How to Determine the Salinity From Conductivity

The equation of state that converts conductivity to salinity along with the definition of all its terms takes up two pages in a UNESCO PDF (see References). Fortunately, several sites have converted this formula into online calculators. The main stumbling block remaining is merely to get your measurements in the proper units. For example, note that the SI unit for conductivity is siemens per meter (S/m). A siemen is the inverse of an ohm, also referred to as a "mho."

Instructions

1. Open the link in the References section below to the Johns Hopkins Applied Physics Lab website. Its default is to have the pressure, temperature and conductivity fields checked off.

2. Uncheck pressure and check the depth field. Enter the depth at which you are measuring conductivity, in meters.

3. Enter the water's temperature and the conductivity measured by your lab equipment in siemens per meter. If your lab equipment rates conductivity in milli-siemens per centimeter (mS/cm), just divide by 10 to convert it to S/m.

4. Move your mouse pointer to a different field and click it. This will activate the JavaScript to update all calculated fields. Read the salinity off the salinity field, in PSU. PSU stands for practical salinity unit. One PSU equals one gram of salts per thousand grams of solution. That's about a gram per liter of seawater.

Tips & Warnings

• To determine salinity from conductivity using a table, use the Envco Global link in the Resources section. To determine salinity from the equation of state, use the UNESCO link below.

References

• UNESCO: Algorithms for Computation of Fundamental Properties of Seawater
• Johns Hopkins Applied Physics Lab: A Seawater Equation of State Calculator

Resources

• EnvcoGlobal.com: Conductivity-Salinity Table
• Martindale Center: Seawater Calculators
• Online Conversion: Salinity to Conductivity Conversion