BrewBug, is a neat little toy that essentially has a "floating bob" that measures the temperature and specific gravity of the wort as it is fermenting, allowing for niceties like changing the temperature based on the specific gravity, and just really nice plots of the temperature and specific gravity.
Has anyone tried to DIY something like this before? I expect that the beer bug works by using a hollow bob with a temperature probe and measuring it's weight over time (with calibration). But I'm not sure where to get the "weight measuring" device: a linear actuator? A hook's law spring (changing weight to location) and then something to measure the position?