A student from the HIGH TIDE project turns on the CTD instrument. High school students use the CTD recorder to measure salinity, temperature and depth of the water in the Lafayette River which is a part of Chesapeake Bay.
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Image courtesy of the HIGH TIDE project
Salinity - Dissolved Salts, Measuring Salinity
When we measure the salinity of water, we look at how much dissolved salt is in the
water, or the concentration of salt in the water. Concentration is the amount (by weight) of salt in water and can be expressed in parts per million (ppm). Here are the classes of water:
- Fresh water - less than 1,000 ppm
- Slightly saline water - From 1,000 ppm to 3,000 ppm
- Moderately saline water - From 3,000 ppm to 10,000 ppm
- Highly saline water - From 10,000 ppm to 35,000 ppm
Ocean water has a salinity that is approximately 35,000 ppm. That's the same as saying ocean water is about 3.5% salt. Sometimes, salinity is measured in different units. Another common unit is the psu (practical salinity units). Ocean water has a salinity of approximately 35 psu. Scientists measure salinity using a CTD instrument (CTD = conductivity, temperature, depth).
Ocean water is about 3.5% salt. That means that if the oceans dried up completely, enough salt would be left behind to build a 180-mile-tall, one- mile-thick wall around the equator. About 90 percent of that salt would be sodium chloride, or ordinary table salt. Chlorine, sodium and the other major dissolved salts of the ocean are listed in this table:
Dissolved salts in
sea water (atoms): |
| 55.3 % Chlorine |
| 30.8 % Sodium |
| 3.7 % Magnesium |
| 2.6 % Sulfur |
| 1.2 % Calcium |
| 1.1 % Potassium |
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