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TRUE NATURAL Salinity Refractometer; Reef Aquarium, w/ Lighted Daylight Plate


  • $ 5499

 THE REEF SEA METER with Lighted Daylight Plate



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

Dual Scale Specifications:

Range:                                          Minimum Division:       Accuracy:                  Light Source:

0-40 PPT of Salinity in NSW         1.00 PPT                        +/- 1.00 PPT               Ambient: Sunlight or Lamp

1.015 to 1.030 Specific Gravity     0.001 SG                        +/- 0.001 SG


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 to test 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.
Our 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.
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.
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.
Step 1: Clean the glass prism and clear daylight plate cover of the Refractometer with distilled or reverse osmosis water to ensure that there are no salt residues that would affect the reading.  These water sources will be pure water with 0ppt.
Step 2: Place a drop of the calibration fluid (distilled or R/O water) onto the glass prism of the unit.  Close the daylight plate over the prism and smooth out any air pockets between the prism and the daylight plate.
Step 3: Wait for 15 seconds for the temperature to acclimate 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.  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 loosening the screw. Your Refractometer is now calibrated.


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.
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 35ppt 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 29ppt as it exerts less osmotic stress on the fish whereas reef systems are normally maintained at 35-36ppt.
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 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’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 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 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













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 35ppt 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 non specific 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 35ppt 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 User's Guide and a LIGHTED DAYLIGHT PLATE 


National Industrial Supply
30777 Rancho California #891420
Temecula, CA 92589, USA
Sales: 951.308.9269  -  Fax: 310.733.5480  -  E-mail:  -  Website: