I think this is the write up your referring to:
What’s different between our NEW refractometer and others you might ask?
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/saltrefractometer 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.
Features:
True calibration for Seawater measurement.
A New finer scale running from 20-40ppt salinity (SG 1.015 - 1.030)
20/20 calibration at room temperature for more consistency and accuracy in everyday use.
Full ATC (auto temperature compensation) ranging from 5 to 30 deg C
I don't know if the 1.5 ppt is worth the extra money but that and the way the viewing lens is set up with larger scale that targets the range we are looking for. Is why I went with it.
