A CTD measures Conductivity, Temperature, and Depth. Despite the name, all CTDs actually measure pressure, which is not quite the same thing as depth. The relationship between pressure and depth is a complex one involving water density and compressibility as well as the strength of the local gravity field. The CTD data can be used to calculate salinity, density, sound velocity, and other parameters of interest. The term CTD is often used today to describe a package that includes the actual CTD as well as auxiliary sensors to measure other parameters (such as dissolved oxygen, pH, turbidity, fluorometer, altimeter, etc.) and a water sampler to collect water samples for later analysis in the lab. The term Sonde is sometimes used as an alternative to CTD.
A Profiling CTD measures water parameters as it travels through the water, whether lowered over the side of a ship with a winch to take measurements of a vertical column of water or integrated with an autonomous vehicle or glider. Common to all Sea-Bird Profiling CTDs:
- Purposeful designs built to perform under the unique dynamic conditions found on varying measurement platforms
- Pumped and ducted constant flow for matched temperature and conductivity response
- Measurements are made on the same sample of water with a predictable delay and predictable flow effects
- Aligning and coordinating measurements done with software or (for some models) automatically
See the following documents for more details on CTDs:
- CTDs Explained provides an introductory overview of CTDs.
- Application Note 82 provides guidelines on specifying a CTD.