Salinity is obviously
important in saltwater aquariums. However, the most economical
ways of testing salinity are not always accurate. There are various
methods to test salinity in a reef tank and they include testing
buoyancy, refraction and conductivity. All of these methods can
be very accurate but what determines their reliability is the
quality of the instrument.
testers work by measuring conductivity in salt water. There are
probes in the tester that send a signal through the water and
detecting the levels of ions in the water. The higher the conductivity
the higher the salinity will be. Conductivity meters are very
reliable although they should be recalibrated once a month. These
are used in our systems due to its convenience and reliability.
It's a simple dip and check unlike hydrometers which require cleaning,
and refractometers which require a good light source and light
cleaning of the prism with a soft cloth.
Refractometers work by passing light
through water and analyzing the refraction. Most people have seen
this phenomenon when placing a straw in drink. When light passes
through spaces of different densities the light will bend the object
in the water. The basis of this idea is that light will travel slower
in spaces of higher density and thus appear to be bent or shifted.
Because the density of air and water is different, the object in
the water will bend. Refractometers measure this bend and determine
the salinity through something called a refraction index.
awareness of it because it's always
off by a consistent amount. In most cases, once properly calibrated
they are extremely accurate and reliable.
are considered to be very accurate once calibrated. However,
there are issues with making sure they are calibrated correctly.
I have found some refractometers to be impossible to be dialed
in correctly. Refractometers have a dial on top that allow
the user to adjust the scale. I've came across a few that
couldn't be adjusted high enough because the dial wouldn't
turn far enough to the proper reading. Therefore it was always
constantly off. This is no big deal as long as there is
Hydrometers work through buoyancy.
The higher the salt content in water the more buoyant it will
be. For example, it's easier to stay afloat in a saltwater lagoon
than a freshwater one. Most hydrometers have a swing arm attached
and will float to the appropriate specific gravity. Specific gravity
is the measurement of density compared to pure water. Therefore,
the specific gravity of reverse osmosis/deionized water is always
1 and salt water will always be greater than 1 because it's denser.
Swing arm hydrometers have issues
with accuracy and consistency. This is partly due to deposits
that often get trapped in the pivot of the swing arm which causes
inaccurate readings. Second, when water flows into the hydrometer
small micro bubbles can sometimes be created and thus cause higher
readings. Temperature of the water may also skew the readings
although it will be minor as long as it's 75-85 degrees. When
mixing salt water and using a hydrometer to check salinity it's
important to check salinity only after the temperature is within
70 degrees or higher. Swing arm hydrometers can be accurate but
it must be properly maintained and constantly checked against
a fluid of known salinity. After using a hydrometer it's important
to rinse it out thoroughly with warm water to get rid of any trapped
particles. For precaution it's a good idea to pour vinegar into
the hydrometer to dissolve any calcium deposits that might be
trapped on the arm.
There are hydrometers that don't
use a swing arm but use a bulb instead. These glass bulbs look
very similar to glass thermometers (often much bigger) and float
in a tube of water. The higher the bulb floats the higher the
salinity. They are much more accurate than the swing arms because
there are no moving pivots and bubbles don't skew readings too