Believe it or not, Daniel Kramnik is only a Junior in High School and has built this Digital Salinometer for a Water Quality Science Olympiad event. The Digital Salinometer built by Daniel took almost 2 weeks and is capable of accuracy within 0.0014%. Daniel's entry ended up winning him first place at the regionals and he's now headed for the State competition in March. Amazing what high school kids can do these days!
"The entire circuit consisted of several parts: an opamp-based salinity-measuring circuit, a linear positive/negative power supply, a digital voltmeter, and a peltier device temperature controller.
In order to save time debugging (inevitably) flawed bread or perfboarded circuits, I designed custom circuit boards in Eagle CAD and etched them at home with ferric chloride. I won’t post my .sch/.brd files as that would make it too easy to copy this project for Science Olympiad, but I will share schematic image files and board image files (if you already know how to reCAD them for etching in Eagle, chances are, you’re already beyond this guide)."