How many Calories will you burn just by evaporating moisture from a garment?

[Tjaard Breeuwer]

I just read the newsletter with this title.

“

Thermodynamics can be used to calculate the amount of heat you can lose through evaporation of moisture that is absorbed into your base layer.

Here’s the strategy:
Estimate how much sweat you generate over the course of a 6 hour hiking day with an air temperature near freezing (RH = 60%).
Determine the water absorption capacity of your base layer.
Calculate the evaporative heat loss based on the latent heat of evaporation for water.
Calculate your net body heat generated while hiking by subtracting evaporative heat loss from heat produced as a result of moving.
Compare this to the same set of calculations for a garment with a different water absorption capacity.’

[Tjaard Breeuwer]

@Ryan,

I have some fundamental questions regarding the model.

Here are some of them:

• Whether or not moisture is absorbed into the garment, it still needs to be evaporated. If you sweat 100 grams of water, it will take a X joules to evaporate, regardless of what intermediate steps happen.
• if you are sweating, it is because you are hot, energy ‘lost’ to evaporation is the whole point! If you are not hot, you would not be sweating

Can you clarify what you meant a bit more?

I think this might be what you you are assuming, please confirm or correct me:

• Assume that even when not overdressed, we still sweat.
• assume that the baselayer will absorb to its max ability during the active parts of the day, and no drying will happen during active moments of the day
• assume that 100% of the moisture in the baselayer will evaporate during rest in the evening/night.

 

[Jerry Adams]

Interesting article

One question i have is, for example, say you sweat some amount of water that gets absorbed into the base layer

If you have a base layer that doesn’t absorb much water, maybe it will be saturated

If you have a base layer that absorbs a lot of water, maybe it will be half saturated.

Then, to dry both base layers, it takes the same amount of heat because they contain the same amount of water

Maybe what’s important is how much you sweat, not how absorbant your base layer is

Maybe with the less absorbent base layer, you’ll notice you’re sweating sooner so you’ll shed insulation or slow down and then sweat less

This is a complicated problem

 

[Jon Fong / Flat Cat Gear]

If you have a base layer that doesn’t absorb much water, maybe it will be saturated

Isn’t that the idea behond sweat suites?  The internal environment saturates and the body does not cool off.  In fact, the body produces more sweat hoping to cool off.  Thinking out loud here.

[Thom]

Someone inhaled something illegal.
Thom

[Roger Caffin]

Whether or not moisture is absorbed into the garment, it still needs to be evaporated. If you sweat 100 grams of water, it will take a X joules to evaporate, regardless of what intermediate steps happen.

Basic physics.
Cheers

[Jerry Adams]

Good points jon

Another thing is if you notice you’re sweating, then maybe you’ll do something to cool down, like remove insulation

yeah roger, the heat required to evaporate an amount of sweat is straightforward, but how much you sweat is complicated

Sometimes I’ll start sweating and ignore it and sweat more

Other times I’ll notice I’m getting warm and shed insulation before I sweat and get everything wet

[Paul McLaughlin]

However: IF the moisture you are generating passes through your clothing WHILE you are active, and cooling is a good thing, and there is effectively waste heat to power the evaporation, you are better off than if the clothing absorbs it, and then you have to power the evaporation later when you are inactive and not generating as much heat. So, while the calorie consumption modeling seems a bit doubtful to me, the benefit of less absorptive fabrics seems beyond dispute. And also the even greater importance of managing one’s layers to minimize sensible perspiration. Which is not easy to do, but a goal well worth pursuing and something that one definitely gets better at with practice.

[Jerry Adams]

Yeah, exactly, minimizing sensible perspiration is more important

If it’s warm weather then you’ll sweat regardless, which is fine

But if it’s cold and you have too much insulation and sweat a lot and get your clothes wet, then stop, you’ll be cold

I seem to remember a story about someone in the arctic that did that and died.  Maybe Jack London.  Or maybe he got wet from falling into water…

[Author]

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