Chris, the choice is only arbitray if there is no temp. gradient across the layers or if they are "clear" to IR and even then it isn't truly arbitrary b/c the surface area changes.

EDIT: changed "clear" to "clear" to IR

Edited by jnklein21 on 05/27/2010 09:21:29 MDT.

@Roger - I looked for this stuff online a while back and fund two published articles that looked reasonable. But I can't find them on my computer right now. One quoted "50%-70%" lost as radiation, the other said "80%". I'll try and find the articles again and post references. I know the numbers seem high but they were for a naked person in *still* air, with an ambient temp in the 20s (C) so I think I can believe it.

@James - yes, if the ground is at 40F the radiative losses will be greater. None of which changes the point that as soon as you lie down on a cold surface, conductive losses become the largest heat loss component. Add an increase in convective losses due to moving air, and some blocking of IR due to clothing, and it's not difficult to see that the radiative losses become *relatively* unimportant.

Stuart, I won't argue about which is more important. One parting shot though...lets say our guy is in his sleeping bag with a layer or two of clothing on and on a sleeping pad. Whatever he has on it is keeping him warm at 32F ambient. Oh, also, he's in a tent under trees and there is a slight breeze out.
I'll make a conservative (@least I think so) est. and say the exterior shell of his bag is 37F. With these conditions radiation losses from the bag to the tent fabric would be ~15W. This is somewhere around 20% of what the guy is generating. Try the numbers yourself (I figured e=.9 and A = 1.5)
No it isn't the majority of losses but it isn't negligle either. Also, I would bet the temp differential would be more like 10F but who knows. The most importand thing is with a IR reflector you cut that number down by something like 90% -- at a weight penalty of 1-2oz. Could this be done with 1-2 oz of high loft down, I doubt it, and definatly not as cheaply or easily

Actually James, in the situation you describe, I agree with you :)

Once you've got reasonable control over the convective and conductive losses, then you can start thinking about radiative losses and a space blanket is a cheap, simple, and effective way to do that. And as you say, it's a lot cheaper than more down - assuming you could find a source of extra down out in the field! Maybe the best solution is to walk with a spare goose...

The problems arise when people don't worry about the convective and conductive losses first and they dive for the space blanket thinking it will magically make them instantly warm even if they are wet, exposed to wind, wearing insufficient clothing, etc. Sadly, it happens - people want (expect?) a magic solution and for some reason, some people have come to view space blankets as just that.

Personally I carry a space bag (mylar bivy) even on day walks. I've never needed it for myself, but I've come across people who did need it.

I see a difficult thermal analysis problem being hacked up into small oversimplified statements with a lot of not quite relevant material being tossed around. And some "true to life" snippents being tossed on top for added flavor.

I used to work where people were doing temperature control of spacecraft with external solar heating, internal heating from electronics heat, thermal conductive blankets and radiative coatings/materials. People have been doing this analysis stuff for a very long time.

I am sure that somewhere a geek or two has used all of this proven technology to analyze the sleeping bag problem if nothing else, for his or her own amusement and education.

The reference below is maybe worthwhile to look at.


Modeling Heat and Mass Transfer
from Fabric-Covered Cylinders
Phillip Gibson
U.S. Army Natick Soldier Research, Development, & Engineering Center, Natick, Massachusetts, USA

Journal of Engineered Fibers and Fabrics 1 http://www.jeffjournal.org
Volume 4, Issue 1 – 2009 – Special Issue: MODELING

www.jeffjournal.org/papers/Volume4/4.1Gibson-Modeling_Heat.pdf

This model appears to not involve the heat transfer with a mattress or ground - but the cylinder model applies somewhat better to the arm or leg in free air.

The references are nice as a selection of what the author, part of the US Army cold weather equipment team, thinks is relevant.

I am sure that intellectual curiosity has led to published and unpublished models of humans (cylinders) in sleeping bags.

Gibson properly notes that the more geometrically challenging situations of asymmetric heat exchanges of an insulated cylinder lying on the ground, or the full body three dimensional heat exchange problem are costly in computer resources.

Still, a talented analysis with a decent home computer and a commercial thermal analysis program could make great headway.

I am suggesting that those interested might try to seek out a thermal analysis wonk who likes backpacking, and see what interest in intellectual curiosity can be excited on this topic.

James, care to elaborate on what was oversimplified? or what was irrelevant?

I have though about coming up w/ some code to iterate through these type of problems, though this process involves simplifications as well. The "technology" really isn't that fancy - the heat transfer equations are the same, the mathematics required for solution are simplified over many iterations to allow for low error.

Most of the previous discussion has been about whether or not one would benefit from IR barriers. Feasabilitly, if you will. If someone doens't believe there is a benefit after considering the Stefan-Boltzman eq. Than some computer program output shouldn't change their mind (as the program is just applying the heat transfer equation).

I would like to encourage your expressed interest in doing modeling to gain understanding.

Part of the oversimplification comes from citing Stephan-Boltzman equations and not understanding exactly to what they do and don't apply. In the climate change problem, this same difficulty arises.

Then, mixing in "personal experiences" which may not actually shed any light on the specific discussions, but these are sometimes given too much argumentative strength as is common, say - with eyewitness identification of rapists only to have DNA testing years later reveal that the semen did not come from the person identified by "eyewitness testimony" from one, two or three or more witnesses. There is a whole (but small) industry on reliability of personal experiences.

One can find limited function demo versions of finite element analysis codes online.
http://www.freebyte.com/cad/fea.htm

Some of these demo versions are said to allow 1000 nodes. Some of these analysis packages have mechanical and thermal modules.

Yes, there is certainly a percentage of the population that won't believe any computer model, and will believe what they interpret of their personal experience over and above anything that contradicts what they already believe.

In other words, there are some people to not bother trying to convince. I am very glad to be retired and not have to face those people.

We have long been bamboozled about the thermal characteristics of our outdoor gear, both by the outdoor industry, and also by the outdoor media, and by ourselves.

Look at the very long time it has taken to get a standard approach to rating sleeping bags, and then look at the holes that were left in the standard.

As you appear to well know, the lack of basic published analysis is stunning.

On the other hand, we can persist in living without the understanding that technology can provide, after all, we've gotten along "just fine" for a long time.

I have a "down and other insulation" database of about 80 Megabytes of downloaded materials, and ran out of interest and patience to read it all. It isn't like the subject of human garment insulation is some very recently discovered need, after all.

So, why is it so seemingly bad?

"I would like to encourage your expressed interest in doing modeling to gain understanding"

How could I model what I didn't already understand; as modeling is the practical application of theory.

"Part of the oversimplification comes from citing Stephan-Boltzman equations and not understanding exactly to what they do and don't apply"

The only stefan-boltzman eq. cited is the one for emitted radiation and it applies to anything with a temperature.

"Then, mixing in "personal experiences" which may not actually shed any light on the specific discussions, but these are sometimes given too much argumentative strength.."

I agree - personal experience is of little use without knowing the specific condition which effected/produced the experience.

"I have a "down and other insulation" database of about 80 Megabytes of downloaded materials"

I would be interested in seeing this...

"So, why is it so seemingly bad?"

?????