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Originally Posted by bobkoure
I'm also curious how you are getting it to -90. The easy/obvious/probably-wrong one is that you are simply boiling off nitrogen (too cold -200C or so - acetone freezes) or maybe dry ice (nah - too warm - only -75C or so) Some kind of condense/evap loop? Even that'd have to be somewhat exotic given the temp.
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Normal phase change would do, or a cascade. The temp depends on the gas you use, with the right combo -90 is quite possible.