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

Product Information

Midas CTD PLUS—Valeport. [EB/OL]. Available online: https://www.valeport.co.uk/products/midas-ctd-plus/ (accessed on 30 May 2022). 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. R. A. Cox, F. Culkin, and J. P. Riley, “The electrical conductivity,” Deep-Sea Res. 4 (17), 679–689 (1970).

Figure 11. 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 can be corrected using a seawater standard. By turning the calibration screw until a seawater standard reads 1.0264, the red line moves onto the green line and the refractometer is properly calibrated. In this case, accurate calibration can also be performed using freshwater. If it does not read correctly, and is off by an amount that is significant relative to your salinity precision requirements, then you need to recalibrate it using this second fluid. I suggest that a salinity error of ± 1 ppt or a specific gravity error of ± 0.0075 is allowable. If the refractometer is off significantly, and you used a do-it-yourself standard made with crude techniques such as Coke bottles, a good next step might be to buy a commercial standard. 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). Other methods of salinity determination are also quite suitable for reef aquarists. These include conductivity using electronic meters (I use this method), and specific gravity using floating glass hydrometers. Plastic swing arm hydrometers can be accurate, but seem to be more prone to inaccuracies than electronic meters and glass hydrometers. In general, it is good to calibrate any device used with a seawater standard at least once to confirm its proper operation before relying on it to gauge the salinity in a reef aquarium.For example, refractive index cannot determine whether a salt in water is potassium sulfate, sodium chloride, magnesium nitrate or calcium bromide, but if you know which of these you have by some other means (such as the name on a chemical’s bottle), then you can determine how much is present in solution by measuring the refractive index, and then looking it up in a table that relates the refractive index to the concentration of that material.

Wang, L.; Xu, Y.; Geng, Y.; Wang, J.; Du, Y.; Yi, D.; Hong, X.; Li, X. High-sensitivity fiber salinity sensor based on an exposed-core microstructured fiber interferometer. J. Phys. D Appl. Phys. 2019, 52, 495402. [ Google Scholar] [ CrossRef]

Light travels through most materials more slowly than it does through a vacuum, so their refractive index is higher than 1.00000. The detailed mathematics and physics behind refractive index are actually quite complicated, because it is often a complex number with real and imaginary parts, but a simple version is adequate for all purposes that a reef aquarist would encounter. Some materials slow light traveling through them more than others, and slower light travel leads to a higher refractive index. Table 1 shows some typical refractive index values for comparative purposes. This type of slope correction turns out to be important for reef aquarists, as slope miscalibration errors seem to abound in inexpensive refractometers. Many aquarists have found that when calibrated using pure freshwater, their refractometers do not accurately read 35 ppt seawater standards. Many read 1 ppt, which is likely acceptable to most aquarists, but some read much further from the actual value. These inaccuracies may be partly because many of these may actually be salt refractometers and not actual seawater refractometers (see next section).

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). Figure 17. The relationship between the real (actual) salinity and the measured salinity (in ppt) for an incorrectly calibrated seawater refractometer (red) and a perfectly calibrated seawater refractometer (green). This red refractometer has a slope error, with values far from the calibration point (freshwater with a salinity of 0 ppt) reading lower than the actual value. The amount of error in measuring seawater is indicated. 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] Rinse off saltwater with RO water. Any residual salt will cause inaccurate readings the next time you use the refractometer.

Why Switch to a Refractometer?

Assuming that a refractometer is made correctly for the fluid it is intended to measure, the way to calibrate a refractometer is to put a liquid of known refractive index on it, and adjust the scale’s position by turning the calibration screw (Figure 3) until it reads correctly. When a refractometer is perfectly calibrated, it will show the fluid’s exact refractive index (assuming that it reports the results in refractive index, but this is not always the case). Figure 4 shows a graph of the measured refractive index vs. the real refractive index for a perfectly calibrated refractometer. At all points these two values are the same. While this graph alone is not particularly enlightening, it forms the basis of later graphs that explain how errors in calibration get corrected. Figure 7. 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 pure freshwater (refractive index = 1.3330) as shown as well as by calibrating with seawater (Figure 8). Cong, J.; Zhang, X.; Chen, K.; Xu, J. Fiber optic Bragg grating sensor based on hydrogels for measuring salinity. Sens.Actuators B Chem. 2002, 87, 487–490. [ Google Scholar] [ CrossRef]