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.
foam and foil are your friends. Typically, you'll have multiple layers
of foam, then foil, then foam, etc. Mass also has an effect (in
increasing the time constant.. the "C", as opposed to the insulation
which increases the R)
John Strong's "Procedures in Experimental Physics" has a section on
thermal design (for furnaces and ovens), and is worth having a copy of.'
Moore, et.al., "building scientific apparatus" is another winner, and
has a whole chapter on precision temperature control
(I have to warn you.. get these two books, and you'll contemplate, or
worse, actually start, a whole raft of really interesting things to do.
Everyone needs a duoplasmatron ion source, don't they? Or a 6 foot
tall Geiger-Muller tube made from copper pipe and mixing bowls.)
And, as you get better insulation, heat leaks (whether conducted by,
say, the wires, or by air, or by IR radiation) become a bigger relative
problem.
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