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

Product Information

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. In the case of a refractometer, the light bends in proportion to the liquid’s refractive index. As the light then travels down the refractometer, it passes through lenses and lands on a scale. The bending of the light at the liquid/prism interface sends the light higher or lower in the scale’s grid. Aquarists then look through the viewfinder on the other end and read where the light is falling on the scale. Light covers a portion of the scale, and the remainder is dark. The dividing line between light and dark is the place to read the scale. Calibration is accomplished by turning the calibration screw, which raises or lowers the reticle (the scale) relative to the path of the light. Correction of slope miscalibration errors should be carried out using a fluid that approximately matches the refractive index of the water being tested, so for reef aquarium water, calibration with 35 ppt seawater solves this problem, while calibration with pure freshwater does not. CTD 115M—Sea & Sun Technology. [EB/OL]. Available online: https://www.sea-sun-tech.com/product/multiparameter-probe-ctd-115-memory/ (accessed on 30 May 2022).

The refractive index of liquids change with temperature and therefore has a major effect on measuring salinity with a refractometer. 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). 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.

Just as was shown for refractive index, recalibration of a refractometer with an offset error can be discussed in terms of specific gravity and salinity. Figure 11 shows what happens when adjusting the calibration screw so that the specific gravity of a 35ppt seawater standard (with a known specific gravity of 1.0264) really reads 1.0264. In this figure, the miscalibrated red line moves exactly onto the green line, and the refractometer is then good to go at all specific gravity values. Similarly, Figure 12 shows what happens when adjusting the calibration screw so that the salinity of a 35 ppt seawater standard really reads 35 ppt. In this figure, the miscalibrated red line moves exactly onto the green line, and the refractometer is then good to go at all specific gravity values. Atkinson, L.P.; Lee, T.N.; Blanton, J.O.; Paffenhöfer, G.A. Hydrographic observations. Prog. Oceanogr. 1987, 19, 231–266. [ Google Scholar] [ CrossRef] Seawater contains approximately 70 chemical elements that include Calcium, Magnesium, Potassium etc. in addition to the Sodium Chloride. Fixing this problem requires simply adjusting the offset. This adjustment is what happens when the calibration screw is adjusted on a refractometer. The scale simply moves up or down inside the refractometer (or in some other way the scale moves relative to the refracted light) as the user turns the screw that moves it. The scale’s apparent reading changes, and the user turns the screw until the scale’s reading matches the known refractive index of the standard being used for calibration. Figure 7 shows how the relationship between the reported refractive index and the real refractive index changes during this type of calibration when using pure freshwater for calibration. Figure 8 shows how the relationship between the reported refractive index and the real refractive index changes during this type of calibration when using 35 ppt seawater for calibration. Both methods work equally well for this type of correction.

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). 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.R. A. Cox, F. Culkin, and J. P. Riley, “The electrical conductivity,” Deep-Sea Res. 4 (17), 679–689 (1970). Standard Loggers—RBRvirtuoso, RBRduo, RBRconcerto, RBRmaestro. [EB/OL]. Available online: https://rbr-global.com/products/standard-loggers/ (accessed on 30 May 2022). A. I. Yurin, G. N. Vishnyakov, and V. L. Minaev, “Measurement of the refractive index using a modified constant deviation method,” Izmeritel’naya Tekhnika, No. 12, 35–39 (2022).

This analysis makes it clear that offset miscalibration is readily corrected by turning the refractometer’s adjustment screw, and that it can be corrected using either pure freshwater or 35 ppt seawater for calibration. 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. Wu, C.; Gao, W.; Zou, J.; Jin, Q.; Jian, J. Design and batch microfabrication of a high precision conductivity and temperature sensor for marine measurement. IEEE Sens. J. 2020, 20, 10179–10186. [ Google Scholar] [ CrossRef]Akter, S.; Ahmed, K.; El-Naggar, S.A.; Taya, S.A.; Nguyen, T.K.; Dhasarathan, V. Highly Sensitive Refractive Index Sensor for Temperature and Salinity Measurement of Seawater. Optik 2020, 216, 164901. [ Google Scholar] [ CrossRef] L. W. Tilton, Prism Refractometry and Certain Goniometrical Requirements for Precision National Bureau of Standards (Gaithersairg, 1929).

International Oceanographic Tables (National Institute of Oceanography, Paris; Great Britain and UNESCO, 1966).

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] Lewis, E. The practical salinity scale 1978 and its antecedents. IEEE J. Ocean. Eng. 1980, 5, 3–8. [ Google Scholar] [ CrossRef]