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What is seawater specific gravity testing?

Seawater specific gravity (SSG) is an indirect measure of seawater density. Saltwater is more dense than pure water because it has a higher content of dissolved salts and minerals. Different species of aquatic life thrive in freshwater and saltwater, with most species being very sensitive – so even very small changes in the salt water density can affect the organisms within the aquatic ecosystem. Seawater specific gravity testing is useful for various applications, including aquaculture, desalination plants and oceanographic research.


Aquaread’s seawater specific gravity meters

Specific seawater gravity is a standard parameter with each of our multiparameter water testing probes, as well as the Aquaplus combined optical dissolved oxygen, electrical conductivity and temperature probe. Our specific seawater gravity meter measures in the range of 0 – 50 ST. ST, or Sigma-t, is a unit used in oceanography for measuring seawater density at a given temperature. Aquaread’s rugged water quality testing equipment is designed for use in the field, and can be used for both fixed and spot monitoring.


The Aquaplus, Aquaprobe AP-5000 and Aquaprobe AP-2000 are all portable devices that are ideal for transporting between multiple sample locations. The Aquaprobe AP-7000 is designed with long term deployment in mind thanks to its built-in self-cleaning facility.

How do seawater specific gravity meters work?

Seawater specific gravity is calculated from electrical conductivity and temperature readings. The electrical conductivity of water is it’s ability to conduct a current of electricity. Seawater contains dissolved ionic salts. These free ions within the water conduct electricity, so the more dissolved salts in the water the higher the conductivity. A water sample with a high conductivity will also be more dense because of the high concentration of dissolved salts. For more information on conductivity, take a look at our electrical conductivity parameter information.


Electrical conductivity is measured by inserting a probe into a water sample and applying a voltage between electrodes. The drop in voltage that occurs is a measure of the resistance of water. Resistance and conductivity are reciprocal; the higher the resistance the lower the conductivity. Electrical conductivity is relative to temperature, which means that the conductivity alters depending on the temperature of the water.

This is why it’s vital to take temperature readings alongside those for conductivity. The word “specific” in seawater specific gravity means that the value given is already corrected to reflect the temperature of the water sample.

Get into monitoring with Aquaread

If you would like any more information about TDS meters and other total dissolved solids testing equipment, or water quality monitoring in general, please Contact Us. Click here to see our product range.