Why Do Oceanographers Measure The Conductivity Of Seawater?

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As conductivity is temperature-dependent and conductivity can be used to determine salinity, oceanographers measure conductivity, temperature, and depth (CTD) when studying seawater. 

Oceanography is the study of the ocean, and oceanographers have a very important job in climate research. Because the ocean stores so much heat, the ocean has a large effect on our world’s climate. With climate change such a “hot topic” in the media, oceanographers are part of the race to save Planet Earth!

To help predict future changes in the Earth’s temperature and warn us of possible sea-level changes, seawater must be frequently analyzed. To measure seawater accurately, a CTD meter (or Sonde) is used to understand the physics, chemistry, and biology. 

Why Measure The Conductivity Of Seawater?

As you probably are aware, seawater contains salt. Measuring salinity is a common practice amongst oceanographers because it can help them understand the water cycle in greater detail. As temperature and salinity directly affect the density in the ocean, measuring the conductivity of seawater is somewhat the missing puzzle piece.

Conductivity measures the ability of a material to transmit an electrical current over a certain distance, usually measured in Siemens (S) per distance. When the number of dissolved ions (charged particles) in a solution increases, so does the solution’s ability to carry an electrical charge.

Oceanographers measure conductivity to see how many dissolved substances, chemicals, and minerals are present in seawater. When there are higher amounts of these impurities in the water, a higher electrical conductivity is usually measured. 

How Do Oceanographers Measure The Conductivity Of Seawater?

To measure the conductivity of seawater, ocean scientists, commonly known as oceanographers, use an oceanography instrument called a CTD (or Sonde). A CTD stands for conductivity, temperature, and depth, with depth closely related to pressure. 

By using a CTD, oceanographers can measure the conductivity of seawater accurately. Salinity can be determined from the temperature and pressure of the same sample of seawater collected inside the sample bottles. The depth measured is derived from the pressure underwater, which calculates the density of water from the temperature and salinity readings. This is why some oceanographers associate the “D” in CTD as “density” not “depth”.  

Oceanographers use a CTD device to provide a better understanding of seawater characteristics through the entire water column, hence the need to drop the CTD device at many depths. 

A CTD is crucial to understanding the physics of seawater. Studying physics in the ocean allows biologists to look at the chemical makeup of seawater and how it changes with depth. Therefore the use of a CTD in oceanographic research is key to investigating the physics, chemistry, and biology of seawater. 

How Does A CTD Measure Conductivity?

Inside a CTD there is a cluster of sensors that measure conductivity, temperature, and depth (pressure). Depth measurements come from the hydrostatic pressure measured, and salinity from the electrical conductivity. 

The sensors are protected by a metal or resin housing which determines the CTD depth limit. For example, when measuring the conductivity of seawater at greater depths of 34,000 ft, titanium housings are used to protect the sensors. 

The sensors can be clustered on a large frame, known as a rosette. A rosette can hold many water-sampling bottles (also known as Niskin bottles), to collect more samples at different depths. 

The rosette frame also contains sensors that can measure extra physical and/or chemical properties by taking samples of the water. These include looking at dissolved oxygen (DO), microscopic organisms (plankton), and chlorophyll fluorescence.

Open Niskin bottles

How To Deploy A CTD?

Firstly, CTDs are massive, so they need to be placed on the deck of a boat/research vessel. The CTD (or CTD rosette) is then deployed over the back of a research vessel and slowly lowered into the water.

CTD on a boat deck

The instrument is dropped down (downcast) to just above the ocean floor, or as close as it can get, usually at a rate of 0.5 m/s to collect samples of seawater. During its descent, the CTD records the water column profile of the downcast.

A conducting wire cable is connected to the CTD, which is then connected to a computer onboard the vessel. This cable allows the CTD to instantaneously upload real-time visualization and data on the oceanographer’s computer screen. 

On the way back up (upcast), the oceanographer uses the water column profile to determine the depths at which the CTD (or rosette) will be stopped. Oceanographers must stop the CTD to allow the bottles to collect the water samples for onboard analysis. The Niskin bottles are open when the CTD is deployed, and are closed when a small weight moves down the hydrographic line, pulling the ends of the bottle shut.

The CTD will detect how the conductivity and temperature changes of the seawater, relative to depth.

Conclusion

Oceanographers must measure the conductivity of seawater. As conductivity is directly related to salinity, oceanographers can use an instrument called a CTD (conductivity, temperature, depth) to detect how the conductivity and temperature changes relative to depth. 

If you have any questions regarding conductivity, temperature, or salinity, the testing kits we have to offer, do not hesitate to contact our world-class team at Atlas Scientific.

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