Salt Water Refractometer

Having a Refractometer is in my view one of the most important pieces of equipment. This means that you can accurately measure the salinity of the water. I recommend using a Deltec Refractometer. 

SEAWATER Refractometer 

INSTRUCTIONS FOR ATC NATURAL SEA WATER REFRACTOMETER Hand held instrument for the measurement of Salinity and Specific Gravity (S.G.) by refractive index.

 

Specifically designed for aquarium use for measuring the salinity of Natural Sea Water.

 

Almost all hand held salt water refractometers available to the hobby are designed and calibrated for the testing of

 

Even though the largest percentage of salt in the sea is sodium chloride, the presence of other significant ions such as magnesium and calcium in natural sea water results in a different refractive index relative to that of brine. A standard salt (brine) refractometer will therefore not give the correct salinity for 

For example a 35ppt solution of NSW has the same refractive index as a 36.5ppt solution of brine. When using a normal brine refractometer the user should therefore look for a reading of 36.5ppt when trying to achieve a reading of 35ppt for 

brine solutions, i.e. sodium chloride, and are not calibrated for the refractive index of natural sea water. natural sea water (NSW) and a conversion factor must be applied. natural sea water. EASIER TO READ

 

One of the other problems with most refractometers is that the scale displayed normally reads from 0-100ppt when we are really only interested in the region from 30-40ppt.

 

Our new Seawater Refractometer has therefore been specifically designed to read from 0-40ppt which gives you 2.5 times the resolution of a normal 0-100 scale. 

AUTO TEMPERATURE COMPENSATION (ATC)

 

There is a lot of misunderstanding concerning the way the ATC feature works and its effect on the refractometer at different ambient room temperatures.

 

Salinity is a measurement of a mass of salt in a mass of water and therefore does not vary with temperature however a refractometer does not measure salinity directly but measures the refractive index which is then displayed as salinity. The refractive index of a solution does vary with temperature therefore the reading that you measure with a refractometer is always temperature dependant. 

An ATC refractometer has a bimetal strip inside the instrument that moves the reading scale as the temperature changes to compensate for the change in refractive index. 

What people do not generally understand is that it is the temperature of the instrument and not the water temperature that is important as the small sample of water used for testing will equilibrate within seconds to the temperature of the refractometer. 

Once correctly calibrated at the set calibration temperature of 20 

oC the refractometer can then be used in environments where the ambient temperature and therefore instrument temperature would heat up or cool down within the range of the ATC which is between 10 and 30 centigrade.

CALIBRATION

 

If you look at the scale above you will see that the refractometer states that it is calibrated at and displays the SG of seawater at 20

 

Calibration should always be carried out with the 

The D-D refractometer is designed with a copper body which is more expensive to manufacture but responds faster to ambient temperature changes than other materials. 

Step 1: Clean the glass prism and translucent cover flap of the refractometer with pure or reverse osmosis water to ensure that there are no salt residues that would affect the reading. 

Step 2: Using the pipette dropper supplied, fill and empty the dropper a number of times in pure or RO water and then whilst holding the refractometer horizontally place a sample of the water on the glass prism. 

Step 3: Close the clear flap onto the sample and wait for 15 seconds for the temperature to equilibrate with the instrument before looking through the eyepiece with the prism end pointing towards a bright light source. The eyepiece can be screwed in or out to adjust the focus and compensate for individual eyesight. 

oC. Most salt refractometers use this temperature and will be marked 20oC or 20/20.

instrument at 20oC which is close to the average room temperature in most cases. Allow the refractometer to stand at this temperature for 30 minutes to equilibrate. Note that the refractive index is also dependant on the wavelength of light too and therefore a light source closest to daylight should also be used

 

You will observe that the top of the view screen is blue and where it changes from blue to clear at the bottom is where the reading should be taken. If this boundary does not line up exactly with the zero mark on the scale then take the small screwdriver provided and having first removed the protective cap adjust the scale position by tightening or slackening the screw. 

.

Your refractometer is now calibrated.

 

Note it is also possible to calibrate the refractometer using a 35ppt reference solution however be wary of any versions that do not quote the temperature of calibration as they may be designed for use or calibration at a different temperature.

 

MEASURING THE SALINITY OF YOUR AQUARIUM

 

Ensure that the prism of your refractometer is clean by washing the prism plate and cover plate after use each time in pure water and drying it with the polishing cloth supplied.

 

Take a representative sample of the water from your aquarium, ensuring that you fill and empty the dropper a number of times first to rinse out any salt deposits from previous use, and then place the sample of water on the prism in the same way that was done during the calibration process. It is better to put more water on the plate and to let the cover plate push the excess away than to use a too small a volume. 

Allow the water to equilibrate to the temperature of the instrument for 15 seconds and then look down the eyepiece to take the salinity reading at the bottom of the blue and clear interface. 

One side of the scale will allow you to read the salinity and the other side the specific gravity calculated at 20 

oC pure water density**.

SALINITY AND SPECIFIC GRAVITY STANDARDS

 

We recommend that you try to get used to thinking in terms of salinity and use the salinity readings on the refractometer as this is the scale that is calibrated against the refractive index standard.

 

SALINITY

 

Salinity as explained earlier in these instructions is generally described as the mass of salt in a specific mass of pure water and is normally expressed in parts per thousand or ppt (‰). 

For example 35ppt could be 35 grams of salt in 965 grams of pure water or 35 tonnes of salt in 965 tonnes of pure water – 35 units by weight in a total of 1000 units by weight. Weight is never affected by temperature. 

In the oceans the salinity varies from 5 ppt in the Northern Baltic Sea to 40 ppt in the Northern Red Sea with a generally accepted global average of 35ppt for NSW. The level that you keep your aquarium at will generally be determined by the type of animals that you keep in the system. 

Often fish only systems are maintained at lower salinities down to 29ppt as it exerts less osmotic stress on the fish whereas reef systems are normally maintained at 35-36ppt. 

is always constant whereas Specific Gravity, although more commonly used in the hobby, is fraught with misconceptions that are explained later.

The parameters printed for H2Ocean Pro+ reef salt are quoted at a salinity of 35.5ppt.

 

SPECIFIC GRAVITY 

As density is temperature dependant due to expansion, which changes the volume for a given mass, then specific gravity is also temperature dependant and varies according to two parameters. 

1 – The temperature of the test sample during measurement – normally the tank temperature at 25 

2 – The temperature at which the pure water density used in the SG calculation was measured at – normally the calibration temperature that should be printed on all good scientific hydrometers. 

is not an exact scale and is defined as the relationship between the density of a solution, in this case seawater, relative to the density of pure water – normally measured directly by hydrometer. oC  

This leads to huge misunderstandings within the hobby as to what the specific gravity should be specified at within the aquarium. Most people believe that the SG of 35ppt seawater is 1.025 at 25oC whereas in reality the specific gravity measured is determined by the calibration temperature of the particular hydrometer owned by the aquarist. NSW   

Calibration Temp.   

S.G Reading at Sample Temp.   

Salinity of Instrument   

25C   

20C   

35ppt   

60F or 15.56C   

1.0233   

1.0257   

35ppt   

68F or 20C   

1.0252   

1.0266   

35ppt   

77F or 25C   

1.0264   

1.0277   

Information taken from D&Ds website to help:- 

When we brought out the D-D H2Ocean Pro+ reef salt a few years ago, we were adamant that we wanted to print the exact levels and tolerances for the main important elements that we would expect to find in our salt on every bucket; information that should be readily available to allow an informed choice for every good reef salt. 

Of course once we gave people levels to test against we received a steady stream of emails and threads from individuals who were measuring readings outside these guaranteed parameters. The main reason that we found for this discrepancy, other than general mixing and test kit errors, was due to low specific gravity, i.e. not enough salt in their water. 

Further investigation brought to light a number of anomalies and misunderstandings regarding the whole issue of S.G. and S.G. measurement and has prompted me to write this article which tries to illustrate some potential problems when using specific gravity as a unit of measurement without a full understanding of what is actually being measured. 

Most people, if asked, would probably tell you that their reef aquarium is running at an S.G. (Specific Gravity) of 1.025. Those with greater understanding may go into further detail and tell you that the S.G. is 1.025 at 25C, but where does this figure come from, and is it the best way to measure or express the amount of salt in your aquarium, is it even the correct level to be running at? 

The salinity of the world’s oceans is not constant and varies substantially from one location to the next with concentrations of just 10-15ppt in the Baltic Sea to around 40ppt in the Red Sea. 

But stop a minute!! Why are we talking about Salinity now instead of S.G. and what is the difference?? 

SALINITY 

Salinity is a true measurement of the concentration of salt in the ocean and is calculated as the total weight of ‘dry’ salt dissolved in a total of 1000 weight units of water or parts per thousand, (ppt).   Salinity is a pure weight per weight measurement and as weight is not influenced by temperature then neither is salinity. 

The generally accepted standard salinity for natural seawater, (NSW), is 35ppt. 

It is not possible to measure the salinity of the ocean or your aquarium directly unless you take a precise weight of seawater and evaporate all of the H20 from it then measure the weight of the residue, (mass solids analysis). Obviously this is not a convenient technique for the average hobbyist, so we use other methods to determine the salinity indirectly. 

SPECIFIC GRAVITY (S.G.) 

Specific Gravity, or relative density, is expressed as the ratio of the density of seawater relative to the density of the same volume of pure water. This of course can be quantified quite easily, however as the density of a liquid is calculated as the weight of a unit volume of that liquid and because liquids expand with temperature, then the volume that we measure and subsequently the weight of that volume will also change. We should always therefore qualify the S.G. by quoting it at a specific measured temperature, e.g. 1.025 at 25C. 

Many people may have noticed on their hydrometers that there is a temperature quoted on the side, normally in small writing. This is the temperature at which the hydrometer was designed/calibrated to measure the water sample at – 

SIMPLES – to use the words of a well known Russian meerkat – All we need to do is to ensure that our sample water is at that temperature and the specific gravity will be exactly right. 

NOT QUITE TRUE – At a simple level this is correct however as you get further into the subject you will find that the S.G. reading that you obtain may be more specific to the equipment you have chosen to use to measure it rather than to the actual salt concentration in your tank. 

I suspect, rightly or wrongly, that the specific gravity levels that we as hobbyists use in our aquariums may be historical from general oceanographic studies and it is not immediately clear when or where the 1.025 at 25C standard originates from. 

If you trawl the internet and scientific literature you will find that the specific gravity measurements and tables for seawater which you come across were first developed long ago by shipping companies and oceanographic bodies like the NOAA. These charts were used to allow a simple way of calculating the changes in seawater density across the various oceans which affects how much cargo a ship can carry on a journey from one destination to another. The higher the density or salt content the more cargo the ship can hold without sinking. 

If you go back to the original definition of specific gravity you will see that we calculate the S.G. as the density of a sample of SEAWATER at a specific temperature divided by the density of PURE WATER at a specific temperature. It is with density of the pure water that the problem or potential for error occurs. 

Unfortunately there are a number of different temperature standards commonly used in oceanography and so printed tables for PURE WATER density can be quoted at 4C, 60F (15.56C), 20C or 25C depending on their intended application. Each temperature standard will result in a different pure water density figure which will give a different S.G. when we use that density in our calculation. 

EXAMPLE: If we calculate the specific gravity for a sample of 35ppt seawater using a standard density temperature of 25C for the seawater sample but vary the density temperature used for the pure water, we can see how this affects the specific gravity result? 

Specific Gravity = (ρs(T)/ρ0(Tx)) X 1000 

ρs is the density of 35ppt seawater at temperature T, in this case 25C = 1.023343 

ρ0 is the density of pure water at a range of commonly used temperature standards (Tx). 

ρ0 (15.56C) = 0.9990166       SG = 1.023343/0.9990166              = 1.02435 

ρ0 (20.0C) = 0.998203            SG = 1.023343/0.998203                = 1.02519 

ρ0 (25.0C) = 0.997047958     SG = 1.023343/0.997047958         = 1.02637 

We can see from the above that by using a pure water temperature standard of 20C that the S.G. of 35ppt NSW does in fact approximate to 1.025 which is perhaps where the figure comes from however you can also see the potential range that becomes available if another standard temperature is used. 

All of these readings are for the same sample of 35ppt seawater at the same sample temperature of 25C. To measure the amount of salt in our aquarium by way of an S.G. reading, we must therefore understand which pure water temperature standard and which sample water temperature standard was originally used when calibrating the hydrometer or refractometer which is not constant between all equipment and manufacturers. 

The Tropic Marin High Precision Hydrometer for example quotes 25C/25C on the instructions which means that it is calibrated using the density of both the sea water and pure water at 25C. With this instrument you should therefore be looking to measure an S.G. of 1.0264 and not 1.025 for the same 35ppt water sample. 

This brings us back full circle to the point that the salinity scale which we talked about at the start is perhaps a much better way of expressing the salt concentration of your aquarium as we do not have the same potential for variation however most glass hydrometers do not even show this scale. 

DOES IT REALLY MATTER 

The main drive behind this article was to enlighten people on the potential variation between different specific gravity measurements, and to show individuals why they often find varying concentrations of major elements such as calcium and magnesium in their tanks compared with what they ‘expect to find’ or have been ‘told that they should find’ in their salt mix. 

If as a consequence of measurement or calibration technique you do end up with a low salt concentration in your tank or salt mix, a salinity of only 33ppt for example. You should then equally expect that the concentrations of ‘all’ elements will be low as a consequence compared with the target concentrations found in natural seawater. 

If the normal calcium level of NSW at 35ppt is 420ppm then at 33ppt salinity this will only be 396ppm and if the aquarist is not aware of his low salinity and adds more calcium to raise the level to 420ppm then they will be putting this particular element out of balance with the rest of the salt whilst ignoring other elements that may be just as important for the proper biological function of the organisms we keep. What they should really do is to add more ‘salt’ to raise the salinity, and bring ‘all’ of the levels up together. 

D-D quote figures on the concentrations of the main elements in our H2Ocean salt at 35.5ppt which is an average of the readings found in the coral seas. With our new found understanding of the relationship between salinity and specific gravity we can expect that the 35.5ppt standard can correctly equate to an S.G. anywhere between 1.0247 and 1.0267 depending on the hydrometer used. 

HYDROMETERS AND REFRACTOMETERS 

We discussed earlier that as aquarists we do not measure salinity or specific gravity directly and instead measure another parameter which has a relationship with the salt concentration such as refractive index in the case of a refractometer, buoyancy in the case of a hydrometer and conductivity if measured electrically. 

Unfortunately all of these alternative parameters ‘are’ affected by temperature and so even when measuring salinity, which is not temperature dependant, we must still make an allowance because of the indirect test method. 

HYDROMETERS 

There are various glass and plastic hydrometers available to aquarists on the market which work on the principle of buoyancy, where the denser the liquid in which it is immersed, the higher it will float. 

 The more expensive glass hydrometers are very accurate but time consuming to use as they require a clear sided vessel in which to float and for the water to be at the exact calibration temperature. They are extremely delicate and easy to break and generally only display an S.G. scale which means you need a complete understanding of both the sample calibration temperature required, and the pure water calibration standard used to be able to relate the reading back to a particular salinity. 

The cheaper plastic swing arm hydrometers are easy to use and robust but are not always intended to give an absolutely accurate reading and not normally used by more experienced reef keepers housing delicate and expensive stock. There can be issues with these units giving false readings over time due to bubbles or deposits attaching to the swing arm or pivot point which change the buoyancy and therefore the reading obtained. Swing arm hydrometers normally show a salinity scale and an S.G. scale but calibration temperatures may vary from one manufacturer to another. 

REFRACTOMETERS 

Refractometers work on the principle that changes in salt concentration affect the refractive index of light and therefore light entering the instrument is deflected by varying amounts onto a graduated scale. They are a convenient, fast and easy method of keeping check on the salinity in your aquarium or when mixing up new batches for water changes. 

The majority of refractometers in the hobby are manufactured and calibrated for use with saltwater and not seawater and intended for measuring sodium chloride, (brine) concentrations. Although sodium chloride is the main constituent of NSW, it varies in refractive index due to the lack of other ions such as calcium and magnesium which are found in seawater. The result of these additional elements is that the refractive index changes so that a 35ppt seawater solution approximates to the same refractive index as that of 36.8ppt brine. Whilst this may seem a small difference we should remember that it does have a significant impact on the true concentration of the dissolved elements in our aquariums. 

When using a conventional ‘saltwater’ or ‘brine’ refractometer for the reef aquarium we must therefore look to achieve a reading of 36.8ppt if our intended salinity is 35ppt or 37.3ppt if our intended salinity is 35.5ppt as quoted for the H2Ocean salt. 

CALIBRATION 

To use a refractometer properly we must first understand the correct calibration procedure. If you look down the lens of most good refractometers you will see 20/20 printed on the screen. This means that it was originally calibrated with both pure water and sample water temperatures set at 20C and so in order to recalibrate the instrument properly you must use the same temperature for your calibration sample. What many people do not understand however is that it is the temperature of the instrument that should be at 20C and not the liquid as the few drops of sample water contains so little heat that it soon equilibrates to the same temperature as the refractometer body. 

So with your refractometer at 20C, which is close to normal room temperature, you add a sample of distilled or RO water to the screen and use the adjustment screw to line up the blue marker with the zero salinity mark. This can now be used to measure the sample salinity accurately but only whilst the refractometer is still at 20C. Remember that even if your aquarium is running at say 27C (80.6F), the refractometer will always read as if it were at 20C.  

If you have an auto temperature compensating model (ATC) you must still calibrate the unit at the calibration temperature of 20C however this version, once correctly set, will auto adjust for environments where the instrument warms or cools away from this temperature. Normally the range is 10-35C and is achieved by a small bimetallic strip in the body of the refractometer which responds to the change and moves the graduated scale accordingly. For this reason copper bodied refractometers are better than the lower cost plastic ones as they conduct the ambient temperature changes faster. 

True seawater refractometers have never really existed in the hobby at an affordable price until now but as part of the development of our salt and salt related products, D-D have brought out a new model of refractometer which is specifically calibrated to the salinity of NSW thus removing the requirement to apply an offset to obtain the true reading for your aquarium. 

These new models are easily identified as seawater refractometers on the viewing screen and have been produced to show the correct S.G. of 1.0266 for NSW at a salinity of 35ppt and a sample/pure water density of 20C and a reduced range of salinity from 0 -40 rather than 0 – 100. 

IN CONCLUSION 

Hydrometers – understand the calibration temperature requirements, and take note that S.G. is not a constant number across all measuring equipment and reference sources. 

Refractometers – understand the calibration requirements and apply an offset to measure the true seawater salinity if using a saltwater/brine model. Understand that the S.G. scale is not a constant number across all measuring equipment and reference sources. 

Remember that the most important thing for your aquarium is consistency and stability. This applies to both the salinity and the relative concentrations of all elements within the salt and so any changes in salinity, salt brand or parameters should be made gradually. 

Top up evaporated loss regularly or fit an auto top up system to reduce swings in salinity to a minimum. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

This entry was posted on Tuesday, October 19th, 2010 at 1:02 pm and is filed under Marine Aquarium Hobby, Salt Mixing. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.