Most refractometers used in the hobby are not specifically designed for reef aquariums and use a brine measurement algorithm (NaCl rather than seawater) for a temperature of 15°C rather than 25°C.
This gives a possible deviation of the result of 1.5 ppt, which can have real effects on coral growth and coloration.
Red Sea Video Seawater Refractometer
Red Sea Saltwater Refractometer Key Features:
- Direct reading of the absolute salinity of seawater at 25°C (no need for seawater or temperature compensation factors).
- Specially developed for the ion content of seawater for more accurate salinity measurement (industrial refractometers are calibrated for brine).
- Calibrated for sea water at 25°C, normal temperature for a reef aquarium (most standard refractometers are calibrated at 15°C).
- Easy-to-read, high-resolution display focused on useful reef aquarium values up to 40 ppt.
- Automatic Temperature Compensation (ATC) for accurate measurement at room temperature.
Red Sea Refractometer Display
The ppt scale of the Red Sea seawater refractometer is calculated using a seawater algorithm and will therefore give you the Absolute Salinity of your seawater.
The ppt scale of conventional refractometers is calculated using an algorithm for brine. Using such refractometers will give you an error of about 1.5 ppt.
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Red Sea Refractometer Display | Display of a classic refractometer |
About Salinity
Salinity is a general term that describes the concentration of salt in water. However different types of salt solutions have different indexes of refraction.
Brine is a solution of Sodium Chloride which contains 2 chemical elements.
Seawater contains approximately 70 chemical elements including Calcium, Magnesium, Potassium, etc… in addition to Sodium Chloride.
Seawater and brine with the same salinity (so the same number of grams of salt per liter of water) have different refractive indices and will give different salinity readings on the same refractometer.
Refraction and refractive index
All transparent materials like liquids refract light.
All transparent materials refract light differently and have what is called a "refractive index" which indicates how well a given material refracts light.
Different concentrations of the same liquid have different refractive indexes.
The scale of a refractometer is calculated using a mathematical algorithm which translates the refraction measured at such concentration for a specific liquid.
Index of refraction and temperature
The refractive indices of a liquid change with temperature which has major effects on the measurement of salinity with a refractometer.
The algorithm that defines the ppt scale of a refractometer is calculated at a specific temperature.
The ppt reading of refractometers that are not calibrated for reef aquariums (i.e. at 25°C) should be adjusted via a conversion table to give the ppt for the temperature of the aquarium. This is often a cause of misinterpretation of refractometer readings.
For example, using a seawater refractometer calibrated at 20°C will give a reading approximately 1.5 ppt lower than the absolute salinity of the same sample at normal aquarium temperature at 25°C.
This is why the Red Sea seawater refractometer calibrated at 25°C allows you to directly read the Absolute Salinity of seawater without applying any compensation factor.
ATC – Automatic Temperature Compensation
Temperature is probably the number one cause of a refractometer reading incorrectly. Calibration and testing of water samples should be done after the refractometer and the water sample are at the same temperature.
The ATC compensates for the few degrees difference between the ambient temperature and the refractometer calibration temperature. Larger deviations will give an error of about 1 ppt.
ATC does not adjust the reading of a refractometer to the temperature of the aquarium.
