Sybon Heavy Duty Model is one of the most robust models available. You can feel the difference in weight by holding a Sybon and other brands at the same time. Sybon also has excellent quality control and it's very rare to see any issues with craftsmanship. Although you will get roughly the same accuracy from model to model, if you just want the best built, you can't go wrong with Sybon.
Sybon the largest manufacturer of salinity refractometers. Their refractometers are of very highest quality and they come with a small bottle of ultrapure water for occasional recalibration, calibration key, properly written user manual, a piece of soft flannel, a hard plastic case and a very attractive packaging box.
The Sybon HD RHS series are designed for testing the concentration of saltwater and brine. It's triple scale provides a direct reading of the specific gravity and concentration. (Parts per thousand) of salt in water and the RHS-10 proves itself invaluable for agriculture, food processing and wildlife management. this instrument is equipped with Automatic Compensation system making it ideal for field use.
*Refractometers are listed to be calibrated with RO/DI or Distilled water. There is some new information stating for best results you should use a Salinity Fluid like the American Marine Salinity Fluid. This will yield you much more accurate results.
How to calibrate with Sybon Salinity Fluid: Take a couple drops and read your unit. You should read 1.0259 salinity or 35ppt. Adjust your unit to match one of these two numbers. Once you adjusted the refractometer is now fine tuned for saltwater salinity/ppt. If you read ro/di after calibration you will have a negative reading and that is completely normal, as the unit is fine tuned now for a saltwater tank.