|WikiProject Medicine||(Rated Start-class, Low-importance)|
space blankets are also used by a lot of gardeners: they're put down as weed control layers (the plant you want have to grow gets planted through a little hole, insect control (many bugs instinctively go to the bottom of a leaf--if light is relecting up, they can't find the "bottom"), reflecting light in shadier areas, and it's bright and light enough to blow if you hang up strips of it to scare birds. This is common enough that you can often find space blankets sold at garden and farm stores.
- I currently work for a company that makes thermal imagers, I've got qualifications in physics, and was into hiking at university.
These 'blankets' reflect most of your radiated heat. But most of your heat loss is not radiated - it is conducted. Most of that through your exposed extremities like hands and head. Look at extreme mountaineers - they lose fingers and toes to frostbite. Feet get the most conduction to the snow. I never heard of other bits getting frostbite, but that's not a mountaineering story you would bring up eh guys?
I keep reading about space blankets being clever technology, but no hiker I met had much faith in them. Most considered them worse than useless, because people got false feelings of security from their magic hi-tech shiny space blanket. I never heard anyone say "My space blanket was lovely and warm!". Maybe because they died of hypothermia before they could demand a refund.
I considered getting a workmate to take an infra-red photo of me with various amounts of clothing, with and without a space blanket. Apart from being a bit kinky, it is effing cold right now.
I looked around for an alternative guinea pig (not a real one), and noticed my lunch: a hot bacon roll in aluminium foil. Different kind of pig, dead and fried. :-) Space blankets are just very thin aluminium foil, with plastic backing.
I can state that it looked very 'dark' in radiated heat, not surprising as shiny aluminium is not very emissive. Unwrapped a bit, the contents were glowing. So thermal radiation was being reduced.
However, it felt hot to the touch, so conduction was not being reduced. Ten minutes and it had gone from piping hot to lukewarm.SpareHeadOne 23:09, 1 December 2010 (UTC)
- OK, so what we need is published, reliable data to say this. Wikipedia is based on things that can be cited as a secondary or tertiary source. Primary sources aren't acceptable, but if you publish in a peer-reviewed journal, or you know someone that has, then we can add all that information in. OwainDavies (about)(talk) edited at 20:23, 29 November 2010 (UTC)
- Do we really need a peer-reviewed reference for information this basic?
I'd have thought this was common sense - I do not know of any source of cold weather clothing that bothers with radiated heat. Everything I have seen involves trapping layers of air next to the body. Nobody wears aluminium foil clothes to keep warm. Space blankets are just very thin aluminium film on a plastic sheet. You can easily prove how ineffective it is by wrapping yourself in foil (or plastic sheet) and going out in the cold. SpareHeadOne 23:09, 1 December 2010 (UTC)
- It doesn't have to be peer reviewed, just reliable. Peer reviewed is the gold standard, but if you've got external reliable sources from other places (text books, reputable news agencies - that sort of thing, but not company promotional websites or other community type websites) then that's absolutely fine. I know it can be frustrating, but key in WP is that we don't print what is true, but what we can prove to be true (see wp:nor, wp:v and wp:cite for more information). I hope that helps, and you may be able to find some sources for it with a simple Google search. If you want more info or help, just let me know. OwainDavies (about)(talk) edited at 13:40, 1 December 2010 (UTC)
- One can take things to absurd levels, like Mr. Lawrence Logic does in Viz Comic.
If I say hydrogen burns with a hot flame, that would be acceptable and credible. If I say hydrogen burns at a particular temperature, then I need a source for that figure. If I say hydrogen makes things cold when it burns (contrary to common belief), then I need backup.
It is common knowledge that on Earth, heat loss by conduction and convection is far greater than radiation. The vacuum flask proves this all the time. The glass ones often have silvering to reflect radiated heat, but stainless steel ones don't take silvering like glass. A boiling hot cup of tea in a typical room goes 'cold' in about 10 to 20 minutes. In a vacuum flask, it is still piping hot 8 or 9 hours later.
Do you really need a scientific citation for that?
You might want one for figures on the actual percentages.
Quoting the article: http://en.wikipedia.org/wiki/Vacuum_flask "At the temperatures for which vacuum flasks are used (usually below the boiling point of water), and with the use of reflective coatings, there is little infrared (radiative) transfer"
Thermal radiation is proportional to the 4th power of the absolute temperature. So one can feel heat radiated from something as hot as an electric fire, or light bulb filament if a few inches away. I don't know anyone who can feels warmth radiating from a human. SpareHeadOne 23:09, 1 December 2010 (UTC)
- BTW I tried calculating the thermal black-body radiation of a human body at 37C and got something like 563W, which didn't sound credible. Then I considered that the surroundings at 21C were radiating back, so the actual power loss would be the difference. Even so, I got 190W. I heard that humans are approximately 100W, so it is not credible that humans radiate twice as much power as they consume. The figures probably need adjusting for that fact that people lose less heat when they are normally clothed. SpareHeadOne 23:09, 1 December 2010 (UTC)
- maybe it is because humans are not perfect black-body emitters. btw, I wouldn't do so much assuming about what is common-sense. E.g. a sufficient reason for most hikers to not rely on aluminium is that that wearing flimsy non-breathable material is unpleasant.
- If anyone can provide authoritative info on the relative importances of convection, conduction, evaporation, radiation, and emissivity, I for one would be very interested.
claims about space blankets
I've seen many claims that space blankets reflect high percentages of body heat(i.e. 80%), but have not seen any scientific literature supporting this, and find it highly questionable if not unlikely. Outdoorvegan 02:41, 23 April 2006 (UTC)
- Agreed. I think a lot of the confusion is that it reflects (supposedly) 80% of radiated heat, whereas often most heat is actually lost through convection or conduction, not radiation... or something like that. I'll try and find a reference for it. — pmcm 22:38, 24 April 2006 (UTC)
Data on space blankets
It's interesting to see almost my exact thoughts on this subject. I am a scientist (and climber) and have been considering using a "space blanket" for an ultralight back-country marathon. However, I don't want to be the guinea pig. I suppose I could test the material by using it in my backyard first, but I thought of all things, there would likely be data available. Alas, I have yet to locate any. I imagine the US patent office probably has the original information but I have yet to search there. Count me in for another person looking for real data on the subject.
See this sight
- This is not a "space blanket" as discussed here (i.e. single layer of aluminium coated plastic). If you look at the site, it is actually several layers of material designed to trap air and so reduce conduction/convention. One layer is coated to reflect radiated heat, but I reckon that might just add a proportion of the total insulating ability.
Blizzard rescue blankets use Reflexcell(TM) ... see Thermal Performance & vs Alternatives, although this source might not be considered Secondary, It does gain good provenance from being a supplier to the UK Military. Exit2DOS2000•T•C• 03:33, 10 October 2008 (UTC) MPIOutdoors is now Grabber Outdoors. It should be noted that "Space Blanket" is a registered trademark of Grabber, Inc. and should be cited appropriately. —Preceding unsigned comment added by 220.127.116.11 (talk) 15:00, 29 January 2010 (UTC)
- I wouldn't cite the military as an endorsement. These are the people who bought millions of pounds to a UK company for 'bomb detectors' that were shown to be completely useless. The manufacturer said they were based on the same principles as 'dowsing'. The military also handed over a huge sum of 'protection money' to a fake Taliban leader! SpareHeadOne 23:11, 1 December 2010 (UTC)
Metalized side advised to be used as the inward layer to reduce radiative heat loss via reflection? Why?
I don't understand the above-indicated contention in the article (marked citation needed currently). If the metalized surface reduces radiative heat loss by reflecting it, then what difference does it make which side is the outer side? The layer will reflect regardless, its not like it disappears if you put it on the outside instead of the inside. Does the article claim it somehow acts as a one way radiation reflector? I could understand its preferred placement on the side of incoming radiation when you wish to wish to use it to prevent radiative energy from raising the temp of an object (on infrared telescopes in space, etc.) as you'd want the reflective layer to be the outermost layer to prevent radiation from reaching a portion of the insulated object(in this case the substrate of the blanket) which could absorb such and pass heat through the metallized layer via conduction, but this mechanism doesn't seem relevant in this application, where heat is sought to be retained and conductive losses greatly surpass radiative losses in magnitude- esp since the metallized surface pointing in would if anything increase conduction to the blanket itself. Does this make sense to anyone? The only explanation I can see for a side-preference is: a) the metallic side facing inward would reduce environmental abrasion of the metallic coating, increasing durability. Hopefully someone can comment or explain any mistakes I've made. --Δζ (talk) 04:07, 21 December 2009 (UTC)
I think that the 'gold' surface on some sheets is there for search and rescue reasons, as gold against white snow is more visible than silver. However, the gold surface used as the user facing surface may also decrease infrared heat loss. What I don't understand, is why make a sheet with two different reflective surfaces, if a gold surface is better than silver ... one obvious reason, I guess, would be cost. —Preceding unsigned comment added by 18.104.22.168 (talk) 09:55, 9 January 2010 (UTC)
The difference in reflection from those two different sides is negligible. Until someone can cite proper research, I'll simply delete the sentence claiming a difference between the two sides 22.214.171.124 (talk) 23:06, 28 January 2010 (UTC)
- My intuition suggests that the metallised side would reflect better, because the IR did not have to travel through the plastic. I know the plastic looks transparent in visible light, but things are not the same in IR. IR lenses made in germanium are transparent to IR and opaque to visible light. Ordinary glass is transparent to visible, and opaque to IR. SpareHeadOne 23:13, 1 December 2010 (UTC)
- I response to Δζ's comment, I can state that it makes an enormous amount of difference which side the coating is on, depending on the wavelength. For example, silver and aluminium are excellent reflectors of visible light, and remain so on either side of a sheet of glass which is transparent to visible light. I took an infra-red camera into my company gent's toilet with the lights off (in the name of science and not some well-dodgy reason). I could see my hand well, but only a very feint reflection in the mirror. This is a reflection from the surface of the glass, not the metal underneath. This is analogous to seeing a dark reflection on the surface of a highly-polished sheet of black glass. In infra-red, optically clear glass looks like black glass, and a metal film on the back surface is irrelevant.SpareHeadOne 01:49, 11 December 2010 (UTC)
- Regarding the unsigned IP's post about gold versus silver, I suspect the vendors have simply added yellow colouring to the mylar to give the appearance of gold. If it looks like a more valuable metal, people will generally perceive it as a higher value item and might pay accordingly more. Even if it means the product is less reflective in practice.
Soviet space program
Does anyone know if the Soviets copied the idea for use by their cosmonauts? I assume they are standard now on all space missions, now that the Cold War is over and they are commonly available in stores. GBC (talk) 15:20, 22 February 2010 (UTC)
"Civilian versions may be ..."
'Civilian' as opposed to what? One would expect this to be used to differentiate from 'military', but that obviously isn't the case for NASA, and other space agencies. Nor would 'commercial' or 'professional' make any sense. I've replaced it with "They may be...".