Refractometers and Salinity Measurement

$: Refractometers and Salinity Measurement$
4/29/13 Refractometers and Salinity Measurement by Randy Holmes-Farley - Reefkeeping.com

reefkeeping.com/issues/2006-12/rhf/ 1/29

Refractometers and Salinity Measurement

Salinity is one of the most important parameters measured in reef aquaria. It controls not onlythe 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.

Fortunately for aquarists, most marine organisms are fairly forgiving of the exact salinity, and high

quality reef aquaria can seemingly have a fairly wide range of salinity. Reef aquarists monitor salinityin a variety of ways. These include specific gravity measurement using hydrometers, conductivity

measurement using electronic meters and refractive index measurement using refractometers. For

many years reef hobbyists have had high expectations of accuracy when using refractometers. Tosome extent this may be because early models may have been more accurate than some of the very

inexpensive refractometers in use today, but the lack of standards available to actually test themprobably also contributed to this acceptance of their accuracy. Now that such standards are both

commercially available and can be DIY projects, many aquarists have come to find that theirrefractometers are not as accurate as they had assumed.

This article describes how refractometers work, what the concerns are with different types of

commercial models that may be less than optimal for reef aquarium purposes, and how best tocalibrate them (which is often not what the directions that come with them claim).

The sections are:

General Salinity Discussion

What is the Index of Refraction?Refractive Index and Salinity

Refractive Index and Ion Imbalances in SeawaterHow a Refractometer WorksTemperature and Refractive Index: ATC

http://reefkeeping.com/issues/2004-05/rhf/index.php

http://www.advancedaquarist.com/issues/jan2002/chemistry.htm

http://www.aquariumfish.com/aquariumfish/detail.aspx?aid=1804

http://reefkeeping.com/issues/2006-12/rhf/#1








Temperature and Refractive Index: ATC

Refractometer CalibrationImperfect Refractometer Calibration: Offset Miscalibration

Imperfect Refractometer Calibration: Slope MiscalibrationImperfect Refractometer Use: Scale Misunderstanding and Salt RefractometersBrix Refractometers

Clinical RefractometersCommercial Refractometer Standards

Do-it-yourself Refractometer StandardsTips on Selecting a RefractometerTips on Calibrating a Refractometer

Other Tips on Using a RefractometerSummary

General Salinity Discussion

As far as I know, little evidence suggests that keeping a coral reef aquarium at anything other than a

natural oceanic salinity level is preferable to natural seawater's salinity. It nevertheless appears to becommon practice to keep marine fish and, in many cases, reef aquaria, at somewhat lower than naturaloceanic salinity levels. This practice stems, at least in part, from the belief that fish are less stressed at

reduced salinity. Substantial misunderstandings also arise among aquarists as to how specific

gravity really relates to salinity, especially considering the effects of temperature.

Seawater's salinity is generally defined in parts per thousand by weight (ppt) or in practical salinityunits (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).

The salinity on natural reefs has been discussed in a previous article. Based on such information, my

recommendation is to maintain salinity at a natural level of about 35 ppt (abbreviated as ‰ and also asPSU, practical salinity units). If the aquarium's organisms are from brackish environments with lower

salinity, or from the Red Sea with higher salinity, selecting something other than 35 ppt may make

good sense. Otherwise, I suggest targeting a salinity of 35 ppt (specific gravity = 1.0264;conductivity = 53 mS/cm; refractive index = 1.33940).

Recommendations aside, high quality reef aquaria exist with a fairly wide range of salinity. Many

highly successful reef aquaria have salinity in the range of 32-36 ppt, or specific gravity in the range of

1.024 to 1.027.

What is the Index of Refraction?

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. Most aquarists do not realize that when usinga 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!

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 areactually 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.















http://web.archive.org/web/20030218193420/www.animalnetwork.com/fish2/aqfm/1997/nov/features/1/default.asp



T able 1. Index of Refraction of Various Materials.

Material Index of Refraction

Vacuum 1.0000

Air 1.0003

Water (pure) 1 .3330

Seawater (35 ppt) 1 .3394

Ethy l alcohol 1 .361

Sugar Ssolution (80% sugar) 1 .49

Glass (soda lime) 1 .510

Bromine (liquid) 1 .661

Ruby 1.7 60

Diamond 2.417

In solutions of two compounds, such as ethyl alcohol in water, sugar in water or salt in water, the

refractive index changes in step with how much of each component is present. 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.

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). Changes in refractive index are not suitable for determining trace levels ofions (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.

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 someother 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.

Refractive Index and Salinity

Aquarists can use the effects that added salts have on the refractive index of a water solution to

determine the salinity of reef aquarium water. As the salinity of seawater rises, the amount of saltadded rises, so the refractive index rises. Figure 1 plots seawater's refractive index vs. its salinity.

Figure 2 shows a similar plot of seawater's refractive index vs. specific gravity. These data are also

summarized in Table 1. These sets of data demonstrate how aquarists can use refractive index to

measure salinity and specific gravity, assuming they have a refractometer that can read in theappropriate refractive index range.

http://www.robinwood.com/Catalog/Technical/Gen3DTuts/Gen3DPages/RefractionIndexList.html

http://www.kayelaby.npl.co.uk/general_physics/2_5/2_5_7.html




Figure 1. A plot of the relationship between the refractive index and the salinity of seawater.

Figure 2. A plot of the relationship between the refractive index and the specificgrav ity of seawater in the range of interest to most reef aquarists. The black circlesrepresent data points for whole values of the salinity (33, ppt, 34 ppt, 35, ppt, etc).

T able 2. Specific gravity and refractive index as a function of seawater’ssalinity of seawater. T he darker blue rows represent the range usually



salinity of seawater. T he darker blue rows represent the range usuallyencountered in the open ocean.

Salinity (ppt) Specific Gravity at 25° CRefractive Index (20°

C)

0 1.0000 1.33300

30 1.0226 1.33851

31 1 .0233 1.33869

32 1.0241 1.33886

33 1.0249 1.33904

34 1.0256 1.33922

35 1.0264 1.33940

36 1.027 1 1 .33958

37 1.027 9 1.3397 6

38 1.0286 1.33994

39 1.0294 1.34012

Refractive Index and Ion Imbalances in Seawater

It turns out that an aqueous solution's refractive index is relatively insensitive to small changes in the

solution's ionic makeup. For example, the usual changes in seawater's major ions that are encountered

in a reef aquarium do not greatly alter the measured salinity. 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.

From refractive index tables found in chemical reference books, we can find that a 10 weight percent

solution of sodium chloride has the same refractive index as a seven weight percent solution of

magnesium chloride, a nine weight percent solution of magnesium sulfate and a 12 weight percent

solution of sodium sulfate. These results indicate that some effects could relate to shifts between these

ions in a reef aquarium, but that these effects are small. We can use these values to roughly predict

how far off salinity measurements might be with some typical changes in the major ions. If we startwith 35 ppt seawater, which normally has the following components,

Chloride 19,350 ppm

Sodium 10,780 ppm

Sulfate 2,700 ppmMagnesium 1,280 ppm

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. 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. So if magnesium is low, the refractive index will be low, and reported salinity will be a bit low.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.

T able 3. T he error in salinity m easurem ent v ia refractive index whenm agnesium is present at unusually high or low concentrations. T he darker

blue row represents natural seawater.

Magnesium(ppm )

Salinity (ppt)Refractive

IndexPredicted

Salinity (ppt)

RelativeError in

Salinity (%)

800 35 1.33925 34.2 2.2

http://web.archive.org/web/20030104132911/http:/ioc.unesco.org/oceanteacher/resourcekit/M3/Converters/SeaWaterEquationOfState/Sea%2BWater%2BEquation%2Bof%2BState%2BCalculator.htm

http://web.archive.org/web/20030104132911/http:/ioc.unesco.org/oceanteacher/resourcekit/M3/Converters/SeaWaterEquationOfState/Sea%2BWater%2BEquation%2Bof%2BState%2BCalculator.htm




800 35 1.33925 34.2 2.2

900 35 1.33928 34.3 2.0

1000 35 1.33931 34.5 1 .4

1100 35 1.33934 34.7 0.9

1200 35 1.33938 34.9 0.3

1280 35 1.33940 35.0 0

1300 35 1.33941 35.1 0.3

1400 35 1.33944 35.2 0.6

1500 35 1.33947 35.4 1.1

How a Refractometer Works

There are several types of refractometers, but this discussion will focus on hand held

refractometers because reef aquarists rarely use any other type. Figure 3 shows the workings of atypical refractometer. In that figure, light enters from the left and passes through the liquid sample.

When the light hits the prism at the bottom of the liquid, it suddenly is slowed more than in the liquid

because the prism has a higher refractive index. 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. The amount it bends or, in technical jargon, the angle of

refraction, depends on the difference in the two media's refractive indices.

Figure 3. A schematic drawing of a ty pical hand held refractometer.

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 thelight at the liquid/prism interface sends the light higher or lower in the scale's grid. Aquarists then lookthrough 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 toread 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.

Temperature and Refractive Index: ATC

It turns out that refractive index is highly dependent on temperature. When using a refractometer

http://en.wikipedia.org/wiki/Traditional_handheld_refractometer



that does not account for this effect, temperature changes can be a large source of errors. Most liquid

materials expand slightly when heated and shrink when cooled. For a given material, light can pass

through it more easily when it is expanded, so the index of refraction falls when materials are warmed.However, the magnitude of this effect is different for every material, and refractometers must

somehow take this into account.

Handheld refractometers account for temperature by employing a bimetal strip inside them. This

bimetal strip expands and contracts as the temperature changes. The bimetal strip is attached to theoptics inside the refractometer, moving them slightly as the temperature changes. This movement is

designed to exactly cancel temperature's effects on refractive index, and generally does a very good jobIF the refractometer is designed to cancel out the temperature effects of the specific material being

analyzed.

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, although it may not beperfect in some situations because salts and other materials in the water can change temperature's

effects on refractive index by a small extent (possibly to a larger extent for very concentratedsolutions, like 750% sugar in water, but seawater is not in that category). Other details of thiscompensation may cause it to be imperfect (for example, the bimetallic strip provides a linear

correction while the true temperature effect may be nonlinear), but those issues are beyond the scopeof this article, and in general automatic temperature compensation (ATC) is a very useful attribute for

aquarists using refractometers.

Refractometer Calibration

Assuming that a refractometer is made correctly for the fluid it is intended to measure, the way tocalibrate 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 isperfectly calibrated, it will show the fluid's exact refractive index (assuming that it reports the resultsin 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 twovalues 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 4. The relationship between the real (actual) refractive index and the measuredrefractive index for a perfectly calibrated refractometer.

For many refractometers used by reef aquarists, the manufacturer calls for pure freshwater to be used

for calibration. With a perfectly made refractometer (that hasn't changed since its manufacture), thatsingle point calibration at the end of the range (Figure 5) would be adequate, albeit not perfect. Abetter single point calibration might be performed in the middle of the range being used, and for higher

accuracy, more than one calibrating solution would be used.

http://www.misco.com/calibration.php



Figure 5. 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.

Imperfect Refractometer Calibration: Offset Miscalibration

If somehow a refractometer is not perfectly made or calibrated, two different types of errors are often

encountered. Figure 6 shows a graph of what I call an offset miscalibration. Essentially, therefractometer reads a refractive index that is either lower or higher than the real refractive index, andthis difference, or "offset," is the same at all values of the refractive index. This type of miscalibration

is, for example, what happens when the calibration screw on a perfect refractometer is intentionallymoved off perfect calibration.

Figure 6. The relationship between the real (actual) refractive index and the measuredrefractive index for an incorrectly calibrated refractometer. This refractometer has an

offset error, with all values reading higher than the actual value.



Fixing this problem requires simply adjusting the offset. This adjustment is what happens when thecalibration screw is adjusted on a refractometer. The scale simply moves up or down inside therefractometer (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 thescale'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 indexchanges 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 duringthis type of calibration when using 35 ppt seawater for calibration. Both methods work equally well forthis type of correction.

Figure 7 . The relationship between the real (actual) refractive index and the measuredrefractive index for an incorrectly calibrated refractometer. This refractometer has anoffset error, with all values reading higher than the actual value. This ty pe of error can

be corrected by recalibrating with pure freshwater (refractive index = 1 .3330) as shownas well as by calibrating with seawater (Figure 8).



Figure 8. The relationship between the real (actual) refractive index and the measuredrefractive index for an incorrectly calibrated refractometer. This refractometer has an

offset error, with all values reading higher than the actual value. This ty pe of error can becorrected by recalibrating with 35 ppt seawater (refractive index = 1 .3394) as shown as

well as by calibrating with pure freshwater (Figure 7 ).

These same issues apply to refractometers that read in units of salinity (ppt) or specific gravity. Inthose 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. Figure 9, forexample, shows the relationship between the measured and actual specific gravity for a refractometerwith an offset miscalibration. 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. Similarly, Figure 10 shows the relationshipbetween the measured and actual salinity for a refractometer with an offset miscalibration. It is clear

that seawater (35 ppt) reads much lower in this case, at about 31 ppt.

Figure 9. The relationship between the real (actual) specific grav ity and the measuredspecific grav ity for a perfectly calibrated seawater refractometer (green) and an

incorrectly calibrated seawater refractometer (red). This refractometer has an offset



incorrectly calibrated seawater refractometer (red). This refractometer has an offseterror, with all values reading higher than the actual value. The error in measuring the

specific grav ity of seawater with a real refractive index of 1 .0264 is indicated.

Figure 10. 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). This refractometer has an offset error, with allvalues reading higher than the actual value. The error in measuring the salinity of

seawater with a real salinity of 35 ppt is indicated.

Just as was shown for refractive index, recalibration of a refractometer with an offset error can bediscussed 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 12shows 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.



Figure 11. The relationship between the real (actual) specific grav ity and the measuredspecific grav ity for a perfectly calibrated seawater refractometer (green) and an

incorrectly calibrated seawater refractometer (red). This refractometer has an offseterror, with all values reading higher than the actual value. The error can be correctedusing 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 properlycalibrated. In this case, accurate calibration can also be performed using freshwater.



Figure 12. The relationship between the real (actual) salinity and the measuredsalinity (in ppt) for a perfectly calibrated seawater refractometer (green) and an

incorrectly calibrated seawater refractometer (red). This refractometer has an offseterror, with all values reading higher than the actual value. The error can be correctedusing a seawater standard. By turning the calibration screw until a seawater standardreads 35 ppt, the red line moves onto the green line and the refractometer is properlycalibrated. In this case, accurate calibration can also be performed using freshwater.

This analysis makes it clear that offset miscalibration is readily corrected by turning therefractometer's adjustment screw, and that it can be corrected using either pure freshwater or 35 ppt

seawater.

Imperfect Refractometer Calibration: Slope Miscalibration

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). Essentially, 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 somepoint 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 thatcase, no amount of moving the scale up or down can make it accurate at all values of refractive index.



Figure 13. The relationship between the real (actual) refractive index and the measuredrefractive index for an incorrectly calibrated refractometer (red) and a perfectly calibrated

refractometer (green). This red refractometer has a slope error, with values far from thecalibration point (here shown as refractive index = 1 .3330 for pure freshwater) reading

higher than the actual value. The error in reading refractive index values as far away as thatof seawater can be significant, as shown.

Can such a refractometer be used? 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. Calibrating using a liquid matching seawater,

for example, can lead to a slope correction as shown in Figure 14. In this type of calibration, therefractometer is accurate at that refractive index, but not necessarily at other values.

Figure 14. The refractometer of Figure 13 (red) has a slope error, with values far from thecalibration point reading incorrectly . This ty pe of error can only be corrected by calibrating with

a solution with refractive index near to the expected measurement point. For use in seawater,



a solution with refractive index near to the expected measurement point. For use in seawater,recalibration with 35 ppt seawater (refractive index = 1 .3394) moves the red line onto the greenline 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.

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).

These same issues apply to refractometers that read in units of salinity (ppt) or specific gravity. Inthose 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 arefractometer 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.

Figure 15. The relationship between the real (actual) specific grav ity and the measuredspecific grav ity for an incorrectly calibrated seawater refractometer (red) and a perfectlycalibrated seawater refractometer (green). This red refractometer has a slope error, withvalues far from the calibration point (freshwater with a specific grav ity of 1 .000) reading

higher than the actual value. The amount of error in measuring seawater is indicated.



Figure 16. The relationship between the real (actual) specific grav ity and the measuredspecific grav ity for an incorrectly calibrated seawater refractometer (red) and a perfectlycalibrated seawater refractometer (green). This red refractometer has a slope error, withvalues far from the calibration point (freshwater with a specific grav ity of 1 .000) reading

higher than the actual value. The amount of error in measuring seawater is indicated.This figure is an expansion of Figure 15 in the region of most interest to reef aquarists.

Similarly, Figure 17 shows the relationship between the measured and actual salinity for a

refractometer with an offset miscalibration. Figure 18 is an expansion of the region of salinity of

interest to reef aquarists. It is clear that seawater (35 ppt) reads much lower in this case, at about 30

ppt.



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, withvalues far from the calibration point (freshwater with a salinity of 0 ppt) reading higher

than the actual value. The amount of error in measuring seawater is indicated.



Figure 18. 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) readinghigher than the actual value. The amount of error in measuring seawater is indicated.This figure is an expansion of Figure 17 in the region of most interest to reef aquarists.

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. 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. Figure 20 is an expansion of the region of salinity of interest toreef aquarists. In this figure, the miscalibrated red line moves onto the green line, and the

refractometer is then good to go at specific gravity values near 1.0264 (say, 1.020 to 1.030), but it is

no longer accurate at a specific gravity of 1.000 (freshwater; Figure 19).



Figure 19. The refractometer of Figure 15 and 16 (red) has a slope error, with valuesfar from the calibration point reading incorrectly . In this figure it has been recalibratedwith seawater and so is accurate in the region around the specific grav ity of seawater,

but not in the region of freshwater (specific grav ity = 1 .000).



Figure 20. The refractometer of Figure 15 and 16 (red) has a slope error, with valuesfar 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 grav ity from1.020 to 1 .030 despite the slope error. This figure is an expansion of Figure 19 in the

region of most interest to reef aquarists.

Similarly, Figure 21 shows what happens when adjusting the calibration screw so that the salinity of a

35ppt seawater standard really reads 35 ppt. Figure 20 is an expansion of the region of salinity ofinterest to reef aquarists. In this figure, the miscalibrated red line moves onto the green line, and the

refractometer is then good to go at salinity values near 35 ppt (say, 30 to 40 ppt), but it is no longer

accurate in freshwater (salinity = 0 ppt; Figure 22).

Figure 21. The refractometer of Figure 17 and 18 (red) has a slope error, with values far



Figure 21. The refractometer of Figure 17 and 18 (red) has a slope error, with values farfrom 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 inthe region of freshwater (salinity = 0 ppt).

Figure 22. The refractometer of Figure 17 and 18 (red) has a slope error, with values farfrom the calibration point reading incorrectly . In this figure it has been recalibrated with

seawater, and so is adequately accurate over the range of salinity of 30-40 ppt despitethe slope error. This figure is an expansion of Figure 21 in the region of most interest to

reef aquarists.

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 usingpure 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 notactual seawater refractometers (see next section).

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.

Imperfect Refractometer Use: Scale Misunderstanding and SaltRefractometers

Refractometers can lead to incorrect readings in additional ways and, again, these issues abound for

reef aquarists. One is that many refractometers are intended to measure sodium chloride solutions, not

seawater. These are often called salt or brine refractometers. Despite the scale reading in ppt (‰) or

specific gravity, they are not intended to be used for seawater. Unfortunately, many refractometers

used by aquarists fall into this category. In fact, very few refractometers used by hobbyists are true

seawater refractometers.

Fortunately for aquarists, the differences between a salt refractometer and a seawater refractometerare not too large. 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. This error is significant, in my

opinion, but not usually enough to cause a reef aquarium to fail, assuming the aquarist has targeted an

appropriate salinity in the first place. Figure 23 shows the relationship between a perfectly calibrated

and accurate salt refractometer and a perfectly calibrated and accurate seawater refractometer when

the units are reported in salinity. This figure shows the measured salinity reading for seawater beingabout 1.7 ppt higher than it really is.

Figure 23. The relationship between the real (actual) salinity and the measured




Figure 23. The relationship between the real (actual) salinity and the measuredsalinity (in ppt) for a perfectly calibrated seawater refractometer (green) and a

perfectly calibrated salt refractometer (red). This salt refractometer effectively has asignificant 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. Saltrefractometers reading in salinity can be recalibrated using seawater to eliminate

nearly all of this error (just as the refractometer in Figures 17 and 18 was recalibratedin seawater to give Figures 21 and 22).

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).

Even more confusing, but perhaps a bit less of a problem in terms of the error's magnitude, saltrefractometers sometimes read in specific gravity. But that value is specific gravity of a sodium

chloride solution with the measured refractive index, not seawater with that refractive index. A sodium

chloride solution with the same refractive index as 35 ppt seawater (which turns out to be 36.5 ppt

sodium chloride) has a specific gravity matching 34.3 ppt seawater. So this type of refractometer, when

perfectly calibrated, will read the specific gravity of 35 ppt seawater to be a bit low, at 1.0261 instead

of about 1.0264. That error (reading 0.0003 or so too low) is, however, probably less than most reef

aquarists are concerned with. Figure 24 shows the relationship between a perfectly calibrated andaccurate salt refractometer and a perfectly calibrated and accurate seawater refractometer when the

units are reported in specific gravity. This figure shows the measured salinity reading for seawater

being about 0.0003 lower than it really is.

Figure 24. The relationship between the real (actual) specific grav ity and themeasured specific grav ity for a perfectly calibrated seawater refractometer (green)

and a perfectly calibrated salt refractometer (red). This salt refractometer effectivelyhas a very small slope error, with values far from the calibration point (freshwater

with a salinity of 0 ppt) reading roughly 0.0003 specific grav ity units higher than theactual value. Salt refractometers reading in specific grav ity can be recalibrated using

seawater to eliminate nearly all of this already small error (just as the refractometer




seawater to eliminate nearly all of this already small error (just as the refractometerin Figures 15 and 16 was recalibrated in seawater to give Figures 19 and 20).

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.

Brix Refractometers

A commonly manufactured type of refractometer is called a Brix refractometer. Its scale usually

reads in Brix, or % Brix (percent Brix). These refractometers are used in many industries to measure

the concentration of sugar in water such as in the soft drink industry. They can be used to measure

seawater's salinity, but are not always precise enough around the range of seawater's refractive index

to be useful. A resolution of 0.2% Brix is common, and that is borderline acceptable for the reasons

detailed below.

Table 4 shows the relationship between seawater salinity, refractive index and % Brix. If arefractometer 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.

T able 4. T he relationship between seawater salinity ,refractive index and % Brix.

Seawater Salinity (ppt) Refractive Index % Brix

0 1.33300 0

30 1.33851 3.8

31 1 .33869 3.9

32 1.33886 4.1

33 1.33904 4.2

34 1.33922 4.3

35 1.33940 4.4

36 1.33958 4.5

37 1.3397 6 4.7

38 1.33994 4.8

39 1.34012 4.9

40 1.34031 5.0

Clinical Refractometers

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. The scale

can read in units familiar to reef aquarists (ppt or specific gravity), but that is ppt or specific gravity ofa protein solution, not a seawater solution. Those units should be ignored, and if they are all that is

available on the refractometer, I'd find another refractometer. 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. Some clinical refractometers read in refractive index, which is okay if you match the refractive

index to the appropriate seawater refractive index (e.g., 35 ppt seawater has a refractive index of

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index to the appropriate seawater refractive index (e.g., 35 ppt seawater has a refractive index of

1.33940). Such conversions of refractive index to salinity or specific gravity are shown in Figures 1 and

2, and Table 1.

Commercial Refractometer Standards

Despite that fact that many refractometers sold to aquarists recommend calibration in pure water,

such a calibration alone will not always ensure accuracy. Consequently, other standards may also need

to be used. These other standards should be solutions with known refractive indices that are close to

the values intended to be measured in the aquarium. For this purpose, seawater with a salinity of 35

ppt is perfect, and such standards can be obtained commercially or made from table salt with

appropriate measurement.

One suitable commercial standard is made by American Marine and sold under the brand namePinpoint. 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.

For example, standard seawater with S=35 (35 practical salinity units, or PSU) is defined as

seawater with the same conductivity as a solution made from 3.24356 weight percent potassiumchloride (KCl), and that conductivity is exactly 53 mS/cm (mS/cm, or milliSiemens per centimeter, is

one of the units used for conductivity). That solution, however, has a refractive index of about 1.3371,

matching seawater just below 26 ppt. So do not assume that all 53 mS/cm conductivity standards are

suitable for refractometer calibration.

Salifert has a product called Refracto-Check that they often give away at meetings like MACNA. It is a

35 ppt seawater refractive index standard, but it is not widely available commercially.

Do-it-yourself Refractometer Standards

In a previous article I have described how to make a do-it-yourself refractometer standard

matching 35 ppt seawater, and I will just summarize that recipe here.

To provide a standard for refractometers requires a solution whose refractive index is similar to

normal seawater. Seawater with a salinity of 35 ppt has a refractive index of 1.3394. Likewise, the

refractive index of different sodium chloride solutions can be found in the scientific literature. My CRC

Handbook of Chemistry and Physics (57th Edition, Page D-252) has such a table. That table has entries

for 3.6 and 3.7 weight percent solutions of sodium chloride that span the value for normal seawater.

Interpolating between these data points suggests that a solution of 3.65 weight percent sodium

chloride has the same refractive index as 35 ppt seawater, and therefore can be used as an appropriate

standard (Table 5).

T able 5. Refractive Index as a function of the concentration of a sodiumchloride solution. T he darker blue row represents the standard.

Sodium Chloride Concentration(weight %)

RefractiveIndex

Equivalent SeawaterSalinity (ppt)

3.3 1 .3388 31.65

3.4 1 .3390 32.8

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3.5 1 .3391 33.3

3.6 1 .3393 34.4

3.65 1.3394 35.0

3.7 1 .3395 35.6

3.8 1.3397 36.7

This 3.65 weight percent sodium chloride solution can be made by dissolving 3.65 grams of sodiumchloride 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.).

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).

For a rougher measurement in the absence of an accurate water volume or weight measurement:

1. Measure ¼ cup of Morton's Iodized Salt (about 73.1 g).

2. Add one teaspoon of salt (making about 79.3 g total salt).

3. Measure the full volume of a plastic 2 L Coke or Diet Coke bottle filled with purifiedfreshwater (about 2104.4 g).

4. 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. The volume of this solution will be

slightly larger than the Coke bottle, so dissolve it in another container.

[Note: the standard described here using soft drink bottles is subject to variation in the volume of

the bottle. It turns out that such bottles can vary in total volume, and this can lead to at least a one

ppt error in the salinity of standards matched to seawater of 35 ppt salinity. Standards made withaccurate measurements of salt and water, however, will accurately match 35 ppt.]

Tips on Selecting a Refractometer

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 isgoing 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.

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.

If selecting a refractometer that reads in % Brix, the range needs to include about 3.8-5% Brix, with a

readability to 0.2% Brix to attain a precision of ± 1 ppt (say, 34-36 ppt) in salinity or ± 0.00075 in

specific gravity (say, 1.0255 to 1.0270).



specific gravity (say, 1.0255 to 1.0270).

It is preferable that refractometers used by aquarists have automatic temperature compensation

(ATC). That feature adds a small amount of cost, but increases the accuracy of measurement and

eliminates concerns about temperature.

Tips on Calibrating a Refractometer

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. So my

recommendation for calibration is as follows:

1. 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.

This calibration should ordinarily be carried out at room temperature using an ATC refractometer. Thedirections with some ATC refractometers insist that the calibration be carried out at a specific

temperature, but I've never understood how that could matter and I would not worry about it. 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.

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). Normally, this step is a quick and easy procedure, and may

often be all that is required IF the refractometer has been verified to have passed the second

calibration step below at least once. This is an offset calibration, as described above.

2. 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. This step involves testing it in a solution matching

the refractive index of 35 ppt seawater (or some similar solution near the range of measurement).

Remember to let it sit for at least 30 seconds so it comes to the same temperature as therefractometer. Suitable commercial and do-it-yourself standards were described earlier in this article.

Using one of them, place a drop onto the refractometer and read the value. If it reads approximately

35 ppt, or a specific gravity of 1.0264, or a refractive index of 1.33940, then the refractometer is

properly calibrated and is set to go.

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 stepmight be to buy a commercial standard.

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.

Again, despite the claims in the directions of some refractometers to have the standard at a particular

temperature, when calibrating an ATC refractometer with this seawater standard, I'd just use it at

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temperature, when calibrating an ATC refractometer with this seawater standard, I'd just use it atroom temperature.

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). 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.

Other Tips on Using a Refractometer

Clean the refractometer between each measurement using a soft, damp cloth. Failure to wipe the

prism can lead to inaccurate results and damage to the prism.

Do not immerse the refractometer in water. If the refractometer looks foggy inside, water has entered

it. You may or may not be able to dry it out without damaging the unit. Do not measure or clean it with

abrasive or corrosive chemicals.

If the scale is completely dark, you may not have added sample to it in the appropriate way. If the

scale is completely light, then the liquid's refractive index is above the refractometer's high end.

Summary

Refractometers are a quick and often accurate way to measure the salinity of reef aquarium water.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. As with many devices, however, you

sometimes get what you pay for, and sometimes less. Very inexpensive refractometers can be prone to

errors and may need to be checked in a solution matching seawater, not just pure freshwater.

Other methods of salinity determination are also quite suitable for reef aquarists. These include

conductivity using electronic meters, 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 aseawater standard at least once to confirm its proper operation before relying on it to gauge the

salinity in a reef aquarium.

Happy reefing!

If you have any questions about this article, please visit my author forum on Reef Central.

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