[time-nuts] temperature stability basics

jimlux jimlux at earthlink.net
Fri Nov 26 15:24:05 UTC 2010


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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