Rebreather underwater time?

[Seth T.]

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

Ha! Awesome.

[ArcticDiver]

May I ask a logistics question?

How do you handle shipping scrubber when you travel? Do you pre-plan and pre-ship? Limt your destinations to those that have scrubber? ??

[lamont]

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

I believe you are mixing up your physics.

When you are dealing with suit inflation gas you are concerned mostly with the thermal conductivity of the gas. Helium has a much higher thermal conductivity than air which is why its a very chilly suit inflation gas.

When looking at breathing gas, however, your lungs heat up the gas that you breathe to a fairly consistent temperature. So what you need to look at is the heat capacity of the gas you are exhaling. Helium has a lower heat capacity than air, so for every breath you exhale slightly less heat will be carried away in that breath (takes less heat to warm up the cold helium coming from the reg / adiabatic expansion to body temperature).

[Joshua Smith]

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

I believe you are mixing up your physics.

When you are dealing with suit inflation gas you are concerned mostly with the thermal conductivity of the gas. Helium has a much higher thermal conductivity than air which is why its a very chilly suit inflation gas.

When looking at breathing gas, however, your lungs heat up the gas that you breathe to a fairly consistent temperature. So what you need to look at is the heat capacity of the gas you are exhaling. Helium has a lower heat capacity than air, so for every breath you exhale slightly less heat will be carried away in that breath (takes less heat to warm up the cold helium coming from the reg / adiabatic expansion to body temperature).

BUT- if there are more molecules per breath being exhaled, they sap your body temperature faster, as is the case with He vs. eanx 21, right?

[lamont]

BUT- if there are more molecules per breath being exhaled, they sap your body temperature faster, as is the case with He vs. eanx 21, right?

PV = nRT

pressure is the same in an exhale of trimix and air/nitrox
volume is the same in an exhale of trimix and air/nitrox
temperature is the same in an exhale of trimix and air/nitrox
R = R

so the number of moles of the gas is the same. there aren't more molecules (atoms technically) of helium compared to molecules of N2 or O2.

and actually that last sentence contains the difference which is critical. N2 and O2 are molecules so they have rotational and vibrational modes. as you start dumping heat energy into N2/O2 it goes into kinetic energy along with rotational and vibrational modes -- but only the portion that goes into kinetic energy produces a temperature increase. when you dump heat energy into He it goes straight into kinetic energy which means that it takes less heat input to warm up a mole of helium compared to a mole of N2 or O2.

the thermal conductivity i think is more related to the velocity of helium and mean free path, which are related to the size of helium -- so heat zips across a container of helium, but the helium itself contains compartively little heat.

[Sounder]

Thank you Lamont!

This is exactly what I am/was looking for.

So if it takes less energy to heat the helium, then would the diver retain more heat using a helium mix compared to an EANx mix, all other things being equal?

[Joshua Smith]

I believe it's the opposite- every trimix diver I ever talked to tells me they're colder diving mix- I'm in way over my head here, but I'm guessing it's because the He transfers heat so efficiently.

[Sounder]

I believe it's the opposite- every trimix diver I ever talked to tells me they're colder diving mix- I'm in way over my head here, but I'm guessing it's because the He transfers heat so efficiently.

Sigh... confused again. Lamont? :hello2:

[Celeus]

When I get the paper/report I mention above, I'll summarize and post here.

[lamont]

I believe it's the opposite- every trimix diver I ever talked to tells me they're colder diving mix- I'm in way over my head here, but I'm guessing it's because the He transfers heat so efficiently.

I doubt the effect is measurable, but unless the temperature of the helium you exhale is significantly warmer than the temperature of the nitrox you exhale (or the helium is for some reason colder going in? ether way you need about double the delta-T for the helium to carry away the same amount of heat) then the physics is very clear. Helium, mole-for-mole, carries less energy away from you when you exhale.

I expect that most trimix divers are convinced that breathing helium makes you cold because they've experimented at one point or another with trimix in suit inflation gas and came to the conclusion that 'helium is a cold gas' so, of course, they get colder when they breathe it. I was never under that impression and i've never felt that i got colder on dives breathing helium.

Even if I'm correct and helium is actually a "warmer" gas to breathe, its only reduces your heat loss to exhaled gas by 17% at most on 30/30 or 21/35, which is an even smaller overall reduction in your heat loss. Diving a CCR eliminates all that loss (to first order, ignoring the diffusion of heat from the loop to the water) and adds in heat from the scrubber reaction, which would be much more noticeable...

Here's wikipedia on the heat capacities:

helium = (25 °C) 20.786 J·mol−1·K−1
nitrogen = (25 °C) (N2) 29.124 J·mol−1·K−1
oxygen = (25 °C) (O2) 29.378 J·mol−1·K−1

which actually doesn't seem quite correct, i thought it was a factor of 2, not a factor of 1.5

but all the monatomic gases like Ar, Ne, etc have heat capacities of around 20 J / (mol K), all the diatomic gases like Cl2, F2, etc have heat capacities of around 30 J / (mol K). diatomic solids like I2 or Br2 do not and you need to use a different model like treating them like an Eintstein Solid (http://en.wikipedia.org/wiki/Einstein_solid) to begin to model how their heat capacities behave.

hmm... yeah, the vibrational mode in a diatomic gas contributes 2 degrees of freedom because of a potential and kinetic energy component (giving a 7:3 ratio) but the vibrational mode tends to be quantum mechanically frozen out at lower temperatures (giving a 5:3 ratio or close to the factor of 1.5 observed in those numbers above... its all coming back to me now...).

[lamont]

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 helium is a low density, low specific heat gas. per that heat loss will be reduced.

the SF6 (sulfur hexaflouride) in that paper is 5 times denser than air, and should have a high specific heat and should be a 'cold' gas to breathe.

[lamont]

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 helium is a low density, low specific heat gas. per that heat loss will be reduced.

the SF6 (sulfur hexaflouride) in that paper is 5 times denser than air, and should have a high specific heat and should be a 'cold' gas to breathe.

hmmm... my edit button is gone so i guess i need to followup to myself...

SF6 apparently has a lot more finely arranged rotational and vibrational modes and has a heat capacity C_p of 97 J / (mol K), which is roughly ~14.5 degrees of freedom.

very chilly gas to breathe...

[Seth T.]

hmm... yeah, the vibrational mode in a diatomic gas contributes 2 degrees of freedom because of a potential and kinetic energy component (giving a 7:3 ratio) but the vibrational mode tends to be quantum mechanically frozen out at lower temperatures (giving a 5:3 ratio or close to the factor of 1.5 observed in those numbers above... its all coming back to me now...).

Whoa, we are WAY beyond casual scuba here.

[lamont]

hmm... yeah, the vibrational mode in a diatomic gas contributes 2 degrees of freedom because of a potential and kinetic energy component (giving a 7:3 ratio) but the vibrational mode tends to be quantum mechanically frozen out at lower temperatures (giving a 5:3 ratio or close to the factor of 1.5 observed in those numbers above... its all coming back to me now...).

Whoa, we are WAY beyond casual scuba here.

its actually only 200-level sophomore physics... been over 100 years since it was leading edge...

[dsteding]

hmm... yeah, the vibrational mode in a diatomic gas contributes 2 degrees of freedom because of a potential and kinetic energy component (giving a 7:3 ratio) but the vibrational mode tends to be quantum mechanically frozen out at lower temperatures (giving a 5:3 ratio or close to the factor of 1.5 observed in those numbers above... its all coming back to me now...).

Whoa, we are WAY beyond casual scuba here.

its actually only 200-level sophomore physics... been over 100 years since it was leading edge...

I've been in lurk mode here, but the scientist in me is having a tough time coming up with a rational explanation for the reports of being colder while breathing helium. Gas physics are all backwards there, for reasons that Lamont has discussed. I wonder if there is another physiological effect associated with a lower PN2 (i.e., decreased narcosis having something to do with perception of being cold).

[Sounder]

hmm... yeah, the vibrational mode in a diatomic gas contributes 2 degrees of freedom because of a potential and kinetic energy component (giving a 7:3 ratio) but the vibrational mode tends to be quantum mechanically frozen out at lower temperatures (giving a 5:3 ratio or close to the factor of 1.5 observed in those numbers above... its all coming back to me now...).

Whoa, we are WAY beyond casual scuba here.

its actually only 200-level sophomore physics... been over 100 years since it was leading edge...

I've been in lurk mode here, but the scientist in me is having a tough time coming up with a rational explanation for the reports of being colder while breathing helium. Gas physics are all backwards there, for reasons that Lamont has discussed. I wonder if there is another physiological effect associated with a lower PN2 (i.e., decreased narcosis having something to do with perception of being cold).

I'm right there with you both - this is why I initially wasn't convinced, and kept asking more. I understand the physics (I love physics - and diving is full of it) although I couldn't articulate them as nicely or plainly as Lamont did.

Steding raises an interesting question. I'd enjoy hearing from other tri-mix divers who have experienced it one way or another. CCR is kind of a beast onto itself with the exothermic sorb reaction. This is very fascinating for me.

Thank you for weighing in on this Lamont. I don't care what Bones says about you, you're an alright guy.

[lamont]

I don't care what Bones says about you, you're an alright guy.

heh... i'm going to have to start practicing stealing equipment off of him... want to get in shape for a tech1 retest and see what i can get off Sherwood...

[lamont]

I wonder if there is another physiological effect associated with a lower PN2 (i.e., decreased narcosis having something to do with perception of being cold).

I know I've described narcosis as "warm and fuzzy" before. I also recall being "stone cold sober" when me + raul ran into a 14 foot sixgill down at olive's den, and wishing i had some warm fuzzies going on..

[dsteding]

I don't care what Bones says about you, you're an alright guy.

heh... i'm going to have to start practicing stealing equipment off of him... want to get in shape for a tech1 retest and see what i can get off Sherwood...

Last dive with Bones I did he insisted on unclipping my SPG every time I checked gas. I'd clip, he'd unclip, I'd reclip.

Later, I found myself staring at his argon bottle, figured it was a nice shelf to put rocks on. Managed to unclip his SPG, and stack probably ten pounds (dry weight) of rocks on the argon bottle. Guy's buoyancy never moved, he instinctively compensated. I swam around in front of him, gave him a hold signal, and then started picking rocks off of his argon bottle and showing them to him. Good times.

I hope you find a way to steal his fins . . .

[Seth T.]

I know I've described narcosis as "warm and fuzzy" before. I also recall being "stone cold sober" when me + raul ran into a 14 foot sixgill down at olive's den, and wishing i had some warm fuzzies going on..

Whoa, that's one big fish.

[Sounder]

I know I've described narcosis as "warm and fuzzy" before. I also recall being "stone cold sober" when me + raul ran into a 14 foot sixgill down at olive's den, and wishing i had some warm fuzzies going on..

Whoa, that's one big fish.

That was a fun story to hear.

[Aquanautchuck]

I wonder if there is another physiological effect associated with a lower PN2 (i.e., decreased narcosis having something to do with perception of being cold).

I know I've described narcosis as "warm and fuzzy" before. I also recall being "stone cold sober" when me + raul ran into a 14 foot sixgill down at olive's den, and wishing i had some warm fuzzies going on..

I know that when me and Sparky say our big 6 gill I almost had some warm and fuzzes in my suit. But I do admit we had to be narced at 120' :occasion5:

[lamont]

I know I've described narcosis as "warm and fuzzy" before. I also recall being "stone cold sober" when me + raul ran into a 14 foot sixgill down at olive's den, and wishing i had some warm fuzzies going on..

Whoa, that's one big fish.

*exactly* what i said to myself...

then the stupid seal blew out the viz and scared me shitless...

[Sounder]

I know I've described narcosis as "warm and fuzzy" before. I also recall being "stone cold sober" when me + raul ran into a 14 foot sixgill down at olive's den, and wishing i had some warm fuzzies going on..

Whoa, that's one big fish.

*exactly* what i said to myself...

then the stupid seal blew out the viz and scared me shitless...

That's the best part of the story!

[Seth T.]

*exactly* what i said to myself...

then the stupid seal blew out the viz and scared me shitless...

Wait a minute! What seal?