The relationship between the real (actual) refractive index and the measured refractive index for an incorrectly calibrated refractometer (red) and a perfectly calibrated refractometer (green). The RFH200 Series is ideal for the measurement of salinity, saltwater, and brine, and for food processing and canning applications. 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. Refractometer is an optical instrument used in measuring liquid concentration (specific gravity). NOTE that this is not necessarily true of all 53 mS/cm conductivity standards. The physics of light is such that when it passes from a medium of one refractive index to one with a different refractive index, the light bends (refracts) at the interface, rather than passing straight through. A refractometer that has a range spanning that value is required. Because many refractometers are designed to use aqueous (water) solutions, the bimetal strip can be designed to account for the change in refractive index of aqueous solutions with temperature, although it may not be perfect in some situations because salts and other materials in the water can change temperature’s effects on refractive index by a small extent (or possibly to a large extent for very concentrated solutions, such as 750% sugar in water, but seawater is not in that category). Fortunately for aquarists, most marine organisms are fairly forgiving of the exact salinity, and high quality reef aquaria can seemingly thrive with a wide range of salinity. If it is going to be calibrated in pure freshwater, the range must extend to 1.3330 (which is almost always the case). Salinity Refractometer S-10E Concentration measurement of sea water Refractometers are commonly used to measure sea water and control the salinity of water from fisheries and fish farms, natural sea water for aquariums, artificial sea water, and the salt water used for fish storage. Principle of salinity measurement using a refractometer The seawater refractive index varies proportionally to density, which is strongly correlated with salinity; therefore, the measurement of seawater refractive index can be used to measure its absolute salinity. Some Brix refractometers have a resolution of 0.5 % Brix or even 1% Brix, and they would not be suitable choices. The bimetal strip is attached to the optics inside the refractometer, moving them slightly as the temperature changes. A refractometer is a laboratory or field device for the measurement of an index of refraction (refractometry).The index of refraction is calculated from Snell's law while for mixtures, the index of refraction can be calculated from the composition of the material using several mixing rules such as the Gladstone–Dale relation and Lorentz–Lorenz equation Figure 16. Figure 20. … In that case, no amount of moving the scale up or down can make it accurate at all values of refractive index. These same issues apply to refractometers that read in units of salinity (ppt) or specific gravity. Fortunately, there are many brands of commercial standards available today, although do not know whether they have all been made correctly. 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). 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. 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. Figure 10. In a previous article I described how to make a do-it-yourself refractometer standard matching 35 ppt seawater, and I will just summarize that recipe here. Figure 5. Consequently, calibrating them for use in marine aquaria is often not best done the way the manufacturer recommends. If you are using a refractometer for hyposalinity, such as when treating a sick fish, I’d just use one calibrated in freshwater, because that is closer in salinity than seawater to the hyposaline solution usually used (say, specific gravity = 1.009). 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. These results indicate that some effects could relate to shifts between these ions in a reef aquarium, but that these effects are small. 2. Scientists have long known this to be true, and refractometers have a long history of use in brewing, sugar refining, analyzing blood and urine protein and many other industries where a quick measure of refractive index can lead to a good assessment of what is present. The second step in calibration should be performed at least once before relying on a refractometer to accurately measure the salinity of a reef aquarium. Figure 21. This refractometer is a portable, precision optical instrument. Such an error can arise, for example, if the scale is not made to exactly the right dimensions. Figure 4 shows a graph of the measured refractive index vs. the real refractive index for a perfectly calibrated refractometer. A plot of the relationship between the refractive index and the specific gravity of seawater in the range of interest to most reef aquarists. Salinity Refractometer, ATC (Automatic Temperature Compensation) Sea Water Refractometer Meter Salimeter Measurement Tool for Aquarium and Seawater Monitoring, 0-10% (ATC Salinity Meter 0-10%) 4.4 out of 5 stars 70 This concentration roughly corresponds to ¼ cup (73.1 g) of Morton’s Iodized Salt dissolved into two liters (2000 g) of water (giving very slightly more than 2 L of total volume). The relationship between the real (actual) salinity and the measured salinity (in ppt) for a perfectly calibrated seawater refractometer (green) and a perfectly calibrated salt refractometer (red). Its scale usually reads in Brix, or % Brix (percent Brix). -Aquarium- Measuring the salinity level of water Automatic refractometer The automatic measurement of the refractive index of the sample is based on the determination of the critical angle of total reflection.An LED light source is imaged under a wide range of angles … Calibration is accomplished by turning the calibration screw, which raises or lowers the reticle (the scale) relative to the path of the light. Seawater with a salinity of 35 ppt has a refractive index of 1.3394. Yes, but only if it is calibrated using a solution known to have a refractive index close to that of the samples to be tested. You may or may not be able to dry it out without damaging the unit. Calibrating using a liquid matching seawater, for example, can lead to a slope correction as shown in Figure 14. 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). At all points these two values are the same. So do not assume that all 53 mS/cm conductivity standards are suitable for refractometer calibration unless that is stated to be the case. Such conversions of refractive index to salinity or specific gravity are shown in Figures 1 and 2, and Table 1. A plot of the relationship between the refractive index and the salinity of seawater. Then, we will discuss what your […] A new standard for hyposalinity can also be made by mixing one part 35 ppt seawater and two parts freshwater, but that is probably overkill. One is that many refractometers are intended to measure sodium chloride solutions, not seawater. We can use these values to roughly predict how far off salinity measurements might be with some typical changes in the major ions. Salinity Measurement Refractometers can be used to measure the salt concentration of soy sauce, other sauces, juice, ketchup, miso soup, other soups, pickling liquid, seasoning liquid, liquids for preserving seafood, etc... Salinity Refractometers, such as the MASTER-S28α, MASTER-S28M, and the PAL-03S are designed for measuring salt and water. In this figure it has been recalibrated with seawater, and so is adequately accurate over the range of salinity of 30-40 ppt despite the slope error. This refractometer has an offset error, with all values reading lower than the actual value. The relationship between the refractive index and the salinity of seawater, showing that the usual point of calibration using pure freshwater is far from the range of measurement used in reef aquaria. This figure is an expansion of Figure 17 in the region of most interest to reef aquarists. However, large differences in the big four ions (chloride, sulfate, sodium and magnesium) will alter the relationship between refractive index and salinity or specific gravity.Refractive Index and Ion Imbalances in Seawater. The amount it bends or, in technical jargon, the angle of refraction, dep ends on the difference in the two media’s refractive indices. If the refractometer is not an ATC refractometer, then careful temperature control or correction is necessary, and such corrections are beyond the scope of this article. 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. Figure 24. 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). 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. Figure 1. First calibrate the refractometer in pure freshwater. Likewise, the refractive index of different sodium chloride solutions can be found in the scientific literature. This refractometer has an offset error, with all values reading lower than the actual value. For any refractometer, the refractive index of seawater with a salinity of 35 ppt is 1.33940. 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. A commonly manufactured type of refractometer is called a Brix refractometer. 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). Suitable commercial and do-it-yourself standards were described earlier in this article. It is designed for the measurement of seawater or very low concentrated salt Imperfect Refractometer Calibration: Slope Miscalibration. ATO refractometers can measure brix and alcohol content, concentration of coolant (antifreeze), salinity, honey, wort specific gravity One suitable commercial standard is made by American Marine and sold under the brand name Pinpoint. 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). These same issues apply to refractometers that read in units of salinity (ppt) or specific gravity. Standards made with accurate measurements of salt and water, however, will accurately match 35 ppt.]. The index of refraction (or refractive index) is the ratio of the speed of light traveling through a vacuum to the speed of light in the material being tested. Despite the scale reading in ppt (‰) or specific gravity, they are not intended to be used for seawater. That requirement is OK in the Milwaukee case, since it is a “True Seawater” refractometer). For a rougher measurement in the absence of an accurate water volume or weight measurement: [Note: the standard described here using soft drink bottles is subject to variation in the volume of the bottle. For example, to measure the salinity of seawater at 35 ppt, calibrate a refractometer using a standard with the same refractive index, and the slope miscalibration error disappears when measuring seawater samples near that salinity (Figure 14). 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. Figure 6 shows a graph of what I call an offset miscalibration. 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). Can such a refractometer be used? 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. These are often called salt or brine refractometers. For many refractometers used by reef aquarists, the manufacturer calls for pure freshwater to be used for calibration. Salinity Refractometer for Seawater and Marine Fishkeeping Aquarium 0-100 PPT with Automatic Temperature Compensation 4.6 out of 5 stars 984 #1 Best Seller in Aquarium Hydrometers However, the magnitude of this effect is different for every material, and refractometers must somehow take this into account. This red refractometer has a slope error, with values far from the calibration point (freshwater with a specific gravity of 1.000) reading lower than the actual value. 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. The liquid measurement is very easy. Figure 11. Most aquarists do not realize that when using a refractometer, they are measuring the speed of light through their aquarium’s water, so having such knowledge might be a good way to impress friends with your technical abilities! For use in seawater, recalibration with 35 ppt seawater (refractive index = 1.3394) moves the red line onto the green line at the refractive index used for calibration (here, 1.33940), and the refractometer now reads accurately in the region of refractive index similar to seawater. These include conductivity using electronic meters (I use this method), and specific gravity using floating glass hydrometers. Brix measurement of jam, liquid sugar, honey, etc. The refractometer of Figure 17 and 18 (red) has a slope error, with values far from the calibration point reading incorrectly. 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. With a perfectly made refractometer (that hasn’t changed since its manufacture), that single point calibration at the end of the range (Figure 5) would be adequate. Salinity is one of the most important parameters measured in reef aquaria. 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 13. A refractometer is an instrument used to measure the refractive index of gases, liquids, and translucent solids like gemstones. If you have any questions about this article, please visit my forum at Reef2Reef. The specialty refractometers have a wide range of applications, from ethylene and propylene glycol to proteins. This is an offset calibration, as described above. This adjustment is what happens when the calibration screw is adjusted on a refractometer. Next, look The amount of error in measuring seawater is indicated. If the scale is completely light, then the liquid’s refractive index is above the refractometer’s high end. 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. The error can be corrected using a seawater standard. These inaccuracies may be partly because many of these may actually be salt refractometers and not actual seawater refractometers (see next section). Figure 7. Substantial misunderstandings also arise among aquarists as to how specific gravity really relates to salinity, especially considering the effects of temperature. This type of problem with a refractometer IS NOT at all corrected by calibrating it with pure freshwater. Recommendations aside, high quality reef aquaria exist with a fairly wide range of salinity. 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. Refractometers can be a very good way to measure salinity, assuming they are appropriately designed and properly calibrated. 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. Figure 2. The error in measuring the specific gravity of seawater with a real refractive index of 1.0264 is indicated. 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. There are several types of refractometers, but this discussion will focus on hand held refractometers because reef aquarists rarely use any other type. 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. By turning the calibration screw until a seawater standard reads 35 ppt, the red line moves onto the green line and the refractometer is properly calibrated. A potentially better single point calibration might be performed in the middle of the range being used, and for higher accuracy, more than one calibrating solution might be used. The effect can be readily understood in that sodium chloride has a smaller effect on refractive index than does the same weight of magnesium chloride. This refractometer has an offset error, with all values reading lower than the actual value. The black circles represent data points for whole values of the salinity (33 ppt, 34 ppt, 35, ppt, etc.). The refractometer read 1.020, the hydrometer read 1.029. The relationship between the real (actual) salinity and the measured salinity (in ppt) for a perfectly calibrated seawater refractometer (green) and an incorrectly calibrated seawater refractometer (red). Figure 17. The digital refractometer eliminates the 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. Unfortunately, many refractometers used by aquarists fall into this category. Figure 18 is an expansion of the region of salinity of interest to reef aquarists. 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). It is clear that seawater (35 ppt) reads much lower in this case, at about 31 ppt. But that value is specific gravity of a sodium chloride solution with the measured refractive index, not seawater with that refractive index. Those units should be ignored, and if they are all that is available on the refractometer, I’d find another refractometer. and substitute more or less magnesium chloride in place of sodium chloride, while maintaining overall salinity at 35 ppt, we get the results shown in Table 3. This salt refractometer effectively has a significant slope error, with values far from the calibration point (freshwater with a salinity of 0 ppt) reading roughly 1.7 ppt higher than the actual value. Figure 1 plots seawater’s refractive index vs. its salinity. The bending of the light at the liquid/prism interface sends the light higher or lower in the scale’s grid. Salinity Refractometers Hr salinityThe Salinity Refractometers are designed to test the concentration of salt water or brine. Based on such information, my recommendation is to maintain salinity at a natural level of about 35 ppt (abbreviated as ‰ and also as PSU, practical salinity units). Remember to let it sit for at least 30 seconds so it comes to the same temperature as the refractometer. 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. Plastic swing arm hydrometers can be accurate, but seem to be more prone to inaccuracies than electronic meters and glass hydrometers. When using a refractometer that does not account for this effect, temperature changes can be a large source of errors. Changes in refractive index are not suitable for determining trace levels of ions (such as the purity of freshwater coming out of an RO/DI (reverse osmosis/deionization) purification system), but it can do a good job when significant amounts of a known material are present. I suggest that a salinity error of ± 1 ppt or a specific gravity error of ± 0.0075 is allowable. Figure 19. Table 1 shows some typical refractive index values for comparative purposes. Many aquarists have found that when calibrated using pure freshwater, their refractometers do not accurately read 35 ppt seawater standards. Reef aquarists monitor salinity in a variety of ways, including specific gravity measurement using hydrometers, conductivity measurement using electronic meters, and refractive index measurement using refractometers. 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). To provide a standard for refractometers requires a solution whose refractive index is similar to normal seawater. 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). It turns out that an aqueous solution’s refractive index is relatively insensitive to small changes in the solution’s ionic makeup. 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). Calibration is usually performed by putting the freshwater on the refractometer, letting it sit for at least 30 seconds so it comes to the same temperature as the refractometer, and adjusting the calibration screw until it reads a value appropriate for freshwater (e.g., refractive index = 1.3330, salinity = 0 ppt, specific gravity = 1.0000). Figure 8. 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. It is clear that seawater (35 ppt) reads much lower in this case, at about 30 ppt. This figure is an expansion of Figure 21 in the region of most interest to reef aquarists. Without salinity, it isn’t saltwater. The relationship between the real (actual) refractive index and the measured refractive index for an incorrectly calibrated refractometer. These data are also summarized in Table 1. The algorithm for defining the PPT (and S.G.) scale for a refractometer is calculated for a specific temperature. Liquids are tested for things such as blood proteins, salinity, fluid concentrations, and specific gravity of urine. Refractive index generally cannot reveal the identity of compounds in water, but when an aquarist knows roughly what material is there he can determine how much of it is there (within the refractive index’s detection capability). Users who would like to measure only salt out of mixed solutions, the PAL-ES2, PAL-ES3, and the ES-421 are the best choice since those are designed for measuring salt only. Magnum Media Salinity 10ATC is a professional brand of salinity measurement instrument. Brix refractometer applications: Measurement for liquid specific density include brix, salinity, antifreeze, soy milk/milk, alcohol/wort, fruit drink sugar, honey Baume (°Bé), coolant (cutting fluid) concentration, etc. 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. Figure 22. 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. The error in reading refractive index values as far away as that of seawater can be significant, as shown. This movement is designed to exactly cancel temperature’s effects on refractive index, and generally does a very good job IF the refractometer is designed to cancel out the temperature effects of the specific material being analyzed. My CRC Handbook of Chemistry and Physics (57th Edition, Page D-252) has such a table. But overall these issues result in a very small error in salinity (in terms of the precision that reef aquarists are typically concerned with, say, ± 1 ppt), so the conclusion is that refractive index is a suitable way to measure salinity regardless of ordinary chemical imbalances. 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. In those cases, the measured and true salinity (or specific gravity) relate to one another in exactly the same way that measured and true refractive index relate to each other in Figures 6-8. It goes without saying that salinity is the most fundamental parameter in a saltwater aquarium. Some materials slow light traveling through them more than others, and slower light travel leads to a higher refractive index. Most liquid materials expand slightly when heated and shrink when cooled. Add one teaspoon of salt (making about 79.3 g total salt). Then, close the plate over the prism carefully and look through the other end of the device. This bimetal strip expands and contracts as the temperature changes. 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). Once checked to be sure that they were made correctly, they may provide years of service, providing they are not dropped onto a hard surface or into an aquarium. This refractometer has an offset error, with all values reading lower than the actual value. Some refractometers, such as the Milwaukee Digital Seawater Refractometer, carry out these same measurement functions automatically, giving a digital readout in any of several units of measure.