Almost all handheld salt water refractometers available to the hobby are designed and calibrated for the testing of brine solutions, i.e. sodium chloride, and are not calibrated for the refractive index of natural sea water.Even though the largest percentage of salt in the sea is sodium chloride, the presence of other significant ions such as magnesium and calcium in natural sea water results in a different refractive index relative to that of brine. A standard salt (brine) refractometer will therefore not give the correct salinity for natural sea water (NSW) and a conversion factor must be applied.For example, a 35ppt solution of NSW has the same refractive index as a 36.5ppt solution of brine.After extensive development, D-D’s new ATC refractometer addresses this issue as the scale is calibrated for seawater giving true salinity results for aquarium use when correctly calibrated and greater accuracy than previously attainable.D-D’s refractometers feature a copper internal body which conducts heat faster than cheaper aluminium and plastic versions for faster and more accurate auto temperature compensation. Easier to ReadOne of the other problems with most refractometers is that the scale displayed normally reads from 0-100ppt when we are really only interested in the region from 30-40ppt. Our new Seawater Refractometer has therefore been specifically designed to read from 0-40ppt which gives you 2.5 times the resolution of a normal 0-100 scale. Auto Temperature Compensation (ATC)There is a lot of misunderstanding concerning the way the ATC feature works and its effect on the refractometer at different ambient room temperatures.Salinity is a measurement of a mass of salt in a mass of water and therefore does not vary with temperature however a refractometer does not measure salinity directly but measures the refractive index which is then displayed as salinity. The refractive index of a solution does vary with temperature therefore the reading that you measure with a refractometer is always temperature dependant.An ATC refractometer has a bimetal strip inside the instrument that moves the reading scale as the temperature changes to compensate for the change in refractive index. What people do not generally understand is that it is the temperature of the instrument and not the water temperature that is important as the small sample of water used for testing will equilibrate within seconds to the temperature of the refractometer.Once correctly calibrated at the set calibration temperature of 20°C the refractometer can then be used in environments where the ambient temperature and therefore instrument temperature would heat up or cool down within the range of the ATC which is between 10 and 30 centigrade.
