There is a much more recent paper called Ultrasound based measurements of sugar and ethanol concentrations in hydroalcoholic solutions.
Was there a reason you rejected ultrasound as an option? Seems that a buoyancy based method would introduce contamination concerns (and extra cleaning).
@Elco any updates on this? What would be the go to solution nowadays with BrewPi?
I did come across https://tilthydrometer.com/products/copy-of-tilt-floating-wireless-hydrometer-and-thermometer-for-brewing which looks like a good option.
Its been a few years and a new release since the last time it was ask, so here I go again 
any progress on the Specific gravity measurment front?
Lots and lots and lots of work invested in this, but we’re deploying a prototype in wine this september. I’m not entirely sure whether I can/should share images of it, so I’ll leave that up to @Elco.
Thanks for the reply. Sounds great, would be great to have an all in one option. while searching I found something similar from Germany (my home country). I guess you already know it, in the unlikely event you havent seen it yet, it may help you in any form 
1 Like