On Fri, Jun 17, 2011 at 07:56:13PM +0930, Steve Potter wrote:
> I don't think your problem is your sleeping bag. From what I remember from
> Physics.. sleeping on an air mattress in zero temperatures is the equivalent
> to sleeping on a block of ice. It has to do with getting rid of the air and
> the ability to heat your surrounds with body temperature. If you can get rid
> of the air, you can actually sleep in packed snow (which is ice)
If you want to get into the physics (and split hairs) - you don't
actually want to get rid of the air. Air has very low thermal
conductivity. The problem is heat conveyed by convection - currents in
the air inside the mattress and heat conveyed by radiation through the
air in the mattress. So what you actually want to get rid of is the
currents in the air and the radiation through the air.
One way to do this is fill the air mattress with down. Air is trapped
into tiny spaces where the viscosity of air at this scale stops currents.
Radiation be intercepted by multiple layers of feathers and will also
be much reduced. Both these mechanism also are important in keeping
birds warm.
Conduction will actually be increased because although keratin thermal
conductivity is low its still 10x higher than air. But despite the
higher conduction the estimates I've seen have the rate of total heat
loss 5-10x less after filling an air mattress with down. Of course you
can use other materials like closed cell foam to similar effect but its
impressive that down is still so widely used for insulation.
Andrew
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