Q:

How to detect ice in thermostat

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?

A:

Use a setup that looks like this:

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.

