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

["R.J.Calliger (510.651.1876)" <calliger@infolane.com>]

	To complete the cooling thoughts in a sleeping bag we have to take
into account your rate of sweating plus the conduction of your body heat to
the ground:


        Evaporation rate is a function of F(specific heat of liquid,
ambient temperature,air flow, etc)
        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 is used for thermal
	conductivity to the ground where air-flow is now zero.

        A wet T-shirt or other wet clothing in a sleeping bag  has a higher
thermal emisivity
        than a dry one and also higher than a non-sweating body.  A hotter
        body also, obviously  has a higher thermal emisivity.


        Therefore, the hotter you are  the faster you loose
        heat and you do it better in a wet shirt than nude.  Of course we
        also have to take into the account of the surface area of the radiative
        body(ies). Those are of course, a secondary affect to the effect of
        overall thermal instability.  A body tends of course again, to achieve
        stability and thermal equalibrium as time advances and T approaches
T(0).

        The actual conductive rates are left as an exercise for the
interested reader.
<smile> However please note for precise results water cooling as well as the
previously discussed evaporative and radiative cooling affects must
be summed with the conductive affects
to achieve total sleeping bag thermal design correctly. Finally, the
equation can be normalized
to compute your own comfort index on a scale of 1 to 10.
I am sure that the dailly professional sleeping bag designer can do a better
job...but this is the fundamental physics behind the bag.

        PS-

        Sorry for all the "hot air" :-)

        --RJ Calliger


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