*** Space Blanket Effectiveness *** Tue Jan 31 18:41:56 1995 Message : #21519601 From: Gary Jensen Address : gjensen@sysdiv.sdl.usu.edu Group : Usenet.rec.backcountry Subject : Re: space blanket Org. : Space Dynamics Laboratory, Utah State University (USU) Has anyone ever heard of putting your space blanket on the outside of your sleeping bag? Correct me if I'm wrong, but doesn't the blanket relect heat from body to the blanket and back to the body? I heard this in a backcountry class at school and was stumped. I had never heard of that and it seemed to defeat the purpose. Oh well. After reading the other posts, I just had to answer... Heat is transfered to or from your body in three ways: Conduction, convection, and radiation. Conduction is (in general) a transfer of heat energy within an object or between two objects in direct contact. If you were to stick your tongue on a cold ski pole, the heat would be conducted out of the water on (and in) your tongue (since water is a great thermal conductor) and the water would freeze. Convection is the transfer of heat energy due to fluid flow, such as when the wind (a fluid) blows across your dry skin. Radiation is the transfer of thermal energy due to the electromagnetic radiation that a warm body gives off. It depends on the temperature of the body, and the temperature of the surroundings that the body is radiating into. If you were go into a cave so that you were surrounded by rock that had a temperature of 4 degees C (40F), your exposed skin would radiate about 20 milliwatts per square centimeter. All of this radiation would functionally be in the infrared region of the spectrum. Humans just don't radiate many visible or radio waves. These are the basic things you must fight in the woods to stay warm. We usually fight conduction with sleeping pads that are made of plastic foam (not very conductive) with lots of dead air space (trapped air that cannot circulate and thus convect heat). Convection we solve with wind- breakers and down coats (again trapped air). Radiation we usually solve just by keeping the body covered so that it radiates into our clothing, and our clothing radiates back into our body. Space blankets are usually made of mylar (a plastic) that has a very thin coating applied to one or both sides. It is used by the space industry to control radiative heat flow. Since in space there is not enough atmosphere to allow convective transfer, and usually your satellite is free floating so that there is no conductive transfer, radiative control is all that is required. Wrapping a satellite in aluminized mylar reflects 96% or so of the energy (visible and infrared) that reaches it from direct solar illumiation, reflected solar off of the earth, and infrared radiation from the earth. So if you wrapped your sleeping bag in yur space blanket it would keep the wind off of you and help solve convective transfer, and the fact that it would reflect back about 96% of the heat that your sleeping bag gave off would help solve radiative transfer. How much would it help? That depends on your sleeping bag, the temperature of the surroundings, how hard the wind was blowing and if you had something else to keep the wind off of you, and so on. But if I was cold enough to be awake wondering about such things, I'd try it and see. Gary Jensen Space Dynamics Lab Utah State University Logan, UT Thu Feb 2 16:12:38 1995 Message : #21605452 From: Woodrow C. Henderson Address : g9uwch@fnma.COM Group : Usenet.rec.backcountry Subject : Re: space blanket Org. : Fannie Mae I've been using one as a ground cloth and wrapping it over me if I get real cold since the mid '60's. They work great! Stake one up behind you (reflecting side towards the fire) and you can bake your "cold side" by an open fire. Put the reflecting side "up" and it makes a cool spot out of the sun. I put one in the stuff sack of every sleeping bag just to ensure I don't forget it. The "space blanket" I'm referring to is the heavy-duty one with the gromets in the corners and the nylon strands meshed with the alum. between to sheets of plastic. Clay Sat Feb 4 16:09:44 1995 Message : #21692912 From: Nick Maclaren Address : nmm1@cus.cam.ac.uk Group : Usenet.rec.backcountry Subject : Re: space blanket Org. : University of Cambridge, England In article , wrote: Radiant heat loss is dependent on the two temperatures that "see" each other, so if you are sleeping out under the stars where your 0 degree outside of the sleeping bag sees -270 degree stars then there will be a lot of radiant heat loss. The difference between your 0 degree bag and -10 degree inside wall of your tent won't make much difference at all. I am sorry, but you have forgotten your elementary physics! What you say is true, but irrelevant. Radiant heat loss is proportional to the FOURTH power of the temperature of the object, which is why the effect can be ignored for humans. The reason that you get colder on clear nights is that radiation cools the earth (with a much larger area), which cools the air, which cools you. Your direct radiation loss is piffling by comparison. Nick Maclaren, University of Cambridge Computer Laboratory, New Museums Site, Pembroke Street, Cambridge CB2 3QG, England. Email: nmm1@ucs.cam.ac.uk Tel.: +44 1223 334761 Fax: +44 1223 334679 Sat Feb 4 10:45:06 1995 Message : #21682959 From: huberman@nando.net Group : Usenet.rec.backcountry Subject : Re: space blanket Org. : News & Observer Public Access There is some confusion here between radiant and evaporative heat loss. Radiant heat loss is dependent on the two temperatures that "see" each other, so if you are sleeping out under the stars where your 0 degree outside of the sleeping bag sees -270 degree stars then there will be a lot of radiant heat loss. The difference between your 0 degree bag and -10 degree inside wall of your tent won't make much difference at all. Evaporative heat loss is caused by evaporation of moisture. Moisture evaporates from your skin because the surrounding air is so dry. If that moisture is kept near the skin with a vapor barrier then the humidity will rise and the moisture loss will become very low. Moisture moving out through the sleeping bag will eventually reach a colder section of the insulation that is below the dew point, and will condense. Then it will evaporate slowly. This moisture in the bag will increase the conductive heat loss, which moves the dew point in closer to you etc. The vapor barrier doesn't have to be perfect. It needs to reduce the evaporative heat loss at the skin to less than the the evaporation rate at the place in the bag where the dew point is. That will keep the moisture from building up in the bag. Just remember that vapor barriers are only comfortable in cold temperatures. If you begin to overheat and sweat then you better ventilate. Also one problem that people frequently encounter when first using vapor barriers is they go to bed when they are cold, so the VB is just right, then in the middle of the night they overheat because they have too much insulation for the temperature.