What is salinity and why should it be measured?

Because of its high sensitivity and ease of measurement, conductivity is said to be the most commonly used method to determine the salinity of seawater. When the Practical Salinity Scale was adopted by oceanographers, they defined salinity as follows:  a seawater of salinity 35 (S = 35) has a conductivity ratio of unity with a solution of 32.4356 grams of potassium chloride in 1 kg of solution at 15C and 1 atmosphere.  This value for salinity was determined by extensive testing of seawater samples.  Therefore, a practical salinity reading is a relative value based on a standard potassium chloride (KCl) solution.  

Since salinity is a ratio, the measured value is dimensionless and has no units. However, salinity is commonly reported in units known as “practical salinity units” or psu, or in the traditional units of “parts per thousand” or ppt.

When properly calibrated, a conductivity probe and meter can be used to determine salinity in seawater and brackish estuarine water.  Salty solutions, such as brines or irrigation water, are better measured by using the TDS mode. HACH Conductivity Meters automatically calculate salinity using oceanographic equations compensated to 15°C per accepted conventions. When using an Hach Conductivity Probe, which has an integrated temperature sensor, and a conductivity meter, like a Orion Star A Portable  Meter or Orion Versa Star Pro Bench Meter, salinity can be reported as practical salinity units (psu) or parts per thousand (ppt), depending on user preference.