We offer salinity and conductivity sensors. Here is some more information about how these sensors work and what they are measuring:
Salinity is the total of all non-carbonate salts dissolved in water, usually expressed in parts per thousand (1 ppt = 1000 mg/L). Unlike chloride (Cl-) concentration, you can think of salinity as a measure of the total salt concentration, comprised mostly of Na+ and Cl- ions. Even though there are smaller quantities of other ions in seawater (e.g., K+, Mg2+, or SO42-), sodium and chloride ions represent about 91% of all seawater ions. Salinity is an important measurement in seawater or in estuaries where freshwater from rivers and streams mixes with salty ocean water. The salinity level in seawater is fairly constant, at about 35 ppt (35,000 mg/L), while brackish estuaries may have salinity levels between 1 and 10 ppt. The Salinity Sensor (SAL-BTA) is appropriate for these types of measurements.
Conductivity is the fundamental property of a material to conduct electric current, usually expressed in microsiemens per centimeter. A Conductivity Probe measures the ability of a solution to conduct an electric current between two electrodes. In solution, the current flows by ion transport. Therefore, an increasing concentration of ions in the solution will result in higher conductivity values.
Because there is a nearly linear relationship between conductivity and concentration of a specific ion or salt, the Conductivity Probe can be used to determine the concentration of an ion. A curve can be obtained if you prepare or purchase standard solutions.
Total dissolved solids (TDS) is often defined in terms of a 2:1 ratio between conductivity in µS/cm and TDS in mg/L. It should be understood that a TDS reading of 500 mg/L can have a different meaning in a sample that is mostly NaCl, such as seawater, compared to another sample that is composed primarily of hard water ions such as Ca2+ and HCO3–. The relationship between conductivity and sodium chloride concentration is approximately a 2:1 ratio.
Vernier offers two conductivity probes: Conductivity Probe (CON-BTA) and Go Direct® Conductivity Probe (GDX-CON). The salinity range of these sensors is 0 to 10 ppt. Seawater has a salinity of 35 ppt, so any seawater samples will need to be diluted before making measurements with this sensor. We recommend that you dilute seawater samples (or other samples that initially give readings above 10 ppt) to 1/4 of their original concentration, then multiply their measured salinity reading by 4 to obtain a final salinity value, in ppt. Brackish water in coastal estuaries is often in the range of 0 to 10 ppt, well within the high range of the probe.