Thermal properties of helium

[wedivebc]

Thanks, so what I surmise from that is helium mixtures remove slightly less heat from the body (166W vs 172W). Is it fair to assume then that for trimix the ratio would fall somewhere in between depending on the amount of helium in the gas?

I believe there no theory to prove here, just simple calculations based on well known parameters

He has higher conductivity than air:
Thermal Conductivity of some common Materials

He has also higher heat capacity than air:
Gases - Specific Heat Capacities and Individual Gas Constants

When breathing both get heated up to same temperature, so we can completely ignore conductivity.

Lots of scuba math below.

one cuft of air weights 0.0807lbs
one cuft of He weights 0.011lbs

one human breath is roughly 1/6th of cuft

Amount of energy required to heat up ~1 lbs of He by 1 degree Kelvin is 2500 Joules. 1 Joule is expending energy of 1 Watt per second.

We are heating up from say 32F to 90F which is 36 K. So total energy to heat up 1lb of He from freezing point to human body temperature is 36 * 2500.

1 lb at normobaric is roughly 90 cuft.
90 cuft is roughly 540 breaths

so to heat up one breath we need 36 * 2500 / 540 Watts total. = 166W.

same calculations for air render = 172 Watts (it has lower heat capacity but each breath weights more)

One breath lasts 15 seconds? 10 seconds? I have seen various estimates of how much radiation human body expels in a second and the lowest I come across was 50W. So not only difference is small, but also it seems that we radiate way more than needed anyways.

 

[rjack321]

Not sure about Lobstah's actual numbers but this was my understanding as well.

Helium takes a little less energy to heat up from outlet temp to body temp. So for a given breath you lose a little less heat breathing trimix vs. air or nitrox.

 

[lobstah]

Thanks, so what I surmise from that is helium mixtures remove slightly less heat from the body (166W vs 172W). Is it fair to assume then that for trimix the ratio would fall somewhere in between depending on the amount of helium in the gas?

It should be Joules not Watts. I got brainfarted after reading some mumbo jumbo on some diet site when looking for heat normally expelled by human. And if you want to make it legible you should probably go all metric or all imperial. And checked for more errors . I'm not even sure if Joules exist in imperial?

And I'm strong believer in empirical testing (or lazy). Never really bothered to calculate it before, let alone find the formula. Statement that He is not really doing anything always seemed to be on a money.

 

[lamont]

the molar specific heat of helium at constant volume is 12.5 J/mol-K

the molar specific heat of oxygen at constant volume is 21.1 J/mol-K

the molar specific heat of nitrogen at constant volume is 20.6 J/mol-K

i get that the ratio should be closer to only 62% of the heat capacity lost in every breath due to helium compared to O2/N2.

this makes sense because helium only has 3 degrees of freedom, oxygen and nitrogen molecules have 5 degrees of freedom, 2 of them internal. ideally, the ratio should be 60%.

i expect that also means that helium is a little colder when it comes out of the tank, too, though -- so it probably more or less comes out in the wash.

 

[TSandM]

Nothing to add to the physics, but a normal ventilatory rate in an adult is about 12 breaths per minute, so that would be 5 seconds per breath. On scuba, it might run a little lower, because we deliberately increase tidal volume.

 

[gsk3]

When breathing both get heated up to same temperature, so we can completely ignore conductivity.

This part is a heck of an assumption and may be driving your conclusions. Without calculating anything, if you assume both gases reach equilibrium (body) temp, then the thermal mass effects will dominate. Since He has lower thermal mass, it will transfer less heat. But what if they don't? Then since He heats up faster, the total heat carried (thermal mass * temperature) is ambiguous without calculation, and may well be higher once calculated.

So...where's the assumption that both gases reach equilibrium temp within the span of one breath coming from?

 

[wedivebc]

It is fairly well established that other than slight variance people breathing air exhale at a slightly lower temp than their normal body temp. The main references I found were studies in pulmonary disease where they have been trying to establish a link between exhaled breath temp and athsma. I have not found any data relating to temp of helium gas exhaled but I don't think it would be a huge assumption that based on the fact that helium is a better condutor of heat it would achieve thermal equilibrium quicker than air therefore the temp should be the same as body and the same as air based exhaled breath

 

[gsk3]

If air makes it to 90 then presumably He makes it to a higher temp. Erefore carries more heat away. Hard to tell how much that matters without calculating.

 

[wedivebc]

If air makes it to 90 then presumably He makes it to a higher temp. Erefore carries more heat away. Hard to tell how much that matters without calculating.

Now who is making the huge assumption?

 

[gsk3]

Now who is making the huge assumption?

Um, lobstah, right after the assumption I was pointing out might be problematic:

When breathing both get heated up to same temperature, so we can completely ignore conductivity.

...

We are heating up from say 32F to 90F which is 36 K. So total energy to heat up 1lb of He from freezing point to human body temperature is 36 * 2500.