlock Holmes stories. Holmes never said, "It's elementary my dear Watson!" Actually, an actor in the part of Holmes did say it, but the phrase never came from Dolye.

 

[The Iceni]

Yes Walter, you're quite right,

But it is "elementary" canuckton!

As Watler will no doubt confirm, when diffusing across a gradient in solution each molecule (element!) of gas acts on its own, the rate being determined by the (partial) pressure gradient and molecular weight of that gas alone.

Isobaric counterdiffusion is the reason helium cannot be used to dilute oxygen for accelerated decompresion from an air or Nitrox dive otherwise we could use 50% heliox from 21 metres instead of fiddly, increasing oxygen mixes.

Dissolved helium diffuses much faster than nitrogen simply because it has is a much smaller molecule.

If, for example, you were to use 50% heliox on a stop instead of 100% oxygen the offgassing of nitrogen from a Nitrox or air dive would be just as rapid as with oxygen alone. (In both cases there is no nitrogen counterpressure). However the on-gassing of helium into the bubbles and micronucleii would be much faster than the offgasing of nitrogen, leading to an increase in the total gaseous pressure within the bubbles, which would grow rather than reduce in size, completely defeating the object of the exercise.

After all bubble size is determined by the total pressure within it, which is made up of the sum of the partial pressures of each and every individual gas.

Argon would be a suitable gas to use in such a scenario but the larger the gas molecule the more they are narcotic and argon is very narcotic even at shallow depths. Xenon could be used but it is so naracotic it causes unconciousness at lower than atmospheric partial pressuers. In my humble opinion, not a very good idea for divers!

If the above explanation is too complicated consider this.

A cylinder is filled to 500 psi with helium and then topped up with nitrogen to 1000 psi. In the gaseous phase diffusion is not affected by molecular weight or size so within a very short time every part of the cylinder will contain an equal amount of helium and nitrogen. (Van der Waals tells us there will be more nitrogen than helium but thats another story.)

However, if one cylinder of a manifolded twinset is filled to 1000 psi with helium and the other to 1000 psi with nitrogen and the isolation manifold then opened, diffusion will eventually cause the two cylinders to equalise, as before, at 1000 psi, but this is through the restriction of the isolation manifold which will delay this considerably. During this process, because helium has a smaller molecule than nitogen the resistance to diffusion through the manifold is less so the diffusion of helium is much more rapid and it will reach equilibrium faster.

At this stage the nitrogen has not yet reached equilibrium. The original nitrogen cylinder will hold more nitrogen than the original helium cylinder because of this. Each cylinder will contain helium at 500 psi but the helium cylinder may only contain 400 psi of nitrogen while the nitrogen cylinder would still contain 600 psi of nitrogen (400 + 600 = 1,000). From Dalton's law we can see that the original helium cylinder will be at 900 psi while the nitrogen cylinder (the bubbles) will be at 1100 psi.

This is how I interpret what happens with the diffusion of gasses in solution. The size of the molecule (not its solubility) determines the rate of diffusion.

Thus helium has much faster half times than either oxygen, nitrogen, argon or xenon.

Canuckton, I hope this helps.

 

[The Iceni]

Originally posted by Dr Deco
It is biophysically difficult to imagine tissues with such long halftimes. One must thing that these are living tissues. Such a long time to bring in oxygen and food, and to eliminate carbon dioxide and waste metabolic products would not readily be conducive to life.


Dr Deco :doctor:

Only active tissue metablises and requires a good blood supply to deliver oxygen and nutients, and to remove waste products.

I do not remember the figures but I imagine that scar tissue, cartilage and mature bone do not have a high metabolic requirement and will in consequence, therefore, have exceedingly long half times.

I am not so sure that the same applies to the epihysial plates in the growing bones of children and young adults.

 

[canuckton]

Excellent, thank you all, and especially Dr. Paul.

I wouldn't call it elementary, but it is logical, and seems to be the same as I learned in the past...however, I guess my understanding was imperfect enough that I couldn't dredge it up for my students. If no one minds, I'd like to use this thread and show it to the class...I think it'll be easier to understand that way than having me muddle it all up again!!

I'd better stick to gear.

Cheers!

 

[The Iceni]

You’re welcome, canuckton.

I am learning a lot myself, otherwise I would not be posting so often! By the way, I was playing with words. I have a very odd (British?) sense of humour. I used the term "elementary" in reference to the natural elements, not suggesting for a moment that it was so easy to understand.

As Bruce Wienke has stated elsewhere on this forum, http://www.scubaboard.com/t10566/s357640e924d492537e370a95e7a2eefd.html by Graham's Law, all gas diffusivities are inversely proportional to the square root of their atomic weight, just as Boyle’s Law states pressure is inversely proportional to volume.

As P is proportional to 1/V so P1V1 = P2 V2

Thus, if my maths is right, as nitrogen has a molecular weight of 28 and helium a molecular weight of 4, helium will diffuse two and a half times as fast as nitrogen;

R1*SQRT(MolWt1) = R2*SQRT(MolWt2)

Rate He*SQRT (4) = Rate N2*SQRT (28)

Rate He * 2 = Rate N2 * 5.29

Rate He = Rate N2 * 5.29/2 = 2.65

 

[Dr Deco]

Dear Readers:

As Dr Thomas stated, gas diffuse in one another independently (at the pressures encountered in diving). The non-interaction of gases (at the pressures encountered in diving) is the basis of Dalton’s Law of Partial Pressures. Stated in words we find that the pressure of one gas species is independent of the presence of another species. The total pressure is thus the sum of each of several components. This has also to do with the diffusion of one gas in another [Graham's Law] or the solubility of several gases in a liquid.

All of these are “limiting cases” and strictly applyonly in infinitely dilute solutions. However, for all practical purposes, at the pressure encountered in diving, the amounts of gases are small and the interactions are minimal.

This is not to say that all combinations of gases are useful and produce no untoward physiological effect. Certainly “isobaric countertransport” is real. You can encounter situations in diving where adding a “fast" gas on top of a “slow” one will produce some forms of DCS.

Dr Deco :doctor: