Red Sea Refractometer
Main features of Red Sea’s Seawater Refractometer include:
- Directly reads the absolute salinity of seawater at 25°C/77 0F ( No need for seawater or temperature compensation factors)
- Specifically designed for the ionic content of seawater (NSW) for more accurate salinity measurement (industry standard refractometers are calibrated for salt brine ).
- Calibrated for seawater (NSW) at 25oC/77oF, the normal temperature range for reef aquariums
(most standard refractometers are calibrated at 15oC/59oF ).
- Easier to read, higher resolution display, focussed to the relevant range for reef aquariums of up to 40ppt
- Includes Integrated Automatic Temperature Compensation (ATC) for accurate measurement at standard ambient temperature
Red Sea’s Refractometer display
The PPT scale on Red Sea’s Seawater refractometer is calculated using an algorithm for seawater and therefore will give a reading of the Absolute Salinity of seawater.
The PPT scale on regular salinity refractometers are calculated using an algorithm for Brine. Using a Brine refractometer for Seawater will give an error of approximately 1.5ppt in the salinity reading.
Salinity is a general term describing the concentration of salt in water. However, solutions of different type of salts have different refractive indexes.
Brine is a solution of Sodium Chloride (table salt) that contains 2 chemical elements.
Seawater contains approximately 70 chemical elements that include Calcium, Magnesium, Potassium etc. in addition to the Sodium Chloride.
Seawater and Brine of the same salinity (i.e. the same weight of salt/liter) have different refractive indexes and will give different salinity readings if measured on the same refractometer.
Refractive index and temperature
The refractive index of liquids change with temperature and therefore has a major effect on measuring salinity with a refractometer.
The algorithm for defining the PPT (and S.G.) scale for a refractometer is calculated for a specific temperature.
PPT readings from refractometers that are not calibrated for reef aquariums (i.e. 25oC) need to be adjusted according to a conversion table to give the actual PPT at the aquarium temperature. This is often a cause for misinterpretation of refractometer readings.
For example, using a seawater refractometer calibrated at 20oC will give a PPT reading that is approximately 1-1.5ppt lower than the absolute salinity of the same water sample at the normal aquarium temperature of 25oC.