One of the primary things I wanted to measure inside my fermenter, and get a better handle on, was specific gravity. A couple of times, my starting gravity's been all over the place, and I'm not sure if it's my measurement methods or my beer making skills. A more trustworthy density measurement, and particularly one that could show me the change over time of specific gravity, would be very useful.
I spent a lot of time trying to figure this out, because it's not a very easy thing. My requirements were to have a sensor that doesn't need to actually touch the beer much (for sanitary reasons), that will accurately measure the tiny changes in density that accompany fermentation, that would preferably be digital. The only way to directly measure the density of a fluid is to weigh a bunch of it, and determine the volume of the fluid at the same time. As it happens, water level sensors are a thing and they're pretty cheap. So are pressure sensors that could be used to measure weight. The only problem with this is that, if you think about it, the density of the entire fermenter never changes, because for it to change some amount of stuff would have to enter or leave the fermenter, and the only thing that leaves the fermenter during fermentation is carbon-dioxide. All the stuff that's settled out and all the dead yeast and stuff that's not going to end up in the final beer gets counted in the measurement and shouldn't.
The other notion I had was pressure sensitive pads. There are such pads that change their resistance when pressure is applied to them. A pair of these, about 1 square inch in area each, one higher up the side of the fermenter than the other, could also be used along with very basic fluid dynamics equations to derive density. The problem with these was that they're analogue sensors, and not really meant to be super accurate. They're more to sense if something is pressing down on them at all, not with a huge amount of accuracy how hard something is pressing on it, which is what I wanted.
The other idea, and this is how many industrial density sensors work, was to simply hook some kind of potentiometer up to a rod attached to the top of a regular hydrometer, changing resistance depending on how high it was floating in the beer. This process isn't as appropriate for my purposes because it would be difficult to fit in the fermenter in the first place, would be difficult to keep clean, and might not accurately measure density during the more vigorous primary fermentation, when the beer is very active and swirling about in the container a lot.
Also, none of these methods would work at all in a glass carboy, and that's the ideal application of this system.
After some research online, I saw that some other industrial density sensors, meant for fluids that are constantly flowing through them, work by measuring the dielectric constant of the fluid, between the plates of a capacitor. A sensor of this nature would only need to be near the beer, not necessarily submerged inside it, so could be glued to the outside of the fermenter and still give accurate results. This can just be two equally sized squares or hemicircles of tin foil contact-cemented to the side of the fermenter very carefully. If this and the leads going to it from the sensor board are small enough, it should be reasonably accurate.
That's gotta wait 'til later though. In other news, my second stout has been bottled, my third racked, and all is well at the Villainous Brewery other than fighting with this capacitance sensor board. Drink up, space cadets!