Is it possible to measure specific gravity (SG) with two pressure sensors? I am looking into the subject of continuous specific gravity (SG) measurement of my beer during primary fermentation. I am wondering if attaching two pressure sensors on different heights on my fermentor wall and then calculating SG from pressure difference would work?
 A: Yes it is absolutely possible; in fact this method is somewhat common among larger breweries.

From the book Brewing Yeast and Fermentation:
'Density may be computed from differences in pressure measured simultaneously at different depths within vessels. Thus, at a constant vertical distance between two points the differential in pressure is a direct function of the density of the liquid phase.'

The book goes on to elaborate on four different practical systems used in commercial fermenters (all based on the same principle), two of which specify the height difference between their two sensors. One was 45 cm, the other was 300 cm. Height differences like this are obviously not going to work for your typical homebrew setup, but it proves the method is sound. Keep in mind the tanks referred to here would be large (probably in the range of 100-500 US barrels).
I do suspect that if you could get sufficiently precise pressure sensors, you may actually be able to make something small enough for whatever you ferment in, and which could be calibrated to hydrometer readings taken simultaneously. However, without knowing some more detailed specifications of the sensors you intend to use (and way more about making stuff like this than I can pretend to know) it's impossible to say how accurate you could get, given the low height/pressure differences you'll experience on homebrew scales.
I agree that you should seek more detailed advice from the Physics SE (or maybe even Electrical Engineering) about the technical aspects of this method, but I thought it would be interesting to know that this is in fact a well-known practice in commercial brewing.
