Two examples of CTD instruments. The CTD on the left is designed for deep ocean measurements. Its sensors for temperature, conductivity and pressure are contained in the small packages at the bottom corners of the frame. The main cylinder houses the control and data processing electronics. The entire package is lowered on a conducting cable and connected through the sets of electrical underwater connectors at the top.

The instrument at the right is a lighter version of the same principle for use on small boats in estuaries and coastal waters. The sensors are seen at the left. The instrument is lowered on a rope, without electrical connection with the boat. When it is brought back on deck it is connected to a computer, and the data are downloaded. The computer screen shows a typical result of a measurement station: Temperature (white), conductivity (green), salinity (blue) and density (red) against depth.

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