Seawater versus Saltwater Refractometers?Visit Manufacturer's Web Site
Keeping water parameters in the right range is crucial for a healthy aquarium and the best way to get an accurate reading of salinity and specific gravity of aquarium water is to use a refractometer. These precision optical instruments are incredibly accurate even at very low salt levels and are equally easy to use.
It is important to note however, that most refractometers used in the hobby are not intended to measure seawater but rather are intended to measure sodium chloride solutions. These are often called salt or brine refractometers. The differences between a salt or brine refractometer and a seawater refractometer are not great, but if they are calibrated with RO/DI water which they often come with when purchased, they can produce inaccurate results. Calibrated with RO/DI water, many of these salt or brine refractometers will give a lower reading (by about 4 ppt) then when they are calibrated with a standard in the range of the water being tested. In other words, calibrated with RO/DI water, your refractometer may give you a salinity reading of 35 ppt when it is actually 39 ppt.
While there is a very technical explanation as to why this happens*, for most of us we simply need to know that it does happen and it is therefore important to calibrate our refractometers using a standard with a known refractive index that is close to the value intended to be measured in our aquariums, i.e. 35 ppt. American Marine makes a 53 mS/cm calibration fluid for their electronic salinity monitors which is also suitable for use with refractometers. Calibrated with an appropriate standard, these salt or brine refractometers will provide accurate and reliable results.
As with any testing device, whether it be a test kit or your newly calibrated refractometer, always question whether the results you're getting are reasonable or even possible in the circumstances. Over the years we have talked to many hobbyists who have made drastic changes to their water chemistry based on inaccurate test results, often doing harm to an otherwise healthy aquarium. Always question your test results before making significant changes.
NOTE: * See Refractometers and Salinity Measurement by Randy Holmes-Farley