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

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V. I. Zaburdaev, “Instability of the salt composition of seawater in reliability of indirect estimates of salinity and density from the electricconductivity, temperature, and pressure,” Morskoi Gidrofiz. Zh., No. 4, 50–56 (1985). 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 specific gravity relate to each other in Figures 13 and 14. Figure 15, for example, shows the relationship between the measured and actual specific gravity for a refractometer with a slope miscalibration. Figure 16 is an expansion of the region of specific gravity of interest to reef aquarists. 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. Aravamudhan, S.; Bhat, S.; Bethala, B.; Bhansali, S.; Langebrake, L. MEMS based conductivity-temperature-depth (CTD) sensor for harsh oceanic environment. In Proceedings of the OCEANS 2005 MTS/IEEE, Washington, DC, USA, 17–23 September 2005; pp. 1785–1789. [ Google Scholar] [ CrossRef] Similarly, Figure 21 shows what happens when adjusting the calibration screw so that the salinity of a 35ppt seawater standard really reads 35 ppt. Figure 20 is an expansion of the region of salinity of interest to reef aquarists. In this figure, the miscalibrated red line moves onto the green line, and the refractometer is then good to go at salinity values near 35 ppt (say, 30 to 40 ppt), but it is no longer accurate in freshwater (salinity = 0 ppt; Figure 22).

Figure 8. The relationship between the real (actual) refractive index and the measured refractive index for an incorrectly calibrated refractometer. This refractometer has an offset error, with all values reading lower than the actual value. This type of error can be corrected by recalibrating with 35 ppt seawater (refractive index = 1.3394) as shown as well as by calibrating with pure freshwater (Figure 7). Atkinson, L.P.; Lee, T.N.; Blanton, J.O.; Paffenhöfer, G.A. Hydrographic observations. Prog. Oceanogr. 1987, 19, 231–266. [ Google Scholar] [ CrossRef] Fortunately for aquarists, the differences between a salt refractometer and a seawater refractometer are not too large. A 35 ppt sodium chloride solution (3.5 weight percent sodium chloride in water) has the same refractive index as a 33.3 ppt seawater solution, so the error in using a perfectly calibrated salt refractometer is about 1.7 ppt, or 5% of the total salinity. This error is significant, in my opinion, but not usually enough to cause a reef aquarium to fail, assuming the aquarist has targeted an appropriate salinity in the first place. Figure 23 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 salinity. This figure shows the measured salinity reading for seawater being about 1.7 ppt higher than it really is. Salinity is a general term describing the concentration of salt in water. However, solutions of different type of salts have different refractive indexes.R. A. Cox, F. Culkin, and J. P. Riley, “The electrical conductivity,” Deep-Sea Res. 4 (17), 679–689 (1970). For example, using a seawater refractometer calibrated at 20 oC 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 25 oC. Seawater contains approximately 70 chemical elements that include Calcium, Magnesium, Potassium etc. in addition to the Sodium Chloride. Dissolve the total salt (79.3 g) in the total water volume (2104 g) to make an approximately 3.65 weight percent solution of NaCl. The volume of this solution will be slightly larger than the Coke bottle, so dissolve it in another container.

Some medical and veterinary labs use a type of refractometer called a “clinical refractometer.” These are normally used to measure proteins in urine, serum and other biological fluids. The scale can read in units familiar to reef aquarists (ppt or specific gravity), but that is ppt or specific gravity of a protein solution, not a seawater solution. Those units should be ignored, and if they are all that is available on the refractometer, I’d find another refractometer. Without a conversion table to seawater salinity or specific gravity, such readings cannot be used to gauge seawater’s salinity as they will be way off. Some clinical refractometers read in refractive index, which is okay if you match the refractive index to the appropriate seawater refractive index (e.g., 35 ppt seawater has a refractive index of 1.33940). Such conversions of refractive index to salinity or specific gravity are shown in Figures 1 and 2, and Table 1.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] Products | JFE Advantech Co., Ltd. [EB/OL]. Available online: https://www.jfe-advantech.co.jp/eng/products/ (accessed on 30 May 2022). If somehow a refractometer is not perfectly made or calibrated, two different types of errors are often encountered. Figure 6 shows a graph of what I call an offset miscalibration. Essentially, the refractometer reads a refractive index that is either lower or higher than the real refractive index, and this difference, or “offset,” is the same at all values of the refractive index. This type of miscalibration is, for example, what happens when the calibration screw on a perfect refractometer is intentionally moved off perfect calibration. A second way that refractometers can give incorrect values is when they are imperfectly made or are made for an application different from seawater. One such error results in what I call a slope miscalibration (Figure 13). Unfortunately, many of the refractometers sold to aquarium hobbyists seem to have this error since they are designed for sodium chloride (brine) solutions, rather than seawater (more on this later). With this sort of error, the refractometer reads a refractive index that is either lower or higher than the real refractive index, and this difference changes with the difference from some point of calibration (here chosen as the bottom left hand corner, matching pure freshwater). In this case, the error becomes larger and larger as the reading moves away from the point of calibration. Such an error can arise, for example, if the scale is not made to exactly the right dimensions. In that case, no amount of moving the scale up or down can make it accurate at all values of refractive index.

Just as was shown for refractive index, recalibration of a refractometer with a slope error can be discussed in terms of specific gravity and salinity. Figure 19 shows what happens when adjusting the calibration screw so that the specific gravity of a 35 ppt seawater standard (with a known specific gravity of 1.0264) really reads 1.0264. Figure 20 is an expansion of the region of salinity of interest to reef aquarists. In this figure, the miscalibrated red line moves onto the green line, and the refractometer is then good to go at specific gravity values near 1.0264 (say, 1.020 to 1.030), but it is no longer accurate at a specific gravity of 1.000 (freshwater; Figure 19). 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).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). Despite the fact that many refractometers sold to aquarists recommend calibration in pure water, such a calibration alone will not ensure accuracy for the reasons described above. My recommendation for calibration is as follows (except for certain digital refractometers which must use pure fresh water): 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]