http://www.youtube.com/watch?v=nMApg9fqqDA
How a Traditional Analog Refractometer Works
Light passing through a liquid is slowed compared to the speed it travels in air. So once a fluid sample is placed on
the measuring surface of a refractometer, the light passing through it slows and is bent.
The refractometer focuses this bent light on a tiny internal scale. The scale is magnified by the eyepiece lenses so it
is easily visible.
The optics are supported by a bi-metal strip that moves lenses in response to temperature changes, ensuring that
readings are accurate regardless of temperature.
Refractometers: General FAQs

What does a refractometer do?

How do I use it?
What does a refractometer do?
The best way to measure the salinity and specific gravity of aquarium water is to use a refractometer. These precision
optical instruments are incredibly accurate even at very low salt levels, and are equally easy to use.
How do I use it?
Place 1 to 2 drops of aquarium water onto the prism glass and close the plate cover. Once a fluid sample is placed on
the measuring surface, the light passing through it slows and bends. The refractometer focuses this bent light on a
tiny internal scale. The scale is magnified by the eyepiece lenses so it is easily visible.
Look through the eyepiece and view the scale. The top portion of the scale appears blue and the bottom part appears
white. Take your reading at the intersection of these two colors. One side of the scale measures salinity and the other
side measures specific gravity.
Salinity & Specific Gravity are easily viewed when looking through the Refractometer
http://reefkeeping.com/issues/2006-12/rhf/
Refractometers and Salinity Measurement
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.
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 salinity in 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. To some 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 them probably 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 their
refractometers 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 to calibrate them (which is often
not what the directions that come with them claim).
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 be common practice to
keep marine fish and, in many cases, reef aquaria, at somewhat lower than natural oceanic 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 salinity units (PSU),
which often is shown simply as S=35, or whatever the value actually is. In this article I will mostly use ppt,
because that more appropriately applies to solutions whose composition deviates greatly from seawater (such as
sodium chloride solutions used to make certain standards).
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 as PSU,
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 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!
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.
Table 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
Ethyl alcohol
1.361
Sugar Ssolution (80% sugar)
1.49
Glass (soda lime)
1.510
Bromine (liquid)
1.661
Ruby
1.760
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 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.
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.
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 salt added 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 the appropriate refractive index range.
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 specific gravity
of seawater in the range of interest to most reef aquarists. The black circles represent
data points for whole values of the salinity (33, ppt, 34 ppt, 35, ppt, etc).
Table 2. Specific gravity and refractive index as a function of seawater’s
salinity of seawater. The darker blue rows represent the range usually
encountered in the open ocean.
Salinity (ppt)
Specific Gravity at 25° C
Refractive 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
1.0271
1.33958
37
1.0279
1.33976
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 start with 35 ppt seawater, which normally has the following components,
Chloride 19,350 ppm
Sodium 10,780 ppm
Sulfate 2,700 ppm
Magnesium 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.
Table 3. The error in salinity measurement via refractive index when
magnesium is present at unusually high or low concentrations. The darker blue
row represents natural seawater.
Magnesium
(ppm)
Salinity (ppt)
Refractive
Index
Predicted
Salinity (ppt)
Relative
Error in
Salinity (%)
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 a typical 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 typical 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 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.
Temperature and Refractive Index: ATC
It turns out that refractive index is highly dependent on temperature. When using a 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 the optics 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 job IF 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 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 (possibly to a larger extent for very concentrated solutions, like 750% sugar in water, but seawater is not
in that category). Other details of this compensation 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 scope of this article, and in general automatic temperature compensation (ATC) is a very useful
attribute
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. The
directions 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 the refractometer. Suitable commercial and doit-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-ityourself standard made with crude techniques such as Coke bottles, a good next step might 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 room
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 a seawater standard at least once to confirm its proper
operation before relying on it to gauge the salinity in a reef aquarium.
Happy reefing!
Reefkeeping Magazine™ Reef Central, LLC-Copyright © 2008