Testing Salinity

Testing Salinity in Saltwater Aquariums

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.

Electronic salinity 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.
They 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
awareness of it because it's always off by a consistent amount. In most cases, once properly calibrated they are extremely accurate and reliable.

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 much.

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