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I think of making precise thermostat based on ice-water phase transition. The idea is to keep 50% ice content, and monitor that instead of temperature.

I am thinking of monitoring dielectric constant via capacitance change, as it differs much between water and ice.

Is this way workable, or is there better ways of monitoring ice content?

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You should be able to measure changes in capacitance by applying a high frequency voltage and measuring the phase delay. I'm not sure this would work as a thermometer though, since it will always be at the ice-water equilibrium temperature. It would tell you how much heat energy the apparatus has absorbed, and you might be able to infer the temperature of the devices environment from that; but it would be an indirect measurement. – Colin K Jun 22 '12 at 14:21
Also, the high heat capacity of the device means it would play a very significant role in the thermal environment you are trying to measure. Usually one likes the measuring device to have minimal impact on the quantity being measured. – Colin K Jun 22 '12 at 14:23

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up vote 4 down vote accepted

Use a setup that looks like this:

Ice

The level of the water in the fine tube changes with the average density of the ice/water mixture so as the ice mets it will go down and as water freezes to ice it will go up.

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Nice idea, but he would have to figure out a way to avoid ice formation in the tube itself, which could be hard depending on the setup. – Forever_a_Newcomer Jun 22 '12 at 17:45
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@Forever_a_Newcomer You can put an oil layer on top of the water. – mmc Jul 19 '12 at 1:11
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The main issues I'd expect is keeping your (frozen) water in place, and keeping it pure.

The problem with freezing water is that the ice can go pretty much anywhere, due to the forces generated by its expansion. You're probably intending to measure the capacitance between two plates, but how do you guarantee the ice stays nicely between them?

Impure water doesn't freeze at 273K, and water in the natural environment attracts impurities.

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