Re: More BTU's was: Re: [pct-l] Insulation above, insulation below

[ROYROBIN@aol.com]

In a message dated 11/10/1999 9:34:41 AM PST, calliger@infolane.com writes:

<< Specifically, P, the heat which can be dissipated is:
 
         P= Q(a) [(T2/T1)-1] where Q(a) is air flow in ft3/min past your body.
         However radiative cooling is a primary consideration as well, it 
being
         P + e(sigma)(T**4-T(0)**4).  Which takes into account the thermal
         emisivity of the radiative body,e, and sigma is the Stefan-Boltzman
         constant  in watts/cm**2xK deg. The same equation [my emphasis] 
         is used for thermal conductivity to the ground where air-flow is now 
         zero.  >>

Beg to differ?  Radiative heat transfer is, as you say, proportional to the 
fourth(!) power of temperature difference.  That's why your sleeping bag 
frosts up when you sleep out on a clear night.  (So get under the trees or a 
tarp.)  However, conductive heat loss is proportional to the simple 
difference in temperature.  

Even so, our main concerns while trying to stay comfortable in our sleeping 
bags are conductive heat loss -- heat transfer from us to the ground --, and 
convection -- heat transfer through the bag to the surrounding air.  A good 
insulating layer reduces heat transfer.  Between us and the ground, this is 
usually a closed cell foam pad, or the equivalent.  Between us and the air, 
it's the dead air space provided by the loft in our down or synthetic filled 
bag.  Moisture anywhere in the system increases the rate of heat transfer, 
which is not good if you are trying to retain body heat.        
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