In a previous article we explained how to use a refractometer to keep track of salinity in your tank and when making batches of saltwater. Did you know you can monitor your reef tank’s salinity with a salinity meter? It’s even possible to track the salinity level using a smart device over Wi-Fi. How does it work? Technically speaking, there’s no such thing as a salinity probe. Confused? Don’t worry. Here’s what you need to know about a salinity probe and meter, their calibration and how to care for one.
Science behind Measuring Salinity with a Probe
Salinity meters and probes are really measuring conductivity. Conductivity is the ability of the water to conduct an electrical current. Marine salts dissolve in water, forming positively and negatively charged ions. The ions dissolved in the water act as the conductors. The major positive ions are sodium, (Na+) calcium (Ca+2), potassium (K+) and magnesium (Mg+2). The major negative ions are chloride (Cl–), sulfate (SO4-2), carbonate (CO3-2), and bicarbonate (HCO3–). Because dissolved salts increase salinity as well as conductivity, the two parameters are related. Even though there are a variety of salts in seawater, sodium and chloride provide most of the conductivity. That’s good because the ratio of sodium and chloride remains very stable while calcium and other ions can fluctuate. This means even though you’re adding supplements like calcium and magnesium, they won’t throw-off the salinity readings.
Salinity Probe Construction
There are many specialized conductivity probes for industrial and research applications. Fortunately, aquarium salinity measurements don’t require an expensive probe. The probes used for measuring salinity (conductivity of saltwater) use two internal electrodes to measure current. The electrodes can be made of stainless steel, titanium, platinum or graphite. The electrodes are encased in a plastic tube or built into the hand-held meter. Not all conductivity probes are designed to measure the relatively high conductivity of saltwater. Your best bet is to use a meter and probe combination designed for aquarium use.
Water temperature is important when measuring conductivity. As water temperature increases dissolved ions move faster and increase conductivity. This will cause errors in calculating salinity. Fortunately, there is a solution. Hand-held salinity meters will have a built-in temperature probe. Aquarium controllers either have a separate temperature probe or the salinity probe has a built-in temperature sensor. Salinity meters, like pH meters, use automatic temperature compensation (ATC) to correct for temperature fluctuations and provide accurate salinity measurements.
Converting Conductivity to Salinity
The salinity meter or sensor module is the brains behind the process. The probe sends the conductivity and temperature information to the meter. Conductivity uses millisiemens (mS) and microsiemens (µS) units of measure. This data is used to convert conductivity into salinity units. The meter has a set of data it uses for this conversion. The ATC feature also contributes to the final calculation, allowing the meter to display an accurate salinity level. If you’ve invested in an automated monitoring system, you’ll be able to record and review fluctuations in salinity over time. You may even be able to see when an automatic top-off system adds water, bringing the salinity back in line.
Calibrating the Salinity Meter
Salinity meters usually come with a calibration solution. The solution is formulated to match the salinity of seawater, typically 35 parts per thousand. Most manufacturers recommend calibrating the meter when you first set it up. This confirms the meter is working properly and allows you to make any necessary adjustments. It takes time for the meter’s ATC system to stabilize. It helps speed the calibration process if the calibration solution is close to room temperature. Probe manufacturers recommend against using a glass or metal container when calibrating the meter. These materials are known to cause electromagnetic interferences (EMC). A plastic container is recommended. When calibrating, make sure there are no air bubbles on the electrodes; this will cause errors. Gently tap the probe on the side of the container to knock off air bubbles. Discard the used calibration solution when you’re finished. If you’re using a separate temperature probe, make sure to place it in the calibration solution so the ATC will work.
Re-calibration and Maintenance
Unlike pH meters, salinity meters don’t require frequent calibration. Depending on use, calibration of a salinity meter can be done once every three or four months or manufacturer’s on how to care for one. If your probe is continuously submerged in your filter sump, it’s especially important to make sure the probe tip is not covered with slime, algae or other debris. The two electrodes are fairly rugged. You can wipe down the outside of the probe body with a soft cloth and RO water. If the electrodes look dirty, gently remove debris with a soft brush and RO water.
Salinity meters, whether hand-held or mounted inside the filter sump, are an easy way to measure salinity in your reef aquarium. Hand-held meters are especially useful if you have multiple tanks and when mixing up a batch of saltwater. Be sure to read the manual that comes with your meter. It will provide up to date information on salinity meter calibration, care and use of the device.