Almost all, if not all, of the refractometers currently available to the aquarium hobby are designed for measuring saltwater (sodium chloride or brine solution) and not true ‘seawater’. Although seawater is mostly sodium chloride it also contains other elements such as magnesium and calcium that change the refractive index slightly and will give a slightly different result.
The most common symptom of this difference in refractive index can be seen when mixing up salt as per salt manufacturer’s instructions and achieving a solution strength where it appears, through testing, that the levels of elements are lower than stated by the manufacturer. In actual fact all of the levels are low as the salinity is not as high as the aquarist believes – we have all seen low Ca or Mg for a given SG with almost all brands of salt.
It turns out that a 35ppt solution of true seawater has the same refractive index as a 36.5ppt solution of brine and so when using a brine/salt refractometer the salinity is likely to be low by 1.5ppt. This can be allowed for by the aquarist but only if they are aware of the difference.
After extensive development, D-D’s new refractometer addresses this issue, giving true salinity results when correctly calibrated and greater accuracy than previously attainable.