CTD InstrumentNowadays, CTD instruments are the main way to measure water characteristics like salinity, temperature, pressure, depth and density. CTD stands for Conductivity-Temperature-Depth.
A CTD instrument is placed in the water it is measuring. As the CTD instrument is lowered through the water (or as it sits still at a given point), measurements of conductivity, temperature and depth are recorded continuously. Some CTD instruments are so fast that they measure each of these quantities 24 times each second! This provides a very detailed description of the water being tested.
CTD instruments measure three important quantities directly - conductivity, temperature and pressure. By measuring conductivity (how easily electric currents pass through the water sample being tested), scientists can get a measurement of that water sample's salinity. This is because electric current passes much more easily through water with a higher salt content. So if we know the conductivity of the water, we know how much salt is in the water. Salinity is measured in psu (practical salinity units).
A CTD instrument also measures the temperature of the water. The sensors used to measure temperature on a CTD instrument are very accurate.
Finally, a CTD instrument measures pressure. Pressure is recorded in decibars. Since depth and pressure are directly related, a measurement in decibars can be converted to depth in meters. Conveniently, the pressure in "X" decibars is almost exactly equal to the pressure found at "X" meters of depth. For instance, at about 500 meters below the surface, the pressure is right around 500 dbars.
The density of water can be calculated if you know the measurements of conductivity (salinity), temperature and pressure of the water.
The conductivity, temperature and pressure measurements are recorded in digital form. They can be stored by the actual CTD instrument and transferred to a personal computer after the CTD has been brought out of the water or the transfer of data can happen continuously through a cord connected from the CTD instrument to a personal computer on ship or on dock. CTD profilers are sophisticated equipment, but they can be used in all sorts of situations. The HIGH TIDE project has high school students using a SeaBird Model SBE-19 CTD to measure water characteristics in Chesapeake Bay. And they lower their CTD from a bridge into the water below!