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"survival" blankets
- Thread starter Hoodoo
- Start date
It is actually fairly correct.
Wrong, function of the emissivity and the 4th power of the absolute temp.
The heat loss is fairly constant as stated but the second part is totally wrong. It is a question of energy balance, lose some gain some, and the surrounding conditions strongly affect the gain part.
By letting a shallow vessel "see" only the clear sky, one can freeze water at temps markedly above freezing. Reason why deserts are cold at night.
TLM
Interesting. Kochanski says exactly the opposite. His theory is that only the true mylar ones (original style super compact space blanket) serve wells as a mirror to reflect the radiant heat. He even recommends replacing them often because they quickly oxidize or otherwise get cloudy as they are used.
He recommends using duct tape to reinforce them at the points where you are attaching them to your cordage or pins.
Of course, he recommends using them as a reflector inside his super shelter bush hippy sweatlodge thing with a nylon tarp and some clear plastic handling the bulk of the "shelter" duties. So the mylar doesn't have to withstand the wind, the wood used in making the shelter, or even the weight of your body laying on it.
I'm somewhat new to the great white north and especially new to Kochanski's ideas for the boreal forest, so I haven't tried this stuff yet.
I think the key is what you're really asking the thing to do, reflect radiant heat from a fire, reflect radiant heat escaping from your body, or just serve as a lightweight tarp.
dogboye
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I've got one Adventure Medical Kits heatsheet survival blanket for 1-2 people that I keep in my hunting kit. I also have one AMK heatsheet emergency bivvy that i keep in the .... survival?... bag of my camping equipment. Never used either, but both seem pretty sturdy. Certainly not like they are going to rip with first use.
Well, I'm not gonna contradict an experienced outdoorsman. What I meant by compact and cheap was the ones that look like a common plastic bag just painted in silver. Like the ones you find in cheap so callled survival kits.
However I gotta say, that the large and sturdy Thermal Blanket from Coghlan's is the only one I've actually seen in use in a true emergency situatios. And the impression it gave me is that it was pretty effective. The EMT's I talked to that day, told me they preferred it to the little cheap one (also from Coghlan's) since it is better constructed and has more thermal capability. It takes a lot more space and it is almost thrice as expensive but I think it's worth it.
Also I think It is important for the blanket to be sturdy and resistant, since I'd be using it for an emergency/survival situation, and that means not only thermal reflection but also, shelter and/or rain cover and building an improvised stretcher or whatever, hence, for me, construction matters, even if it takes more space.
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Absolute loss of photons will not change as you state. I kind of get what you are saying about net loss of heat if the surrounding environment is providing heat. This may occur through scattered photons and or convection. However, gain of heat by either of the above mechanisms does not match your analogy statement in the second sentence.
Your analogy is better explained by a different mechanism. You can get freezing water (if it is a very shallow puddle) on a clear night that is slightly above zero degrees celsius. This has to do with fact that on clear sky nights (especially in the desert) you have very low to near zero humidity. The puddle of water evaporates and the loss of water molecules from puddle surface has a cooling effect. If you are close, but still slightly above zero degrees Celsius, the cooling effect associated with evaporation can drop the water temperature below freezing. Evaporative cooling uses over 500 calories per gram of water lost to vaporization. The rate of evaporation and hence the cooling effect is inversely correlated to humidity, such that zero evaporation occurs at 100% humidity. The above mechanism is thus largely about relative humidity of the air and not about having a 'clear view of the sky'. The same process happens at the surface of our skin.
I should clarify my comments on photon characteristics being related to temperature and elemental composition since at a given temperature a characteristic wavelength spectrum or set of emission lines is produced that are peculiar to the atoms making up the body. This is the quantum effect of electrons dropping into lower energy orbitals and the wavelength being produced is a function of the change in orbital that takes place. The possible valance states that can exist vary from one atom to the next and hence individual atoms produce characteristic emission lines of photons when they radiate energy. This is why we use lamps of different materials to produce different wavelength spectra even though the different types of lamps operate at relatively speaking over a fairly narrow range of temperatures. It is correct that at higher temperature more photons are produced, but this is just sort of nitpicking on my error made for a general simplicity - the assumption I made in my earlier post was that humans are largely made of the same stuff and exist at the same temperature so their IR emissions are generally the same. This is not changed by being able to see the sky!
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It does, the tray is made to "see" only the sky. The radiation balance is then counted to the space through the atmosphere. The data I have seen shows that on clear dry nights the water freezes when ambient temp is upto 282 K.
The emission spectrum pretty well averages out on clothes and skin so the simple radiation law holds remarkably well.
As I said the loss is independent of the surroundings but the what one even feels on the skin is the energy balance, that is also what keeps one warm. This effect can easily be observed on winter nights indoors when the house has IR selective glazing installed.
TLM
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I'm being a little bit mean here using the quotation feature to demonstrate that you have changed your original assertion switching from clear nights causing an increase in radiation loss to clear nights being deficit in heat inputs. It is mean and a bit unfair of me to point this out because the first quote is 9 years old (2001) and the next quote is yesterday. Heck, I change my mind faster than I do my underwear so this is a really cheap shot on my part (but sometimes I'm not above that)
Alright, now lets get back to your data that indicates water freezes at 9 degrees Celcius. If we agree that net radiation loss is independent of surroundings and a function of the body itself and its temperature, something we both said, then you have to logically concede that the mechanism I proposed for evaporative cooling is the best explanation for your example of ice freezing on a clear night. To deny this would be to suggest that it is the 'lack of heat inputs' on the clear night is what causes the ice to freeze and that the actual freezing point of water is 9 degrees Celsius not zero degrees Celsius. This would be factually incorrect.
However, I'm willing to open my mind to your point of view when you are talking about potential heat inputs being related to environmental conditions. Lets produce the following thought experiment.
You are in an empty barn at night. The barn walls act as black body radiators themselves so you receive IR inputs from them. Initially, the walls may be at a higher temperature than the air because of their heat capacity and absorbing sunlight. In this case, I suggest you are better off hugging the walls to attain heat by conduction then you would soaking up its IR by sitting in the center of the barn. After the barn has achieved equilibrium with the outside air then the only source of heat left would be the small amount of IR still lost from the barn because it is still above absolute zero. Likewise, if its cloudy (and the same humidity as a clear night). Then the clouds will contribute some IR radiation back to the earth's surface because the water vapour of the clouds are also above absolute zero. The question is, do you get more IR sitting in the center of the barn on a cloudy night compared to the clear night when temperature and air humidity at the earth's surface are the same? I would agree it is plausible that the barn can intercept IR from the clouds and this could facilitate a small relay of additional IR from the barn walls back to you sitting in the center of the barn. I'm not sure this would be all that apparent though.
Finally, with the IR-reflective coating on insulation made mention of. I agree that this works and it works great. The question is does it work all that much better when it is cloudy compared to when it is clear? I think the difference under the two scenarios would be trivial.
As to the original elevation argument. How does this change heat input? Elevation has very strong effects on the saturation capacity of air for water altering humidity for a given water content of air, but this reflects the evaporative cooling mechanism not the radiation mechanism. The question is, do the clouds contribute more IR when you are closer to them than further away from them? Perhaps, depending on how much IR is absorbed by the small amount of gas molecules present in air in between you and the clouds.
Anyhow, I'm having fun with this debate and appreciate your making me think TLM...It is a fun exercise and this why I like coming to this place!
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Replace the original with "net heat loss", I perfectly well know the physics involved, not much point in trying to explain it in too many words.
Not really it still freezes at 0 C, just that the conditions allow it to cool well below ambient air temp.
Water vapor is itself a IR radiator, small droplets are even better, that is the reason why the sky is colder from high up, less water between you and the cold space.
As to the windows, there is not much difference between a clear and cloudy weather, just as long as the outside is at -20 and the inside at +20. The 4th power does it's work as the windows mostly see solid matter, gases have low emissivities.
TLM
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I think this is one of the oldest thread revivals I've seen! But it's a good topic. I have used the space blankets (non survival) and they work, but you have to be gentle with them. I sometimes carry the red/silver non-disposable version which is much more heavy duty but takes up more room.
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Hi TLM....I think we are achieving agreement here.
As to the 'too many words', we are having a public debate on an open forum and not a private conversation. It would be rude if neither of us tried to present some of our argument in a manner that could be followed by more than just the two of us. Also it took me a few posts to tease out additional information from you to figure out what you mean. If I suffer from verbal diarrhea, I think you must be constipated
On the Clouds bouncing back IR - I think we've achieved agreement here.
On water puddle freezing above zero degrees celcius we still seem to be at odds. As I indicated in my prior post, the freezing above zero phenomena has to occur because of accelerated heat loss not due to heat inputs. I do not believe that water at 9 degrees Celsius would lose heat by radiation fast enough for this to account for surface freezing. I think it is more plausible that evaporative cooling will account for this under conditions of light to moderate wind and zero humidity which maximize water evaporation rates. Recognizing that >500 calories are extracted from the water surface per gram of water evaporated, and it is not unrealistic that a fairly large mass of water is evaporated if the surface area is high. Again, we probably would have to both do some computational calculations to demonstrate the types energy and mass balance conditions necessary to realize a drop in temperature of the boundary layer at the water surface from 9 degree C to 0 degrees C.
I'm game for developing the volatilization model scenario and running an actual computation if you set up an equivalent radiation model and then we can run a parallel set of calculations. We of course have to establish an agreement of physical characteristics of the system we are modelling. I am too lazy to run both simulation sets by myself and quite frankly I would have a harder time setting up the radiation calculations. The volatilization model is easier for me to do.
I have used one of those cheap type of space blankets in a "survival" situation. I use the term loosely here since I always go prepared. I went to the cascades as usual in the fall, with a few people. One of them was new to backpacking and camping all together really. Not including car camping. He said he just got a new tent and we could share it.
Turns out it was his little girls tent and was about 5ftx5ft. I'm 6'3" and I have to stretch out. The first night I slept out on the ground, but the second night it was snowing so I snuggled up to a log and used the blanket as a shelter top. I used some 550 cord and rocks in the corners of the tarp. It worked really well. Yes it was a POS i was surprised how thin it was. I don't know if it saved my life but it was at least 30of outside and my old bag was rated at about 32of.
I will never trust gear that I have never seen. I now carry a one person, one pole rain fly/footprint set up that I like very much, and I still carry an emergency cheap-O blanket if only to put on my survival shelter for some extra rain proofing under the pine bows. I'd say anything is better than nothing!
Turns out it was his little girls tent and was about 5ftx5ft. I'm 6'3" and I have to stretch out. The first night I slept out on the ground, but the second night it was snowing so I snuggled up to a log and used the blanket as a shelter top. I used some 550 cord and rocks in the corners of the tarp. It worked really well. Yes it was a POS i was surprised how thin it was. I don't know if it saved my life but it was at least 30of outside and my old bag was rated at about 32of.
I will never trust gear that I have never seen. I now carry a one person, one pole rain fly/footprint set up that I like very much, and I still carry an emergency cheap-O blanket if only to put on my survival shelter for some extra rain proofing under the pine bows. I'd say anything is better than nothing!
xbxb
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I'm a little older than you guys I suppose so my Space blanket is a Poncho and a Danish army wool blanket. Yes it's heavy, but I don't walk as far as I used to and when I stop I want to be warm and dry. Maybe age will determine some of these answers. It will be interesting to see. I have enjoyed the comments.
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That is a great combo no doubt! While heavy, it meets insulation needs. The typical mylar (or better Heatsheats because of their durability) blankets were designed specifically to be able to fit in a front pocket or your tacklebox so compactness and lightness is the primary attribute of them. They aren't nearly as warm as a blanket/poncho in the same way that one of those dogtag knives isn't quite as good as fixed blade survival knife. However, when faced with an unexpected dunking and that blanket is the only thing you have dry then they can work. I think those thicker ones present some interesting options as a tarp the improve their versatility but at the expense of compact/light weight carry.
when day hiking I prefer to carry both a 2 person heatsheet and a AMK thermo bivy
my most likely shelter scenario is debris shelter- I would use the heatsheet over the frame to keep mositure out (rain or snow) and help w/ keeping heat in, the bivy would be used to crawl into (w/ a very thick layer of natural materials underneat!!
)
the heatsheet could also serve as a poncho w/ a little cutting and some cordage- for a couple of ounces and a $5 bill, there really isn't any reason not to have one anytime we venture into the backcountry
get the 2 person one- it's only a fraction heavier and a less than a dollar more
my most likely shelter scenario is debris shelter- I would use the heatsheet over the frame to keep mositure out (rain or snow) and help w/ keeping heat in, the bivy would be used to crawl into (w/ a very thick layer of natural materials underneat!!
)the heatsheet could also serve as a poncho w/ a little cutting and some cordage- for a couple of ounces and a $5 bill, there really isn't any reason not to have one anytime we venture into the backcountry
get the 2 person one- it's only a fraction heavier and a less than a dollar more
Honestly I didn't bother to read all the details of your conversation but as far as I know the whole radiation thing is governed by the Stefan–Boltzmann law (considering for simplicity solids to be "black bodies") so energy loss by radiation is:
j=sigma*T^4
sigma being the Stefan–Boltzmann constant.
Temperature being measured in kelvin degrees (starting at absolute zero which is -270 something Celsius degrees).
Multiplied by some surface factor.
Now difference between clear sky is and cloudy sky is that:
* with cloudy sky, cloud do radiate back, even if those where near or below freezing, that's still 270^4 something
* clear sky doesn't radiate anything back: all radiation is lost to outerspace. That's almost 0.
Really don't want to redo the computing but I did the exercise during my physics studies time and bottom line was yes it was significant.
In the same idea, having something above your head, even if the thing is very cold does make the difference. And from my experience that's something easily verified.
Now about mylar blanket, they'll radiate back a large portion of the energy you radiate away. As radiation is an exponential of temperature, it will be much better near skin or some energy source, less efficient on top of insulation.
So I'm rather with TLM on this one (whatever he exactly said). -although not so sure about the altitude thing (except if it means sky is less cloudy, or something about "air thickness").
Hope this make some sense.
j=sigma*T^4
sigma being the Stefan–Boltzmann constant.
Temperature being measured in kelvin degrees (starting at absolute zero which is -270 something Celsius degrees).
Multiplied by some surface factor.
Now difference between clear sky is and cloudy sky is that:
* with cloudy sky, cloud do radiate back, even if those where near or below freezing, that's still 270^4 something
* clear sky doesn't radiate anything back: all radiation is lost to outerspace. That's almost 0.
Really don't want to redo the computing but I did the exercise during my physics studies time and bottom line was yes it was significant.
In the same idea, having something above your head, even if the thing is very cold does make the difference. And from my experience that's something easily verified.
Now about mylar blanket, they'll radiate back a large portion of the energy you radiate away. As radiation is an exponential of temperature, it will be much better near skin or some energy source, less efficient on top of insulation.
So I'm rather with TLM on this one (whatever he exactly said). -although not so sure about the altitude thing (except if it means sky is less cloudy, or something about "air thickness").
Hope this make some sense.
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Actually the whole point would be that "small amount" is not that small. The temperature difference between air and the barn plays almost no difference in the regard of radiation (considering that air itself does radiate -and/or reflect- very little energy).
In that respect I would rather be concerned about how much "solid angle" the barn will cover (better being in the middle of the barn than near some openings (and I'm not considering convection here).
Actually water will always freezing temperature will remain about 0°C. The fact air is say 9°C doesn't mean water can't be 0°C.
Fact is if water is loosing energy through radiation it will start sucking heat from the air. Now air/water thermal interface isn't that great so it is perfectly possible that water looses energy quicker it can get some from the air. Loss energy would mean (see the thermodynamic temperature formula related to brownian movement) temperature drop.
Answer would be: it's mostly about "solid angle" intercepted by clouds.