Moisture & Conductivity in Air

[Akimbo]

I just ran across an interesting article that indicates that the effect of humidity on thermal conductivity of air at temperatures found inside drysuits is negligible.

The Thermal Conductivity of Moist Air Electronics Cooling

Based on this, the practice of purging moist gas (usually perspiration) from a drysuit before jumping has no practical effect. I was unable to find equivalent data for Argon. These relative numbers on different gasses may also be interesting:

Thermal conductivity relative to air:
Air (gas) = 1.000x
Argon (gas) = 0.666x
Carbon dioxide (gas) = 0.608x
Helium (gas) = 5.916x
Hydrogen (gas) = 7.000x
Nitrogen = 1.000x
Oxygen = 1.000x
Water = 24.166x

Values converted from W/(m•K) posted on Thermal Conductivity of some common Materials​

It may be interesting to note that Nitrogen and Oxygen are virtually the same as air and therefore Nitrox. Trimix is another matter. The Helium number makes it obvious why hot water suits are mandatory for saturation divers.

I have not noticed the actual thermal conductivity value of Argon posted on this forum, but the one-third reduction over air or Nitrox looks to me like the added weight of the bottle and regulator would be better invested in lead and additional underwear.

 

[Ana]

First thing that comes to mind is that initially one purges the air out of the suit, for buoyancy reasons not to improve thermal protection.

As far as the benefits of Argon, if I'm reading your data correctly, then it makes sense. Argon's ability for heat transfer is less than air's, hence you keep your body heat longer.

 

[bleeb]

I think you have a few typos. The "%" should probably be replaced by "x".

The figure you quoted is also for liquid water (i.e. 24x more conductive than air). Interestingly, water vapor is less conductive than air (2/3), which would actually be a benefit, insulation-wise. However, I don't think the process is quite as straightforward as saying purging before jumping in has no effect. Let me do some hand waving here.

Evaporative cooling has a much larger effect than conduction. If it's warm enough that someone is perspiring on the surface and the water is cool enough to need a dry suit, once they jump in, the water vapor will start condensing on the inside of their dry suit. This allows increased perspiration evaporation from their skin, cooling the person off much faster than straight conduction. So the possible benefit of purging moist gas from a suit at the last minute wouldn't be from getting rid of the moist gas. It's from drying off the skin while the person is still warm or has a chance to warm back up before cooling down the outside of the suit.

At least that's my theory.

 

[Akimbo]

I think you have a few typos. The "%" should probably be replaced by "x"….

I stand corrected. It was correct in my spreadsheet and I hosed it up transferring to text.

 

[Akimbo]

First thing that comes to mind is that initially one purges the air out of the suit, for buoyancy reasons not to improve thermal protection.

As far as the benefits of Argon, if I'm reading your data correctly, then it makes sense. Argon's ability for heat transfer is less than air's, hence you keep your body heat longer.

The reason for the initial suit purges are for thermal conductivity. The weight difference, thus buoyancy, between Argon and Air in such small volumes at the surface is minute. Even if the weight was significant, substituting the added weight of Argon in the suit doesn’t change displacement, thus the total weight you have to carry.

However, keeping the more conductive air in the suit will dilute Argon's thermal conductivity. In round numbers, not purging air out of the suit would cost about a quarter of the one-third reduction in thermal conductivity at 99’. I could not find any relevant quantitative data on thermal conductivity of gasses at elevated pressures.

Consider that under ideal conditions you get about a one-third improvement in insulating value from Argon. Compare that weight, cost, and complexity to simply adding underwear. For easy calculating, let’s assume you need 32 Lbs of lead to get your trilam drysuit and underwear neutral in salt water. That would be half a cubic foot of displacement. A 33% increase in insulation thickness would be around 10.6 Lbs more displacement — not far from the weight of the pony bottle, valve, regulator, and clamp. You can buy a lot of underwear and lead for the cost of an Argon system — and avoid the continuing expense and hassle of filling the cylinder.

Of course, the decision is a value judgment and depends on your circumstances. I can see it for exceptional and extreme dives. It strikes me as hard to justify for burning through a set of doubles in 45° F water.

 

[Akimbo]

…
The figure you quoted is also for liquid water (i.e. 24x more conductive than air). Interestingly, water vapor is less conductive than air (2/3), which would actually be a benefit, insulation-wise. However, I don't think the process is quite as straightforward as saying purging before jumping in has no effect. Let me do some hand waving here.

Evaporative cooling has a much larger effect than conduction. If it's warm enough that someone is perspiring on the surface and the water is cool enough to need a dry suit, once they jump in, the water vapor will start condensing on the inside of their dry suit. This allows increased perspiration evaporation from their skin, cooling the person off much faster than straight conduction. So the possible benefit of purging moist gas from a suit at the last minute wouldn't be from getting rid of the moist gas. It's from drying off the skin while the person is still warm or has a chance to warm back up before cooling down the outside of the suit.

At least that's my theory.

Looking at the chart, thermal conductivity in the temperature range of sea water is virtually unaffected by humidity. The graphed paths don’t diverge significantly until you get well above body temperature.

It would be interesting to determine how much evaporative cooling occurs once you hit the water. My main reservation to your hypothesis is purging water vapor from the suit is a relatively small volume compared to the volume of liquid/perspiration in the underwear. Wouldn’t the water vapor soon be replaced?

It is clear that drysuit underwear that wicks liquid away from skin feels significantly warmer. It would be interesting to determine how much water vapor is produced in the water with this type of underwear.