FearGod
Freedom Of Mind
I haven't used all of them to know which one is best. Perhaps you should ask that professor which one he used when he tested the salinity around the under ocean springs (or not).
The model on this website would likely do a suitable job: Salinity Refractometer for Reef Aquarium Ocean - Bulk Lot of (100) Units $19.99 each!
Next question.
You're the one who adviced me to check it myself and not the professor.