One of the most important parameters to monitor in our reef aquariums is salinity. There are many ways to test salinity, however I have stuck with a specific type of refractometer due their accuracy and ease of use, since they were released. These units have proved more reliable than even the more expensive electronic counterparts, and are certainly light years ahead of any swing-arm hydrometer.
Refractometers generally available for our hobby are offered with a scale designed for measuring saltwater/brine solution (scale range from 1.000-1.070 / 0-100ppt), which only measures sodium chloride (NaCl) content, and does not account for other elements. These other elements provide a different refractive index compared to brine, which should be taken into account by applying a conversion factor. For example, a 35ppt sample of natural seawater has the same refractive index as a 36.5ppt sample of brine (NaCl and water, only). Therefore anyone using a saltwater/brine solution refractometer should actually be shooting for a measured level of 36.5ppt.
A better option is a refractometer designed specifically to measure seawater, NaCl and all additional elements. These refractometers utilize a scale range of 1.015-1.030 / 20-40ppt offering increased readability which in turn makes our measurement more accurate.
Pinpoint 53.0 mS calibration solution (or similar) should be used with saltwater/brine refractometers as this increases their accuracy by calibrating the instrument into our niche range, reducing slope error. Manufacturers of seawater refractometers recommend to use distilled or RO water since 35ppt solution is not the same as natural seawater, and their scales and calibration standard is designed for seawater measurement.
Currently I know of two companies offering seawater refractometers, Marine Depot and D-D. Both are readily available and can generally be found for under $50 USD. Make sure to follow directions for your specific refractometer, including paying close attention to necessary temperature range for using the instrument and sample of water.