How does the Beer Bug measure Specific Gravity?

[MatthewMoisen]

I have a raspberry pi with DS18B20 temperature sensors to measure the temperatures of my wort. I would love to incorporate a specific gravity sensor, but do not want or need to pay $250 for the Beer Bug.

How does the Beer Bug measure gravity?

 

[ross_daly]

I think it is by measuring the weight of a known-density sensor that is immersed in the beer, and connected to the upper beerbug airlock part by a thin string.

Here are a few photos.
http://bluffcitybrewers.com/2013/07/31/beerbug-review-part-1-brew-day/