Funny thing. I'm an engineer with a few decades of experience with electronics and sophisticated testing and measurement equipment. Even had some college physics classes in light wave refraction way back when. However, until I started visiting this site, I had no idea there was an inexpensive, hand held refractometer made for aquarium use.
As mentioned, I have the refractometer being discussed in this thread and it is very cool. Interestingly enough, my Instant Ocean hydrometer is pretty dang accurate when compared to my refractometer.