Re: [pct-l] Insulation above, insulation below

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

I will have to dig up my old physics book, as this problem is very quantifiable
in that there is a coefficient of thermal heat transfer/loss of down, the
various ground pads,
your clothes, thru radiation, convection currents, thermal conduction to
the ground and also
thru the bag where you touch it.  Then, given a sleeping body (average)
produces
about 100 BTU's per hour one can calclulate what is needed (well- *that* is
what
is done by most design engineers)  (I am one BTW) given the various
ambients one
will experience.  Finally the sublimation type affects of evaporative cooling
is a fixed calculation added (or subtracted depending on relative humidity and
the dew point) to the overall equation of heat transfer.

So when I find it the equation will be something like:

	Comfort index = body weight*btu/lb * t sub pad * t sub down * t
down - humidity factor
	* t ambient or some such which will give you an idea on a scale of
1-10 which
	bag and pad combination would fit you more analytically given that
we can
	make a table of coefficient of heat transfer for the major bags- or
work from the
	temperature rating and fold in humidty and ambient temperature ranges
	expected and try to quantify how much each person perspires
(overall moisture
	loss at night can be up to 1 liter = about 1.5 lbs of water.)..

As an aside:

This is why, for example, that big monster fans are needed on a lot of PC's
today
as the thermal conductivity to the case from the CPU chip is insufficent to
transfer
all the generated heat. Thus convection cooling is required.

Throw this together and it comes out to the following:

	1- A really neat thermal equation ,
	2- All the posts are right!
	3- Sleeping in the nude is most comfortable if you are used to at
	home, if not, than it isn't.  Also depends if you zip two bags
together;
	then the requried thermal equation modifications are left as an
exercise
	for the occupants as they are not linear but exponential two body
problems
	in nature, speaking mathematically, of course.
	4- "Common sense" (if it is so common, why don't more people have
it = joke :) )
	says use a 20-30 degree bag with a good pad and where what you feel
warm
	enuf sleeping and air your bag out for an hour or so in the sun if
you are able.
	5- Leave your PC at home.




At 9:00 AM -0700 11/9/99, Brick Robbins wrote:
>At 06:39 AM 11/09/1999 +0000, Brett Tucker wrote:
>
>>I find that most of the insulation needs to come from above the reposing
>>hiker, since this is where it does the most good. Insulation above the
>>body traps the body's heat, which rises. Insulation beneath the hiker
>>traps less heat,
>
>You are leaving out one VERY important factor: Thermal conductivity. The
>ground below you conducts heat away from your body much better than the air
>around you does. Still air is a very poor thermal conductor (ie good
>insulator), which is why 70F air seems warm, but 70F water (a very good
>thermal conductor) seems cold. Moving air is a good conductor, because your
>body has to continually heat different air molecules.
>
>Most of what a down bag does is trap a layer of still air around your body,
>just like double pane glass traps a layer of still "air" (poor conductor)
>between two layers of glass (good conductors).
>
>That is why an insulating pad is so important. Even if the ground is warmer
>than the air, one can lose much more heat down into the ground. This, of
>course, depends on the thermal conductivity of the ground which depends on
>the soil type and moisture content. As "Ray" says, if you can find a bed of
>nice insulating pine duff, you don't need a pad. Unfortunatly, there just
>aren't that many good pine duff beds between Mexico and Canada.
>
>Crushed compressable insulation below you (such as down) doesn't do you any
>good, since it relies on its loft (which is gone when crushed) to trap
>still air to provide the insulation. The "air" in a insulating pad is held
>in place by a more rigid structure so it doesn't compress (much) under body
>weight, and still provides insulation. That is why a quilt is such a good
>idea. No down that is not doing its job.
>
>-Brick
>  Posting as list member, not list admin
>
>
>
>
>
>
>
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