A Conductivity-Temperature-Depth (CTD) instrument is used to measure salinity, temperature, and depth in the water. A conductivity probe measures induction in a parcel of water, which is dependent upon the salinity of the water. Temperature is measured with a thermistor. Depth is inferred by a pressure measurement, as 1 decibar (dbar) of pressure approximately equals 1 meter of depth. Thus a pressure of 2500 dbar is roughly equal toa depth of 2500 m.

Data from the CTD can be used to create a vertical profile of the water at a specific station. An example of a Temperature vs. Salinity (TS) plot is shown at left.

The graph shows how temperature and salinity change from the sea surface to the bottom. In most areas, temperature tends to decrease with depth while salinity tends to decrease, although this is not always the case. The region of rapidly changing temperature near the surface is called the thermocline, and the region where salinity changes the most is called the halocline. The density of seawater is dependent upon temperature, salinity, and pressure.

If researchers steer a ship along a transect in the ocean, and deploy the CTD at several different stations, they can obtain several measurements of pressure. Depths of similar pressure are termed isobars. The isobars are compared to depths with similar geopotentials, or level surfaces with respect to the earths gravity. If these are sloped with respect to each other, there is a horizontal pressure gradient. Water in the ocean flowsfrom high pressure to low pressure; similar to the situation in which you turn on the faucet in your sink. Therefore, researchers can calculate the speedand direction of the oceans major currents.