Forch, C.; Knudson, M.; Sorensen, S. Reports on the determination of the constants for compilation of hydrographic tables. Det K. Dan. Vidensk. Selsk. Skrifter. Naturvidenskabelig Og Math. Afd. 1902, 6, 1–151. [ Google Scholar] Correction of slope miscalibration errors should be carried out using a fluid that approximately matches the refractive index of the water being tested, so for reef aquarium water, calibration with 35 ppt seawater solves this problem, while calibration with pure freshwater does not. Akter, S.; Ahmed, K.; El-Naggar, S.A.; Taya, S.A.; Nguyen, T.K.; Dhasarathan, V. Highly Sensitive Refractive Index Sensor for Temperature and Salinity Measurement of Seawater. Optik 2020, 216, 164901. [ Google Scholar] [ CrossRef] Refractive index generally cannot reveal the identity of compounds in water, but when an aquarist knows roughly what material is there he can determine how much of it is there (within the refractive index’s detection capability). Changes in refractive index are not suitable for determining trace levels of ions (such as the purity of freshwater coming out of an RO/DI (reverse osmosis/deionization) purification system), but it can do a good job when significant amounts of a known material are present. In the early days of the hobby, we’d use a floating glass hydrometer to measure the specific gravity in our tanks and when mixing up saltwater. The strength of the seawater determines how high the hydrometer floats in the water. Glass hydrometers worked best by taking a water sample with a tall cylinder and placing the hydrometer into it. If you put the hydrometer into your aquarium, it would bounce up and down making readings impossible. Imagine how long it took to mix a batch of saltwater! The latest generation of aquarium hydrometers use a pivoting pointer inside a water collection chamber to indicate specific gravity and salinity levels. Hydrometers are calibrated to be most accurate when the water is 77°F (25°C). While hydrometers are still used today, many reef aquarists have made the switch to a refractometer.So, if you're considering the switch, here’s what you need to know about saltwater refractometers and how to use them. What’s a Refractometer?

Assuming that a refractometer is made correctly for the fluid it is intended to measure, the way to calibrate a refractometer is to put a liquid of known refractive index on it, and adjust the scale’s position by turning the calibration screw (Figure 3) until it reads correctly. When a refractometer is perfectly calibrated, it will show the fluid’s exact refractive index (assuming that it reports the results in refractive index, but this is not always the case). Figure 4 shows a graph of the measured refractive index vs. the real refractive index for a perfectly calibrated refractometer. At all points these two values are the same. While this graph alone is not particularly enlightening, it forms the basis of later graphs that explain how errors in calibration get corrected. All transparent materials refract light differently and have what is called a “refractive index” which means how much a particular material distorts light.Sayed, H.; Aly, A.H. Salinity optical sensor by using two-dimensional photonic crystals: Computational study. Mater. Sci. Eng. B 2021, 269, 115169. [ Google Scholar] [ CrossRef] Figure 11. The relationship between the real (actual) specific gravity and the measured specific gravity for a perfectly calibrated seawater refractometer (green) and an incorrectly calibrated seawater refractometer (red). This refractometer has an offset error, with all values reading lower than the actual value. The error can be corrected using a seawater standard. By turning the calibration screw until a seawater standard reads 1.0264, the red line moves onto the green line and the refractometer is properly calibrated. In this case, accurate calibration can also be performed using freshwater.

A. I. Yurin, G. N. Vishnyakov, and V. L. Minaev, “Measurement of the refractive index using a modified constant deviation method,” Izmeritel’naya Tekhnika, No. 12, 35–39 (2022). SeaGuard Single Point Platform. [EB/OL]. Available online: https://www.aanderaa.com/seaguard-single-point-platform/ (accessed on 30 May 2022). It turns out that an aqueous solution’s refractive index is relatively insensitive to small changes in the solution’s ionic makeup. For example, the usual changes in seawater’s major ions that are encountered in a reef aquarium do not greatly alter the measured salinity. However, large differences in the big four ions (chloride, sulfate, sodium and magnesium) will alter the relationship between refractive index and salinity or specific gravity. Refractive Index and Ion Imbalances in Seawater R. W. Austin and G. Halikas, The Index of Refraction of Seawater. SIO Ref. N 76-1 (Scripps Institution of Oceanography, La Jolla, California, 1976), pp. A4–A21.Fortunately for aquarists, most marine organisms are fairly forgiving of the exact salinity, and high quality reef aquaria can seemingly thrive with a wide range of salinity. Reef aquarists monitor salinity in a variety of ways, including specific gravity measurement using hydrometers, conductivity measurement using electronic meters, and refractive index measurement using refractometers. Refractometers can be a very good way to measure salinity, assuming they are appropriately designed and properly calibrated. Unfortunately, manufacturers of refractometers have often not designed them for measuring seawater, but for measuring sodium chloride brines. Consequently, calibrating them for use in marine aquaria is often not best done the way the manufacturer recommends. Seawater contains approximately 70 chemical elements that include Calcium, Magnesium, Potassium etc. in addition to the Sodium Chloride.