I. V. Gritsenko, M. S. Kovalev, N. G. Stsepuro, Yu. S. Gulina, G. K. Krasin, S. A. Gonchukov, and S. I. Kudryashov, “The optical refractometry using transport-of-intensity equation,” Laser Phys. Lett. 19, 076201 (2022). Figure 14. The refractometer of Figure 13 (red) has a slope error, with values far from the calibration point reading incorrectly. This type of error can only be corrected by calibrating with a solution with refractive index near to the expected measurement point. For use in seawater, recalibration with 35 ppt seawater (refractive index = 1.3394) moves the red line onto the green line at the refractive index used for calibration (here, 1.33940), and the refractometer now reads accurately in the region of refractive index similar to seawater. It turns out that this is a slope miscalibration in the sense that a perfectly made sodium chloride refractometer necessarily has a different relationship between refractive index and salinity than does seawater. This type of problem with a refractometer IS NOT at all corrected by calibrating it with pure freshwater. If you have this type of refractometer, and it was perfectly made and calibrated in freshwater, it will ALWAYS read seawater to be higher in salinity than it actually is (misreporting an actual 33.3 ppt to be 35 ppt). For example, standard seawater with S=35 (35 practical salinity units, or PSU) is defined as seawater with the same conductivity as a solution made from 3.24356 weight percent potassium chloride (KCl), and that conductivity is exactly 53 mS/cm (mS/cm, or milliSiemens per centimeter, is one of the units used for conductivity). That solution, however, has a refractive index of about 1.3371, matching seawater just below 26 ppt. So do not assume that all 53 mS/cm conductivity standards are suitable for refractometer calibration unless that is stated to be the case.

From refractive index tables found in chemical reference books, we can find that a 10 weight percent solution of sodium chloride has the same refractive index as a seven weight percent solution of magnesium chloride, a nine weight percent solution of magnesium sulfate and a 12 weight percent solution of sodium sulfate. These results indicate that some effects could relate to shifts between these ions in a reef aquarium, but that these effects are small. We can use these values to roughly predict how far off salinity measurements might be with some typical changes in the major ions. If we start with 35 ppt seawater, which normally has the following components, SBE 911plus CTD | Sea-Bird Scientific—Overview | Sea-Bird. [EB/OL]. Available online: https://www.seabird.com/sbe-911plus-ctd/product?id=60761421595/ (accessed on 30 May 2022).

Conflicts of Interest

Therefore, Red Sea’s Seawater Refractometer calibrated at 25 °C/77 0F directly reads the absolute salinity of seawater with no need for temperature compensation factor) Even more confusing, but perhaps a bit less of a problem in terms of the error’s magnitude, salt refractometers sometimes read in specific gravity. But that value is specific gravity of a sodium chloride solution with the measured refractive index, not seawater with that refractive index. A sodium chloride solution with the same refractive index as 35 ppt seawater (which turns out to be 36.5 ppt sodium chloride) has a specific gravity matching 34.3 ppt seawater. So this type of refractometer, when perfectly calibrated, will read the specific gravity of 35 ppt seawater to be a bit low, at 1.0261 instead of about 1.0264. That error (reading 0.0003 or so too low) is, however, probably less than most reef aquarists are concerned with. Figure 24 shows the relationship between a perfectly calibrated and accurate salt refractometer and a perfectly calibrated and accurate seawater refractometer when the units are reported in specific gravity. This figure shows the measured salinity reading for seawater being about 0.0003 lower than it really is. F. J. Millero, R. Feistel, D. G. Wright, and T. J. McDougall, “The composition of standard seawater and the definition of the reference-composition salinity scale,” Deep-Sea Res., No. 55, 50–72 (2008). Striggow, K.; Dankert, R. The exact theory of inductive conductivity sensors for oceanographic application. IEEE J. Ocean. Eng. 1985, 10, 175–179. [ Google Scholar] [ CrossRef] Lewis, E. The practical salinity scale 1978 and its antecedents. IEEE J. Ocean. Eng. 1980, 5, 3–8. [ Google Scholar] [ CrossRef]

Kaidarova, A.; Marengo, M.; Marinaro, G.; Geraldi, N.; Duarte, C.M.; Kosel, J. Flexible and biofouling independent salinity sensor. Adv. Mater. Interfaces 2018, 5, 1801110. [ Google Scholar] [ CrossRef] Wang, L.; Xu, Y.; Geng, Y.; Wang, J.; Du, Y.; Yi, D.; Hong, X.; Li, X. High-sensitivity fiber salinity sensor based on an exposed-core microstructured fiber interferometer. J. Phys. D Appl. Phys. 2019, 52, 495402. [ Google Scholar] [ CrossRef] Table 4 shows the relationship between seawater salinity, refractive index and % Brix. If a refractometer has a resolution (not accuracy, but resolution, which is the finest amount it can distinguish) of 0.2 % Brix, then that translates to about +/- 1 ppt. So the best resolution would translate to 35 ppt seawater reading 34-36 ppt, which may be adequate for reef aquarists. A Brix refractometer that reads 0 to 10 % Brix with a resolution of 0.1% Brix might be a fine choice for determining seawater salinity in a reef aquarium, (although they are not inexpensive). Some Brix refractometers have a resolution of 0.5 % Brix or even 1% Brix, and they would not be suitable choices.Millero, F.J.; Feistel, R.; Wright, D.G.; McDougall, T.J. The composition of Standard Seawater and the definition of the Reference-Composition Salinity Scale. Deep Sea Res. Part I Oceanogr. Res. Pap. 2008, 55, 50–72. [ Google Scholar] [ CrossRef] One suitable commercial standard is made by American Marine and sold under the brand name Pinpoint. It is sold as a 53 mS/cm calibration fluid for the company’s electronic salinity probe (a conductivity probe), but it also is suitable for use in a refractometer. NOTE that this is not necessarily true of all 53 mS/cm conductivity standards. The Pinpoint fluid happens to be made to match seawater in other respects, not just conductivity, but other brands, or do-it-yourself 53 mS/cm standards, may not be appropriate to use with a refractometer because, while they have the same conductivity as 35 ppt seawater, they may not have the same refractive index. Products | JFE Advantech Co., Ltd. [EB/OL]. Available online: https://www.jfe-advantech.co.jp/eng/products/ (accessed on 30 May 2022).