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$46.99 TRUE NATURAL SEA WATER Salinity Refractometer Aquarium, Marine Seawater, Hydrometer
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$46.99 TRUE NATURAL SEA WATER Salinity Refractometer Aquarium, Marine Seawater, Hydrometer
$46.99 TRUE NATURAL SEA WATER Salinity Refractometer Aquarium, Marine Seawater, Hydrometer
$46.99 TRUE NATURAL SEA WATER Salinity Refractometer Aquarium, Marine Seawater, Hydrometer

$46.99 TRUE NATURAL SEA WATER Salinity Refractometer Aquarium, Marine Seawater, Hydrometer

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THE REEF SEA METER NATURAL SEA WATER REFRACTOMETER - The MOST ACCURATE way to measure Salinity in marine habitats!

Specifically designed for aquarium use for measuring the salinity of Natural Sea Water (NSW)

Range: 0-40 Parts per Thousand of Salinity in NSW (Natural Sea Water) and 1.015 to 1.030 Specific Gravity
Minimum Division: 1.00 PPT and 0.001 SG +/- 0.001 SG
Accuracy: +/-1.00 PPT
Light Source: Ambient Sunlight or Lamp

Almost all hand held salt water refractometers available to the hobby are designed and calibrated for the testing of brine solutions, i.e. sodium chloride, and are not calibrated for the refractive index of Natural Sea Water.

This is a very important distinction.

Other Salinity Refractometers used for marine aquariums are designed totest salt in water – nothing else!

The Reef Sea Meter is designed to measure Salinity of Natural Sea Water taking into account the other elements that comprise salinity in the marine water sampled. 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 Natural Sea Water and a conversion factor must be applied. 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 Natural Sea Water.


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. The Reef Sea Meter 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. There is a notch on the scale at 0ppt for calibration too!

AUTO TEMPERATURE COMPENSATION (ATC) Salinity is a measurement of amass 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 dependent. 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’C/68’F 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 10-30’C/50-86’F.

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’C (68’F). Most salt Refractometers use this temperature and will be marked 20’C (68’F) or 20/20. Calibration should always be carried out with the instrument at 20’C/68’F which is close to the average room temperature in most cases. Allow the Refractometer to stand at this temperature for 30 minutes to acclimate. This allows for the best calibration possible. The Reef Sea Meter Refractometer is designed with a copper body which is more expensive to manufacture but responds faster to ambient temperature changes than other materials.


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. Why? The prevent corrosion of the unit and to ensure that your readings are accurate (residual salts will skew future readings if you do not clean properly). Take a representative sample of the water from your aquarium or other marine habitat. Allow the water to acclimate 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’C/68’F.


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 is always constant whereas Specific Gravity, although more commonly used in the hobby, is fraught with misconceptions that are explained later. 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 35 ppt could be 35 grams of salt in 965 grams of pure water or 35 tons of salt in 965 tons 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 29 ppt as it exerts less osmotic stress on the fish whereas reef systems are normally maintained at 35-36 ppt.

SPECIFIC GRAVITY 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. As density is temperature dependent due to expansion, which changes the volume for a given mass, then specific gravity is also temperature dependent and varies according to two parameters. 1 - The temperature of the test sample during measurement – normally the tank temperature at 25’C/77’F 2 – The temperature at which the pure water (distilled or R/O) density used in the SG calculation was measured at – normally the calibration temperature that should be printed on all good scientific hydrometers. This leads to huge mis-understandings within the hobby as to what the specific gravity should be specified at within the aquarium. Most people believe that the SG of 35 ppt seawater is 1.025 at 25’C/77’F whereas in reality the specific gravity measured is determined by the calibration temperature of the particular hydrometer owned by the user.

NSW Salinity Calibration Temp SG Reading at 25’C/77’F SG Reading at 20’C/68’F
35 ppt 15.5’C/60’F 1.0233 1.0257
35 ppt 20’C/68’F 1.0252 1.0266
35 ppt 25’C/77’F 1.0264 1.0277

The specific gravity readings in the table above range from 1.0233 – 1.0277 however they are all correct in describing the same sample of water at 35 ppt salinity. The difference between the readings is due solely to the calibration temperature which is used to set the scale for the instrument. From this we can see how nonspecific S.G. is as a way of describing the salinity of the water in your aquarium. The Reef Sea Meter Refractometers are calibrated at 20’C/68’F and so the calculated specific gravity at 35 ppt displays a reading of 1.0266. This is the equivalent to a specific gravity reading of 1.0264 for a water sample temperature of 25’C/77’F using a standard hydrometer that has a calibration temperature of 25’C/77’F

Included are: A plastic, foam-lined hard case, a pipette for dropping test fluids onto the prism (never dip the refractometer into the test fluid!), a mini screwdriver for scale calibration, and a Users Guide.
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