Is Ice water a constant temperature?

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Cool! Excuse the pun.  Even in the flask, that lasted longer than I thought it would.

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If a pool noodle is what I think it is (it isn't a common expression in the UK!), foam-based floating support, then I'd think it would make an excellent insulator with all the air trapped in it.  That was a good experiment and I'm surprised to see that ambient warms to such a degree - 1.34W seems like a lot.  I know of some transistors that wouldn't hold up as well as that ice.

If a pool noodle is what I think it is (it isn't a common expression in the UK!), foam-based floating support, then I'd think it would make an excellent insulator with all the air trapped in it.  That was a good experiment and I'm surprised to see that ambient warms to such a degree - 1.34W seems like a lot.  I know of some transistors that wouldn't hold up as well as that ice.

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It's easy to forget that the air pockets make for a large surface area.  Your graph, for the 4 degree point looks fairly linear - was it like that all the way?  If not your thermal resistance is just a snapshot in time isn't it?  There is another option: most domestic fridges are set at around 4 degrees (perhaps 5) but it may not be clear or exact (mine has a circular knob that can move between "cool" and "eco" but without degree C markings.)  However, it should be constant as long as you don't keep opening the door.  If you can measure from within there - e.g. stuff your DMM, thermistor and flask in the fridge then ambient would be controlled - you could also use a second thermistor to measure how constant that fridge ambient was (how much hysteresis does it exhibit?): you could verify that thermal resistance!

If nothing else, these are 'outside of the box' experiments with thermistors and good fun.

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scottiebabe said:

LM399

Continuing on the tangent: I have a Keithley DMM6500 with an LT SL40057 reference. Rumours say it's an LM399 subset for (selected by) Keithley.

image: blurry photo from my device