Hi,
I have been looking at a similar problem. What I have found is:
many plastic foam materials have very low conduction but are
transparent to long wavelength radiation, so thermal heating/cooling
through them is mainly by thermal radiation.
If you wrap an item in plastic foam, then a radiation barrier like
aluminium foil, then more plastic then more foil etc. you can
seriously reduce the heat transfer.
Air is an excellent insulator if it is in cells too small to allow
convection, (less than say 5mm). However light air filled materials
transmit thermal radiation. The CFC gasses are used in some foams to
partly absorb this radiation, but reflective foil is even better as
it is shiny and emits less radiation in the first place.
The sandwich idea works like this; the amount of radiative transfer
(in Watts) depends on the temperature difference of two layers of
foil. The distance between the layers does not affect the quantity of
energy radiated. So if a shiny box with one inch of air insulation
around it losses 8 watts by radiation to a surrounding box, then by
putting a layer of foil in the middle you halve the temperature
differences and so only have 4 watts of radiative transfer. Place 3
layers of foil (with intermediate foam layers) and it drops to 2
watts. Still in the same one inch space.
Find some closed cell polyethylene that is quite thin and some very
light aluminium foil and you could make many layers. Just make sure
that the foil is always normal to the thermal gradient.
The project is not finished yet but the thermal insulation is now
going to be many times better than with just thick slabs of foam.
cheers,
Neville Michie
On 26/11/2010, at 6:24 PM, beale wrote:
In an attempt to educate myself about temperature stability, I put
a temperature sensor in a 1" cube of brass wrapped in plastic
packing-type bubble wrap, and compared that with another sensor
outside the bubble wrap, with the whole combination in a thin nylon
case just to slow down direct air drafts. I put it on the bench in
the office where the ambient temperature varies up and down by a
few degrees over the day. I recorded both temperatures with milli-
degree resolution.
Looking at the resulting plots, it looks like my thermal mass and
thermal insulation on the "inside" sensor gives me only about a
half hour lag at most relative to the "outside" sensor (hard to
say exactly, it doesn't look like a simple one-pole filter). Note,
I am not attempting any kind of ovenized control as yet, just
measuring some time constants.
I've read that plain bubble wrap has an "R value" of about 2 ft^2·°
F·h/(BTU·in), while some types of rigid foam building insulation go
up to R=8 (at least until the CFC gases used to blow the foam leak
out). What is done in real instruments that need good thermal
insulation? I assume dewar flasks are limited to aerospace
applications.
Photo of the block prior to bubble wrap:
http://picasaweb.google.com/bealevideo/2010_11_18TempExperiment
(live) plot of temperatures:
http://www.pachube.com/feeds/12988
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