Refractometer with Seawater Calibration

What
is different about the D-D Seawater Refractometer?

Almost
all handheld 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.

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 natural sea water has the same refractive index as
a 36.5ppt solution of brine.

After
extensive development, D-Ds new ATC refractometer addresses this issue as the
scale is calibrated for seawater giving true salinity results
for aquarium use when correctly calibrated and greater accuracy than previously
attainable.

Our refractometers
feature a copper internal body which conducts heat faster than cheaper
aluminium and plastic versions for faster and more accurate auto temperature
compensation.

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 20oC 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

Most salt
refractometers use this temperature and will be marked 20C or 20/20.
Calibration should always be carried out with the instrument at 20C which is
close to the average room temperature in most cases. Allow the refractometer to
stand at this temperature for 30 minutes to equilibrate.

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.

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.

D-D
refractometers are calibrated at 20C 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 25C using a
standard hydrometer that has a calibration temperature of 77F or 25C.