Salinity Refractometer for Seawater and Marine Fishkeeping Aquarium 0-100 PPT with Automatic Temperature Compensation

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First calibrate the refractometer in pure freshwater. This can be distilled water, RO (reverse osmosis) water, RO/DI water, bottled water and even tap water with reasonably low TDS (total dissolved solids). Calibrating with tap water that has a TDS value of 350 ppm introduces only about a 1% error in salinity, causing readings in seawater to read a bit low. So 35 ppt seawater (specific gravity = 1.0264) will read to be about 34.7 ppt, and will show a specific gravity of about 1.0261. Keep the prism free of fingerprints. Use a soft eyeglass cloth to keep it clean. Never use a paper towel or regular cloth. It will scratch the prism.

This calibration should ordinarily be carried out at room temperature using an ATC refractometer. The directions with some ATC refractometers insist that the calibration be carried out at a specific temperature, but I’ve never understood how that could possibly matter (unless the ATC function is not accurate) and I would not worry about it. If the refractometer is not an ATC refractometer, then careful temperature control or correction is necessary, and such corrections are beyond the scope of this article. Figure 19. The refractometer of Figure 15 and 16 (red) has a slope error, with values far from the calibration point reading incorrectly. In this figure it has been recalibrated with seawater and so is accurate in the region around the specific gravity of seawater, but not in the region of freshwater (specific gravity = 1.000). Products | JFE Advantech Co., Ltd. [EB/OL]. Available online: https://www.jfe-advantech.co.jp/eng/products/ (accessed on 30 May 2022). 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,

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These same issues apply to refractometers that read in units of salinity (ppt) or specific gravity. In those cases, the measured and true salinity (or specific gravity) relate to one another in exactly the same way that measured and true refractive index relate to each other in Figures 6-8. Figure 9, for example, shows the relationship between the measured and actual specific gravity for a refractometer with an offset miscalibration. It is clear that seawater (35 ppt) which has an actual specific gravity of 1.0264 reads much lower in this case, at about 1.0235. Similarly, Figure 10 shows the relationship between the measured and actual salinity for a refractometer with an offset miscalibration. It is clear that seawater (35 ppt) reads much lower in this case, at about 31 ppt.

C. Forch, M. Knudsen, and S. P. Sorensen, Berichte uber konstantenbestimmungen zur Aufstellung der hydrographischen Tabellen, D. Kgl. Danske Vidensk., Selsk., Skrifter, 6 Raekke Naturvidensk. OgMath. 1902. Afd. 12, H. 1. P. 1–151. Figure 9. 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 in measuring the specific gravity of seawater with a real refractive index of 1.0264 is indicated. Dropping or rough treatment can damage the internal ATC system or misalign the optics. Handle with care.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. CastAway CTD—Handheld Deployable CTD with Integrated GPS and LCD Display. [EB/OL]. Available online: https://www.sontek.com/castaway-ctd/ (accessed on 30 May 2022). The index of refraction (or refractive index) is the ratio of the speed of light traveling through a vacuum to the speed of light in the material being tested. Most aquarists do not realize that when using a refractometer, they are measuring the speed of light through their aquarium’s water, so having such knowledge might be a good way to impress friends with your technical abilities! Broadbent, H.A.; Ivanov, S.Z.; Fries, D.P. A miniature, low cost CTD system for coastal salinity measurements. Meas. Sci. Technol. 2007, 18, 3295. [ Google Scholar] [ CrossRef]

Thermodynamic Equation of Seawater-2010 TEOS-10. Canada, 2022. http://www.TEOS-10.org. Cited September 20, 2022. Brown, W.S. Physical properties of seawater. In Springer Handbook of Ocean Engineering; Springer: Cham, Switzerland, 2016; pp. 101–110. [ Google Scholar] Recommendations aside, high quality reef aquaria exist with a fairly wide range of salinity. Many highly successful reef aquaria have salinity in the range of 32-36 ppt, or specific gravity in the range of 1.024 to 1.027.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). Atkinson, L.P.; Lee, T.N.; Blanton, J.O.; Paffenhöfer, G.A. Hydrographic observations. Prog. Oceanogr. 1987, 19, 231–266. [ Google Scholar] [ CrossRef]

This article describes how refractometers work, what the concerns are with different types of commercial models that may be less than optimal for reef aquarium purposes, and how best to calibrate them. Some of the sections in the middle of the article, which detail exactly how refractometers are calibrated and potentially miscalibrated, can be skipped by folks only interested in using them, rather than the exacting details of how they work, but those sections do give the rationale for why certain issues arise with refractometers.

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As you look through the eyepiece, you’ll see the scale for salinity and specific gravity with a distinct shift in color between the upper and lower section of the scale. That lateral line of color separation is the salinity level in specific gravity and PPT measurements (as seen in photo below). Figure 22. The refractometer of Figure 17 and 18 (red) has a slope error, with values far from the calibration point reading incorrectly. In this figure it has been recalibrated with seawater, and so is adequately accurate over the range of salinity of 30-40 ppt despite the slope error. This figure is an expansion of Figure 21 in the region of most interest to reef aquarists.