This is a Digital Salinometer which [Daniel Kramnik] built as a Science Olympiad entry. He’s a Junior in High School and when looking for a project to enter into the Water Quality event he was interested in achieving greater accuracy than a mechanical hydrometer provides.
We think the circuit design is very impressive for anyone who hasn’t complete formal training as an engineer, and outstanding for someone as young as [Daniel]. Measurements depend on two main parts, a temperature control and a salinity sensor. These are both necessary because fluctuation in sample temperature will affect the salinity reading.
A Peltier element is used to heat the water sample if it doesn’t fall within a set range of temperatures. From there, an Op-Amp circuit conditions a signal running through the sample, passing an output to the ADC converter chip which drives the three-digit readout. [Daniel] calculates an accuracy within 0.0014%. He must be on the mark because he’s won his regional competition and will soon compete at the state level.