BetterSense

Recipes 

I think it might work to dangle a float into the center of the fermenter on a piece of fishing line. This would require being able to measure the very small downward force, to calculate the buoyancy change. Ideally, I would be able to read the gravity with a microcontroller, but a mechanical/visual scale would also be sufficient.

I can't think of a sensor that would work for this and also be usable.

Maybe it would work to hook the fishing line to a long lever to multiply the force and then a standard strain gauge would be able to register.

crane

Recipes 

The biggest problem I see with this is krausen buildup on your float. At my last job I worked on some wireless issues on a set of fermentation controllers at UC Davis and they had originally tried this with wine and it didn't work due to the second main problem - CO2 bubbles sticking to the float. They ended up implementing a system using a differential pressure sensor with each end mounted at different heights in the fermentor. The larger the change in pressure resulted from a higher specific gravity. For this to work however they did have to continuously pump a small amount of air into both of the sides of the pressure sensor similar to how the brewtroller level sensors work. Seeings how they were fermenting 40 gallons or so in one fermentor they didn't run into problems oxidizing the wine, but on a homebrew scale I don't see how you could get away with this without contamination or oxidization issues.

passedpawn

Recipes 

You can measure gravity from inside the wort, not at the top. Put a piece of bouyant (i.e., sp.gr. < 1) on an arm and measure the angle that it hangs at. A little accelerometer device (I like the ST Micro devices) can easily measure this.

The device needs a level bubble to be held exactly level inside. Hold it inside with a magnet on the outside.

Transmit the gravity with RF (lots of simple small solutions there) to a display on the outside. Maybe the "holding" magnet and the simple display are on the same assy.

I use something similar for my saltwater fish tank.

__________________
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and others take delight in the hurling and the bowling
but I take delight in the juice of the barley
and courting pretty fair maids in the morning bright and early