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

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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). 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. Rahman, H.A.; Harun, S.W.; Yasin, M.; Phang, S.W.; Damanhuri, S.S.A.; Arof, H.; Ahmad, H. Tapered plastic multimode fiber sensor for salinity detection. Sens. Actuators A Phys. 2011, 171, 219–222. [ Google Scholar] [ CrossRef] 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). To correct errors using these seawater standards, simply adjust the calibration screw on the refractometer until it reads the correct value for the standard (35 ppt, or a specific gravity of 1.0264, or a refractive index of 1.33940). This type of slope calibration makes the refractometer suitable to read solutions whose salinity is close to seawater’s. After such a calibration, refractometers may not read freshwater correctly.

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. Minato, H.; Kakui, Y.; Nishimoto, A.; Nanjo, M. Remote refractive index difference meter for salinity sensor. IEEE Trans. Instrum. Meas. 1989, 38, 608–612. [ Google Scholar] [ CrossRef]Skinner, A.J.; Lambert, M.F. An Automatic Soil Pore-Water Salinity Sensor Based on a Wetting-Front Detector. IEEE Sens. J. 2011, 11, 245–254. [ Google Scholar] [ CrossRef] X. Quan and E. S. Fry, “Empirical equation for the index of refraction of seawater,” Appl. Opt. 34 (18), 3477–3480 (1995). Selecting a suitable refractometer to use to measure salinity requires first determining whether it covers the appropriate range of interest. For any refractometer, the refractive index of seawater with a salinity of 35 ppt is 1.33940. A refractometer that has a range spanning that value is required. If it is going to be calibrated in pure freshwater, the range must extend to 1.3330 (which is almost always the case). If the range is too wide, or the precision is too low for other reasons, then the uncertainty of a particular measurement will be too high. From Table 2 we can see that an uncertainty of ± 0.00018 in refractive index corresponds to an uncertainty of about ± 1 ppt in salinity (say, 34-36 ppt) or ± 0.00075 in specific gravity (say, 1.0255 to 1.0270). So, readability of a refractometer to 0.0002 refractive index units or better is reasonable for most reef aquarium applications. 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). Salinity is one of the most important parameters measured in reef aquaria. It controls not only the salt balance between an organism and its surrounding environment, but also the levels of a host of ions in seawater that aquarists neither measure nor control independently. Consequently, aquarists must monitor salinity to ensure that organisms are not stressed by moving between aquaria of potentially different salinity, and that the salinity of the aquarium itself is controlled within ranges that organisms thrive in.

IOC; SCOR; IAPSO. The International Thermodynamic Equation of Seawater-2010: Calculation and Use of Thermodynamic Properties; Intergovernmental Oceanographic Commission: Paris, France, 2010. [ Google Scholar]If selecting a refractometer that reads in ppt or specific gravity, it is important to be sure that it is either a true seawater refractometer, or a salt (brine) refractometer, and not a clinical refractometer. For either a true seawater refractometer, or a salt (brine) refractometer (recognizing the differences and potential inaccuracies of salt refractometers that were described earlier in the article), the range needs to include about 30-40 ppt and/or a specific gravity of about 1.022 – 1.029. If it is going to be calibrated in pure freshwater, the range must extend to 0 ppt and specific gravity = 1.0000 (which is almost always the case). If the range is too wide, or the precision is too low for other reasons, then the uncertainty of a particular measurement will be too high. Readability to ± 1 ppt (say, 34-36 ppt) in salinity or ± 0.00075 in specific gravity (say, 1.0255 to 1.0270) is desirable. Figure 21. 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 accurate in the region around the salinity of seawater, but not in the region of freshwater (salinity = 0 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).