What Is R Value
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 Ken Helwig (kennyhel77) - MLife Locale: Scotts Valley CA via San Jose, CA What Is R Value on 01/26/2006 16:39:45 MST I know that a numerical number is used for determining an "R" value when talking about sleeping pads and such. What is that scale and what is considered cold and on the other side of the spectrum, what is warm?I know it is a strange question but it has me wondering, anyone?
 Michael Martin (MikeMartin) - BPL Staff - MLife Locale: North Idaho Re: What Is R Value on 01/26/2006 21:23:15 MST Ken-I won't attempt to answer what R-value is cold or warm. But, I think I can help with the definition.The units for R-Value are 1/[Btu/(hr sq ft deg F)].BTU/hour is rate of heat transfer through the material.Sq ft is the surface area in square feet of the material.Deg F is the difference in temperature on each side of the material in Fahrenheit degrees.So, say you had a 2 ft square piece of material, that passed 50 BTU of heat per hour from one side at 70 degrees to the other side at 40 degrees.The R-Value would be 1/(50/((2*2)*(70-40))) = R2.4R-values are convenient to use because they can simply be added together for various layers. E.g. if you put an R2.2 pad on top of an R3.1 pad, you get an R5.3 pad. (This ignores edge effects, air boundary layers, etc.)Also, for a given material, if you double the thickness, you double its R-Value. (Again, ignoring edge and boundary effects.)Hope this helps.Cheers,-Mike[edited to correct error found by Bill and Paul in following posts. /mm] Edited by MikeMartin on 01/27/2006 16:29:44 MST.
 Bill Fornshell (bfornshell) - MLife Locale: Southern Texas What isn't R value on 01/26/2006 23:49:10 MST Mike: I once asked Thermarest this question. I wanted to use two of their pads and asked if I could add the "R" value of each for the total, They said no it didn't work like that. What is your source for this? ==============================="R-values are convenient to use because they can simply be added together for various layers. E.g. if you put an R2.2 pad on top of an R3.1 pad, you get an R5.3 pad. (This ignores edge effects, air boundary layers, etc.)Also, for a given material, if you double the thickness, you double its R-Value. (Again, ignoring edge and boundary effects.)" ????==============================Thanks
 paul johnson (pj) - F Locale: LazyBoy in my Den - miss the forest Re: Re: What Is R Value on 01/27/2006 04:23:40 MST Mike, are you intending to calculate "big R" (Thermal Resistance) or another thermodynamic quantity? Using your formula, double the thickness and see what happens. Did you get the answer ("big R") that you expected to get?------------------------------Bill,Essentially, Mike is correct. Keep in mind that as Mike says, certain "effects" are ignored, as is air movement/exchange between the two layers of pads. Two identical pads can trap a bit of air (which is then warmed) between them which can be pumped out/exchanged with cooler outside air as a sleeping hiker moves about on the pad during the night. This will have some effect on the system as a whole.Bill, I've got to run for a few hours. It's been over 30yrs since I looked at Heat Transfer equations. I remember them, but I'm very rusty and I'm trying to collect my thoughts so that my post turns out to be shorter than my SMD Essence pack review. If no one else posts back, I'll do so later around lunch time. I'll try to explain a simple equation R = d / k and how Mike's equation fits into it.This stuff is basic Heat Transfer (which is good because I only understand it on a basic level). Any undergraduate level Thermodynamics book should cover it.Bill, if a reply isn't necessary, just post back and I won't burden the Forum with another post on this subject. Many thanks, pj Edited by pj on 01/27/2006 10:44:27 MST.