You do have to be very careful on refractometers. Some of the cheaper ones are just refractometers that were modified to measure saltwater.Some are modified medical units that were meant to measure mineral levels in urine. In my opinion the best ones are made by Milwaukee. There is two models. One that has ATC (auto temprature calibration). In other words it's calibration is not affected by a change in temprature. The other model does not have this feature. These are quailty refracftometers and are way ahead in terms of quality and accuracy over the cheap china made and modified units that have flooded the market lately.