The refractive index of liquids change with temperature and therefore has a major effect on measuring salinity with a refractometer. 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. Dropping or rough treatment can damage the internal ATC system or misalign the optics. Handle with care.

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): 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. 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] L. W. Tilton, Prism Refractometry and Certain Goniometrical Requirements for Precision National Bureau of Standards (Gaithersairg, 1929).Fofonoff, N.P.; Millard, R.C. Algorithms for Computation of Fundamental Properties of Seawater; Unesco Technical Papers in Marine Science; UNESCO: Paris, France, 1983. [ Google Scholar] Figure 6. 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. Monitoring the salinity of your reef aquarium is an important step in maintaining a stable environment for your fish and corals. There are two ways to define the strength of saltwater; one being Salinity, which uses parts per thousand (ppt) as the unit of measure, and the other is specific gravity which is the density of a substance (seawater) in ratio to a standard (pure water) - both are measured by a refractometer. Seawater has a salinity that measures about 35 ppt, or in specific gravity around 1.026. Fresh RO/DI water measures 0 ppt salinity or 1.000 specific gravity. We recommend that you form a habit of measuring salinity in parts per thousand (ppt). This 3.65 weight percent sodium chloride solution can be made by dissolving 3.65 grams of sodium chloride in 96.35 grams (mL) of purified freshwater. This recipe can be scaled to any appropriate size if suitable instruments are available (36.5 grams in 963.5 grams (mL) of water, 0.365 grams in 9.635 g (mL) of water, etc.). P. A. Pavlov, Yu. V. Filatov, and I. B. Zhuravleva, “Calibration of rotary encoders with different interfaces by means of a dynamic goniometer,” Opt. Eng. 60 (7), 074105 (2021).

Open the cover and wipe the prism with a soft cloth to remove fingerprints. There should be no salt residue on the prism or cover. Rinse with RO water if necessary and dry with a soft cloth. Okkonen, S.; Weingartner, T.; Danielson, S.; Musgrave, D.; Schmidt, G. Satellite and hydrographic observations of eddy-induced shelf-slope exchange in the northwestern Gulf of Alaska. J. Geophys. Res. Ocean. 2003, 108, 1–15. [ Google Scholar] [ CrossRef]

Supplementary material

Figure 20. 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 adequately accurate over the range of specific gravity from 1.020 to 1.030 despite the slope error. This figure is an expansion of Figure 19 in the region of most interest to reef aquarists. 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).

Regardless of a salt refractometer’s scale reading (ppt or specific gravity), aquarists can get around this problem by calibrating this type of refractometer in a seawater standard (see below). Because that type of calibration also gets around important manufacturing errors (slope calibration defects due to the scale being the wrong dimensions), it solves both problems at once (however, certain digital refractometers such as the Milwaukee can only be calibrated with pure fresh water. That requirement is OK in the Milwaukee case, since it is a “True Seawater” refractometer). Seawater’s salinity is generally defined in parts per thousand salt by weight (ppt) or in practical salinity units (PSU), which often is shown simply as S=35, or whatever the value actually is. In this article I will mostly use ppt, because that more appropriately applies to solutions whose composition deviates greatly from seawater (such as sodium chloride solutions used to make certain standards).OS320Plus—Idronaut. [EB/OL]. Available online: https://www.idronaut.it/multiparameter-tds/oceanographic-ctds/os320plus-oceanographic-ctd/ (accessed on 30 May 2022).