Kegodometer

I am looking for suggestions on how to build a keg-o-dometer. The intention is to measure how much beer is left in a keg, which is in a kegerator (keg fridge). The initital three ideas:

• turbine flow meter

I've not been able to find one that could go inline with beer, that is food safe and can handle carbonation, for under $100.

• temperature/sonic strip

• weigh scales

This seems like the most likely option. The trick is that one keg is larger than the other in the confined space of the fridge (a 50L and a 20L keg), and the scales would need to be able to handle sustained heavy load (~170lbs for the full 50L). From there, I would run the output from the digital scales to the serial port (db9) of a computer. After going through a few kegs, I would be able to determine the weight of an empty keg as well as a full keg, and could, with software, determine how much beer is left in each keg.