Time to come out from lurking mode I suppose! I did an experiment a while back using a differential pressure sensor:
http://www.digikey.com/product-detail/en/SSCDRRN025MDAA3/480-3660-ND/2704715
.. and a couple of tubes going in to a very ad-hoc-and-experimental fermentor vessel:
This is the result:
The wort OG was 1.043, and it ended at 1.018 (it was fermented in a too cold basement with too little WLP400, which is probably why it stopped early. No oxygen added or shaking wort either..) The values on the graph is voltage * 1000 so there is a 300 mV difference from start to finish.