Rebreather underwater time?

[Sounder]

So John,

Assuming one was on CCR, and using argon for suit inflation, I'm interested in the "heat-zapping" properties of helium in the body at such high levels.

I'm trying to get my head around how helium IN the body removes heat faster than nitrogen. I get the physics of having it as a suit-inflation gas, but am interested in the body angle when the helium is effectively "in solution."

The reason I indicated the diver was on CCR was because of the cost of helium, seeing that it would be fairly cost-prohibitive for the average diver (that is unless our taxes were paying for it) on OC.

Why is it, or would it be, less advantageous to use 100% helium as your dil (figuring you'd use other gas for BC inflation due to cost)?

(Sitting down ready for class...)

[John Rawlings]

So John,

Assuming one was on CCR, and using argon for suit inflation, I'm interested in the "heat-zapping" properties of helium in the body at such high levels.

I'm trying to get my head around how helium IN the body removes heat faster than nitrogen. I get the physics of having it as a suit-inflation gas, but am interested in the body angle when the helium is effectively "in solution."

The reason I indicated the diver was on CCR was because of the cost of helium, seeing that it would be fairly cost-prohibitive for the average diver (that is unless our taxes were paying for it) on OC.

Why is it, or would it be, less advantageous to use 100% helium as your dil (figuring you'd use other gas for BC inflation due to cost)?

(Sitting down ready for class...)

The NPS team did not use CCRs - they were on Open-Circuit.

You would NOT use 100% of anything for your diluent. Diluent can be air or Trimix. I have not heard of anyone diving Heliox as a diluent, but I suppose that it could be done. The point, though, is that you cannot use 100% helium as your diluent as the diluent must contain a breathable percentage of O2.

Not being a trained physiologist, I cannot adequately explain the specific mechanics of how it occurs, but helium will draw heat from your tissues at a greater rate than will nitrogen. Suffice it to say that as a smaller molecule, helium will eliminate itself from your tissues at a far higher rate than will nitrogen. Since it does this by means of the bloodstream, and the bloodstream is the primary means of heat traveling thoughout the body, heat will leave the tissues via the bloodstream and exit the body through respiration from the lungs at a faster pace than it would with nitrogen.

The use of a CCR negates some of this, however, as you are breathing/recycling warm, moist gas that not only is warmed by your body it is also warmed by the chemical reaction of CO2 and the sorb. Your exhalations are returned into the scrubber and so at least some of the heat loss is decreased. You thus will feel the cold less than if you were diving on OC and breathing cold, dry gas, and exhaling it out into the water column.

In my opinion, and I'm sure that others will disagree, the use of Argon will also contribute much to the lessening of the chilling effects caused by breathing helium at a high percentage, (such as the 20/80 Heliox mix I described above).

When you get your CCR and take your class a lot of this will begin to make more sense to you.

- John

[Sounder]

Yeah, it definitely will make sense once I start training... but these are the kind of questions that burn in my mind until answered.

Stupid of me to say 100% helium... I know there must be enough O2 in the mix to support life. Homer Simpson moment - doh!

I can see how during the ascent the gas would carry heat out of the body, but just can't still get my head around the physics and physiology of it in the body... it "sort of" makes sense, but not totally. Bottom line is that those who use it say it does so I'm sure it does - I'm just curious about the mechanics of it all.

Thanks!

[Dmitchell]

Think of it this way, If you were enveloped in a warm atmosphere but spent hours breathing cold air so 20 degrees. You would over time (hours maybe even days) chill to the point of being cold even though your body is in a warm environment. Because breathing the cold air will rob you of heat.

Now, Breathe a gas that is capable of carrying that heat away 5x faster than air and put your body in cold water. The helium will cool you. Breathe a hi HE mix and you can get really cold.

I made the mistake of putting 10/50 in my suit last summer. It felt like ICE WATER. When I switched hoses and flushed the suit with air it felt like it was hot going in.

Dave

[Seth T.]

Think of it this way, If you were enveloped in a warm atmosphere but spent hours breathing cold air so 20 degrees. You would over time (hours maybe even days) chill to the point of being cold even though your body is in a warm environment. Because breathing the cold air will rob you of heat.
Dave

So, it's pretty much the same principle as drinking a cup of hot chocolate when it's freezing outside? Or the other way, drinking a freezing cold bottle of water when it's 90F outside?

Layman's terms baby!

[Celeus]

There is a very lively discussion on rebreather scrubber life, breakthrough, testing methodology, etc. over on rebreatherworld. Just look for the thread related to "a chart".

I have a question, which I probably should ask there, but you all are friendlier.

Why does depth affect scrubber utilization/perfromance? Temp I get, workload I get, but does the pressure actually affect one of the steps in the reactions?

[John Rawlings]

Good points, Dave!

Sounder....here's a "real-world" analogy that might help make it "click" for you. When we think of how humans cool down we normally think of sweating. That is NOT the only way that we do so, however. We also lose some of our body heat through our exhalations.

Dogs cannot sweat. The only way that they can cool themselves down is by their exhalations - they do this by panting. This is why, even on a cold day, after chasing a ball or whatever we will see a dog panting hard to bring its inner body heat downward. Humans do the same, only it isn't as obvious. After atheletes finish a race they are panting as well - part of this is to get higher levels of O2 into their systems, but the other part of it is to cool down from their exertions.

Now, understanding that exhaling is a means by which the body can and will lose heat, does it not make sense that if we have gas molecules warmed by our tissues exiting the human body by exhalation that we will subsequently lose the heat that is attached to those molecules? Basically, molecules of gas do not give the heat they have absorbed back before they are exhaled through the lungs, they take it with them when they leave.

Taking it a step further, if you have MORE of those molecules leaving the body at a FASTER rate (as helium does when compared to nitrogen), does it not then follow that MORE heat will also leave the body at a FASTER rate?

The first time that I ever dived with a CCR was in Puget Sound, and I was noticeably and pleasantly warmer throughout the entire dive. There have been many occasions since when I have been quite comfortable in the warmth department while my buddy diving on OC was physically shaking with the cold.....all part of the ability to retain more body heat with a CCR.

- John

[Sounder]

Your (John) description makes sense, but it is a completely different discussion than the suit warmth one.

Suit warmth is worse with helium because the smaller molecules move faster which is why argon helps so much (it's a massive molecule).

I also get that the molecules do not give back the warmth they take when exhaled. But... do all molecules have the capacity to hold the same amount of heat? If so, then your description of helium having more molecules (which I get) works. If not however, the helium retention of the heat (as is within the suit) is diminished when compared to larger molecules such as nitrogen and argon, and there by I would guess that nitrogen exhalation would take more heat than helium would... which obviously is not the case.

What about this...

Since helium is so small, and diffuses into tissues so quickly, the body can readily become saturated with helium must faster and easier than with other inert gasses.

Because there is so much more gas (helium), presumably absorbing heat, in the body, there is more gas exhaled upon ascent. Because there is more gas coming out of tissues, and at such an accelerated rate from other inert gasses, the acute heat loss is greater. However now we're back to the issue of more molecules absorbing more heat versus larger molecules storing heat longer than smaller ones.

This brings me to another question - so long as the warmed-up molecules are in solution (in the body), they shouldn't be removing heat during descent and bottom time, but only during ascent and deco, right? Wrong, because in practice and experience this isn't the case.

I love theory versus reality. Sometimes things that should work, don't and sometimes things that shouldn't have happened, do. Eventually I suspect I'm just going to have to accept it as "the way it is" and go with it... but then that doesn't satisfy my desire to understand the mechanics of how it works.

My brain hurts.

Aaah, if only I could get this interested in learning my Managerial Accounting material for Monday's test.

[John Rawlings]

Your (John) description makes sense, but it is a completely different discussion than the suit warmth one.

Silly me.....I could have SWORN it was you that asked this:

I'm trying to get my head around how helium IN the body removes heat faster than nitrogen. I get the physics of having it as a suit-inflation gas, but am interested in the body angle when the helium is effectively "in solution."

It must've been space aliens that asked the question....yeah, that's it....space aliens! HA!

- John

[Sounder]

Your (John) description makes sense, but it is a completely different discussion than the suit warmth one.

Silly me.....I could have SWORN it was you that asked this:

I'm trying to get my head around how helium IN the body removes heat faster than nitrogen. I get the physics of having it as a suit-inflation gas, but am interested in the body angle when the helium is effectively "in solution."

It must've been space aliens that asked the question....yeah, that's it....space aliens! HA!

- John

Well, let me clairify that statement (it wasn't space aliens, it was the voices in my head that tell me what to write)...

I'm trying to get my head around how helium IN the body removes heat faster than nitrogen. I get the physics of having it as a suit-inflation gas, but am interested in the body angle when the helium is effectively "in solution."

What I was saying what that I already understand the physics of the heat transfer when helium is used as a suit inflation gas.

I'm seeking the answer from the Wise One (that's you) regarding the gas behavior when it is in the body, not when it is in the suit.

It makes sense to me, but perhaps it wasn't (maybe still isn't?) clear... if it's not, it's not my fault because the voices say it's not.

[John Rawlings]

????? Does anyone speak "Sounderese" Can anyone translate for me? ](*,)

I believe that my response (tissue absorption, tissue releasing, blood flow to lungs, exhalation, body heat loss, etc.) dealt strictly with physiology, not suit issues.

I guess that I am confused as to why you are confused. No offense meant, but I don't know where to take it from here

I shall bid you a fond farewell on this topic. Let's see if anyone else wants to give it a shot.

- John

[Joshua Smith]

I'll give it a shot: Helium is really good at transfering heat away from your body. Through respiration, helium grabs heat from the rich blood supply in your lungs, and makes your core temperature drop. Through a combination of radiation and convection, helium robs your skin of heat when it's in your drysuit.
As far as dissolved helium in your tissues/ blood, I don't think it affects your body temperature, but I could be wrong about that.

[Seth T.]

Sounderese: Helium is Cold ASS!

[Sounder]

I guess that I am confused as to why you are confused. No offense meant, but I don't know where to take it from here.
- John

I'm confused too

Alright, here we're go... (and thank you for your patience with me John!!)

Sounder....here's a "real-world" analogy that might help make it "click" for you. When we think of how humans cool down we normally think of sweating. That is NOT the only way that we do so, however. We also lose some of our body heat through our exhalations.

Makes sense to me.

Dogs cannot sweat. The only way that they can cool themselves down is by their exhalations - they do this by panting. This is why, even on a cold day, after chasing a ball or whatever we will see a dog panting hard to bring its inner body heat downward. Humans do the same, only it isn't as obvious. After atheletes finish a race they are panting as well - part of this is to get higher levels of O2 into their systems, but the other part of it is to cool down from their exertions.

I'm totally with you here too!

Now, understanding that exhaling is a means by which the body can and will lose heat, does it not make sense that if we have gas molecules warmed by our tissues exiting the human body by exhalation that we will subsequently lose the heat that is attached to those molecules? Basically, molecules of gas do not give the heat they have absorbed back before they are exhaled through the lungs, they take it with them when they leave.

100% clear - still right there with ya!

Taking it a step further, if you have MORE of those molecules leaving the body at a FASTER rate (as helium does when compared to nitrogen), does it not then follow that MORE heat will also leave the body at a FASTER rate?

Time-out!! ](*,)

Here's the question: Let's talk about 1 molecule of helium, 1 molecule of nitrogen, and 1 molecule of argon...

Do all three molecules have the ability to store the same amount of heat? If so, then it's clear why helium robs the body faster than the others... there is simply more molecules per x-volume of gas because they're smaller.

IF all three molecules DON'T hold the same amount of heat, then we're talking about a ratio between the molecule size and the amount of heat it can store. We also have to switch to volume of gas now too - i.e. can 1 cu' of helium store more heat than 1 cu' of nitrogen etc? IF helium DOES store more heat per cu' than the other gasses, then it makes sense.

Note: I might have just had a eureka moment in processing this...

Because 1 cu' of helium is filled with many more (because they're smaller) molecules than 1cu' of nitrogen is, it is denser in terms of the number of molecules. The denser the volume of gas, the more energy it takes to warm it. Hence, helium takes more energy to warm because of it's density in volume when compared to the other inert gasses in question (and because it moves faster than the others as the molecules bounce against one another). SO, the denser gas (helium) takes more heat with it upon exhalation than the others do per equal volume.

How'd I do?

The first time that I ever dived with a CCR was in Puget Sound, and I was noticeably and pleasantly warmer throughout the entire dive. There have been many occasions since when I have been quite comfortable in the warmth department while my buddy diving on OC was physically shaking with the cold.....all part of the ability to retain more body heat with a CCR.

I can't wait to get mine!!


Thanks for your help and patience here John. I can see where my questions were getting confusing and apologize for the Sounderese. ](*,)

Thanks Nailer for your help too - helped me to reach the eureka moment.

Whew. Back to Managerial Accounting.

[John Rawlings]

How'd I do?

Better than I woud have!

[Joshua Smith]

SO, the denser gas (helium) takes more heat with it upon exhalation than the others do per equal volume.

How'd I do?

Helium is actually less dense than air, due to the size of the molecules- therefore decreasing Work Of Breathing at depth, be it CC or OC.

[Celeus]

Note: I might have just had a eureka moment in processing this...

Because 1 cu' of helium is filled with many more (because they're smaller) molecules than 1cu' of nitrogen is, it is denser in terms of the number of molecules. The denser the volume of gas, the more energy it takes to warm it. Hence, helium takes more energy to warm because of it's density in volume when compared to the other inert gasses in question (and because it moves faster than the others as the molecules bounce against one another). SO, the denser gas (helium) takes more heat with it upon exhalation than the others do per equal volume.

How'd I do?

Well, this does make sense, but unless I completely mis-recall my gas laws, it's actually incorrect.

The number of atoms in a given volume of gas, at a given pressure and temperature is the same no matter which gas it is- that is, 95 Liters of gas, at 100kPa, at 30 degrees Celsius (I use these units because it makes the math easier) is approximately 37 Moles of gas, or 37 X 6.022 X 10^23 atoms of gas.

Whether the gas is N, He, H, Ar or O does not matter.

I believe what accounts for the effective heat loss when using He in a drysuit compared to with Nitrox is the difference between the specific heat of the two gas mixes. Actually, the more relevant value may be the thermal conductivity of the gas. In any case, gasses with a higher atomic weight are generally going to insulate better.

Now, I have been told that a great part of energy usage/heat loss while diving is related to having to heat the cold air you inspire. This makes sense, and would explain why rebreathers use less energy and keep you warmer.

But, based on that, given a cold gas, it would seem that one with a _low_ specific heat (similar to the R factor of home insulation) would be a better choice- for the same temperature, pressure and volume of gas, it should take less energy to heat something with a low specific heat (say He) than one with a higher specific heat (say Nitrox).

This would only be relevant on open-circuit- once the gas is heated to close to body temperature, you no longer have the large energy loss from the system when using a rebreather.

Anyway, I would like to hear if this is empirically true or not- on Open Circuit is it in fact warmer to use a highly thermally conductive gas (e.g. Ar) for insulation and a low thermally conductive gas (e.g. He) as breathing gas?

I guess some googling is in order.

[Sounder]

Crap. You're right.

Uugh. I give up - I'm having beers with a doc friend who's a physiology ninja next week anyway. I'll just ask him then.

[Sounder]

I think I went down the wrong road thinking relative size and not thermal conductivity.

This explaination seems to be the strongest I could find (Warning - nerdy physics stuff ahead):

http://www.decompression.org/maiken/Why_Argon.htm

[Celeus]

Was just about to post that link!

I also found the following paper:
Respiratory heat loss with high density gas mixtures
Which says in the abstract- "Heat loss was directly proportional to respiratory minute volume and to the product of density and specific heat of each gas mixture"

So, I've forked over some hard-earned (can't you tell I'm working real hard this afternoon) cash to the guvmit and should have a copy in a week or two.

Should be interesting reading. I have a hard time believing I'm not missing something.

[Tangfish]

SO, the denser gas (helium) takes more heat with it upon exhalation than the others do per equal volume.

How'd I do?

Helium is actually less dense than air, due to the size of the molecules- therefore decreasing Work Of Breathing at depth, be it CC or OC.

Actually, helium is *more* dense than air, per given volume due to the fact that the smaller atomic/molecular size allows them to pack in more tightly. Protons are much heavier than the (largely) empty orbitals occupied by electrons - meaning that the more protons (or neutrons) you can pack into the same space, the denser that material will then be.

(this is complete bullshit, designed to confuse)

[Celeus]

Erm, Calvin, I hate to disagree, but the number of atoms in any gas is the same across type of atom- it varies by pressure and temp. Check out Avogadros Law

So, at a constant temp and pressure, the density of any gas is directly proportional to its atomic weight.

[Joshua Smith]

Erm, Calvin, I hate to disagree, but the number of atoms in any gas is the same across type of atom- it varies by pressure and temp. Check out Avogadros Law

So, at a constant temp and pressure, the density of any gas is directly proportional to its atomic weight.

You missed the super tiny post script under Calvin's post.

[CaptnJack]

You missed the super tiny post script under Calvin's post.

I hate when my party ballons fall off the table onto the floor!

[Celeus]

Erm, Calvin, I hate to disagree, but the number of atoms in any gas is the same across type of atom- it varies by pressure and temp. Check out Avogadros Law

So, at a constant temp and pressure, the density of any gas is directly proportional to its atomic weight.

You missed the super tiny post script under Calvin's post.

My eyes, the goggles they do nothing!