Red Sea’s Seawater Refractometer offers better accuracy for reef tanks than most refractometers. Others use an algorithm for the measurement of brine, which is table salt (NaCl) and water solution, and a temperature of 15° C (59° F). This can cause a measurement deviation of up to 1.5 ppt. This deviation can result in improper salinity, which may result in a shift of Ca-KH- Mg concentrations, is harmful to corals. Red Sea’s Seawater Refractometer uses an algorithm specific for sea water in temperature of 25° C (77° F), making it more appropriate for reef tanks. Using an algorithm specifically for seawater gives a reading of the absolute salinity of the water sample. Red Sea was able to develop this technology after spending so much time testing water’s salinity themselves during quality control testing of their reef tank salts.

Product Details

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Red Sea’s Seawater Refractometer features a high resolution display that is focused on the relevant range for reef aquariums; up to 40 ppt. It gives a sharp line for the salinity reading, making it easier to read than other refractometers. Red Sea’s Seawater Refractometer includes Integrated Automatic Temperature Compensation (ATC) for accurate measurement at standard surrounding temperature. Ambient temperature can influence the refractive index, so the ATC compensates for a few degrees difference between ambient temperature and calibrated temperature of the instrument. However, large temperature variations will result in an error of approximately 1.0 ppt in the salinity reading. The ATC will not adjust the PPT of the refractometer to the temperature of the tank, though.

It is important to use a refractometer designed to measure salinity in seawater rather than one designed to measure brine because seawater contains approximately 70 chemical elements including calcium, potassium, magnesium, and bi-carbonates. Corals need a proper balance of these elements and salts to grow and maintain health. Different kinds of salts have different refractive indexes due to differences in their chemistry, therefore seawater and brine of the same salinity will have different readings on the same refractometer. A refractive index is how much a material distorts light. All transparent materials such as water refract light, and different concentrations of the same liquid have different refractive indexes. The refractive index of a liquid will vary based on its ionic composition. Refractometers work by detecting the refractive index and using an algorithm to calculate its related concentration. Red Sea’s Seawater Refractometer uses calibration based on specific ionic content of seawater according to the most advanced international laboratory standards for practical salinity measurements.

It is also important to use a refractometer designed for the proper temperature of your aquarium because temperature can affect refractive index. Refractometers that are not calibrated for reef tanks do not give accurate readings. For example, using one calibrated for 20° C will result in a reading that is 1-1.5 ppt lower than the absolute salinity of a water sample at 25°C, which is the normal temperature for reef aquariums. These readings can be converted into the actual readings for the tank, but this can be confusing and time consuming. Red Sea’s Saltwater Refractometer is calibrated for 25°C, so there is no need for conversions in reef aquariums.

How to Use

Using Red Sea’s Seawater Refractometer is basically the same as a typical refractometer without the conversions. First, clean the glass prism and translucent flap of the instrument with deionized water 22°- 25° C (71.6°- 77° F) and dry thoroughly with a soft cloth. Next, hold it horizontally and open the flap. Then, place a small sample of the water to be tested on the glass prism with the pipette provided. Do not immerse the refractometer in the water. Then, close the flap and wait 15 seconds to allow temperature of the sample and the refractometer to equalize. Then, point the prism end at a light source and look through the eyepiece. For best results, use sunlight for this step. The dividing line between blue on top and white on the bottom is where the reading should be taken. The focus can be adjusted by rotating the eyepiece until the image is sharp. After taking the reading, clean and dry the glass prism and flap again. The refractometer should be stored in a dry environment.

The optical elements inside refractometers can sometimes become misaligned, so it is necessary to regularly check the alignment. Alignment should always be done with the refractometer at 22°-25° C. It should sit at this temperature for 30 minutes before starting the alignment. To begin the calibration, remove the cover from the alignment screw. Then, place a sample of deionized water on the glass prism and check the salinity reading. If the reading is not exactly 0 ppt, adjust the position of the reading by rotating the alignment screw with the screwdriver provided until the reading is 0 ppt. Then, replace the cover to the alignment screw.


Improper salinity levels can be harmful for corals. It can cause problems with their growth and coloration. Red Sea’s Seawater Refractometer offers an easy way to be sure your tank is at the right salinity.



(2014). Red Sea Seawater Refractometer. Red Sea. Retrieved March 31, 2016 from