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reflective coating in sleeping bags
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- This topic has 54 replies, 4 voices, and was last updated 2 years, 10 months ago by Edward John M.
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May 26, 2010 at 7:51 am #1613993
"radiation accounts for about two-thirds of thermal energy loss in cool, still air."
Unless of course the subject is lying down on a sleep mat.
May 26, 2010 at 7:56 am #1613995Please note that the Stefan Boltzmann law which determines the difference between radiation absorbed and emitted by an object only depends on the area of that object, the temperature of that object, the emissivity of that object, and the temperature of the surroundings. In our reflective sleeping bag example, the object is the person + sleeping bag and the surroundings is the ground and air.
It makes absolutely no difference whatsoever if the object is standing up or lying down nor is there any physical reason why that should matter. The radiation leaving your skin doesn't care if you are hiking, sitting, sleeping, or recently deceased. All that matters is skin temperature which admittedly does change depending on activity level.
May 26, 2010 at 7:58 am #1613997"Unless of course the subject is lying down on a sleep mat."
Right. That portion of your body doesn't radiate to the surroundings because there is no gap. That area of your body transfers heat by conduction only. Side sleepers will experience more radiative heat transfer than will back/stomach sleepers.
EDIT: Oops, let me correct myself. The portion of a person in contact with the ground will still transfer heat through radiation to the ground provided there is a temperature difference between the object and the surroundings. So if you warm the mat/pad/ground under you to exactly your skin temperature, then radiation ceases. Most likely this is not the case though and so you continue to pump out heat to the ground via radiation.
May 26, 2010 at 8:05 am #1614001Some guy somewhere told me he uses an emergency blanket on snow for a groundcloth and it is very warm. Any validity? I know I'm forgetting "facts" from other threads, but doesn't it have to be close to you to reduce radiation heat loss. (VBL properties aside)
In other words… Space blanket, then pad, then bag wouldn't make much of a difference?
May 26, 2010 at 8:17 am #1614005Chris,
Aerogel is a very poor block of IR. Aerogel's primary benefit is that the average cell size is smaller than the mean free path of air. This dramatically reduces conduction.
Another material, such as carbon black, etc., is commonly added to aerogel to achieve a better IR block. Carbon is an effective absorber of infrared radiation and, in some cases, actually increases the mechanical strength of the aerogel.
May 26, 2010 at 4:19 pm #1614167Chris,
When applying the Stefan–Boltzmann equation for the situation you described (hiker in sleeping bag) the immediate surroundings would be his jammies (assuming he's not naked). His jammies would exchange IR with the first layer of his sleeping bag, the inside layer would exchange with the bags insulation…..
James
May 26, 2010 at 6:07 pm #1614211> he uses an emergency blanket on snow for a groundcloth and it is very warm.
A bit of creative imagination there imho. In contact with the ground? One degree from the space blanket, if lucky.
Cheers
May 26, 2010 at 6:28 pm #1614220What I figured…
May 26, 2010 at 9:10 pm #1614314"A bit of creative imagination there imho. In contact with the ground? One degree from the space blanket, if lucky."
Roger, I think they have their minds made up differently.
That sounds like another creative marketing victory to me.
–B.G.–
May 26, 2010 at 9:42 pm #1614340I think the guy that told me (a co-worker) has won a victory in creative BSing
May 27, 2010 at 1:39 am #1614389James,
Not so. One can arbitrarily define the system boundary wherever they would like. You just need to be aware of what you are solving for when you do so. We could set the system boundary at the skin, the jammies, the bag's interior surface, the bag's exterior surface, the tent wall, etc.
Chris
May 27, 2010 at 3:19 am #1614394There seems to be confusion about absolute and relative losses.
Maybe this will make things clearer…
A naked person in still air loses (depending on the experimental conditions) 50% to 80% of their body heat by radiation. The absolute amount is of the order 100 Watts by radiation, the remainder by conduction and convection.
Now lay that person down on the cold ground. They will now be losing somewhere of the order of 400 Watts or more by conduction with the ground if the ground is at, say, 40F. So even though they are still radiating 100W, that is now a much smaller proportion of their total heat loss. So when a person is laying on the ground, it's much more important to deal with the conductive heat losses before worrying about the radiative losses.
As soon as you add moving air (increasing convective losses), clothing, the layers of sleeping bag, mat, etc. into the equation it all gets complicated of course, but clothing reduces radiative losses too. The essential point is this: in real world conditions, radiative losses are (relatively) small. For most purposes, you are best to focus on reducing heat lost by conduction and convection.
Hope that helps.
May 27, 2010 at 4:59 am #1614408Hi Stuart
Those numbers seem very high to me. Can you quote a good reference for them?
Otherwise I agree about the real world bit.
Cheers
May 27, 2010 at 6:48 am #1614433Stuart, a nude person laying on 40F ground would be losing much more than 100W via IR losses. Even assuming the radiation exchange, for skin not in contact w/ ground, is occuring at the immediate air temp (must be ~32F for 40F ground temp)….the total IR radiation would be much closer to 300W.
May 27, 2010 at 6:54 am #1614437Yeah Bob, I curse these creative marketing depts are every time I wake up to a frosted tarp even though ambient temps are above freezing. I wish I had been smart enough to invent radiation heat loss so I could have patented various magical products that block it.
May 27, 2010 at 7:22 am #1614446Chris, 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
May 27, 2010 at 5:03 pm #1614632@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.May 27, 2010 at 6:43 pm #1614672Stuart, 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 easilyMay 27, 2010 at 7:13 pm #1614678Actually 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.
May 28, 2010 at 6:06 am #1614785I 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, USAJournal of Engineered Fibers and Fabrics 1 http://www.jeffjournal.org
Volume 4, Issue 1 – 2009 – Special Issue: MODELINGhttp://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.
May 28, 2010 at 11:54 am #1614866James, 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).
May 31, 2010 at 7:19 am #1615382I 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.htmSome 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?
May 31, 2010 at 11:06 am #1615438"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?"
?????
May 14, 2021 at 2:35 pm #3712553I realize that this is an old thread, but I recently have tried making a “shoe box” for my feet in my sleeping bag with reflective polyester batting from JoAnn and was very happy with the result. I am posting because there are many references in the medical and physiology literature that indicate radiant heat loss is the predominant form of heat loss, and what is useful to know is that as the ambient temperature rises, the extent that radiant loss occurs diminishes. The colder it gets, the greater the extent that radiant loss is occurring. A typical value cited is 60 percent of heat loss is radiant.
May 14, 2021 at 5:49 pm #3712608Those of you who are using reflective emergency bags as a VB – what do you wear inside it?
Any sleep clothing is surely going to get soaked with sweat, but Mylar would be very unpleasant against the skin. The WarmLite folks suggest sleeping naked inside a VB (as famously illustrated in their old catalogues), but they have an inner fabric with a nicer texture.
One poster suggested allowing some ventilation at the top of the quilt, but that would lead to heat loss which would surely defeat the whole purpose?
So how does it work in practice?
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