Conductivity, temperature, depth

A CTD or Sonde is an oceanography instrument used to determine the conductivity, temperature, and depth of the ocean.

The CTD may be incorporated into an array of Niskin bottles referred to as a carousel or rosette. The sampling bottles close at predefined depths, triggered either manually or by a computer, and the water samples may subsequently be analyzed further for biological and chemical parameters.

The CTD may also be used for the calibration of sensors.

The instrument is a cluster of sensors which measure conductivity, temperature, and pressure. Sensors commonly scan at a rate of 24 Hz. Depth measurements are derived from measurement of hydrostatic pressure, and salinity is measured from electrical conductivity. Sensors are arranged inside a metal or resin housing, the material used for the housing determining the depth to which the CTD can be lowered. Titanium housings allow sampling to depths in excess of 10,000 metres (33,000 ft). Other sensors may be added to the cluster, including some that measure chemical or biological parameters, such as dissolved oxygen and chlorophyll fluorescence, the latter an indication of the concentration of microscopic photosynthetic organisms (phytoplankton) contained in the water.

Deployment of the rosette is from the deck of a research vessel. The instrument is lowered into the water in what is called the downcast to a determined depth or to a few meters above the ocean floor, generally at a rate of about 0.5 m s-1. Most of the time a conducting wire cable is attached to the CTD frame connecting the CTD to an onboard computer, and allows instantaneous uploading and real time visualization of the collected data on the computer screen. The water column profile of the downcast is often used to determine the depths at which the rosette will be stopped on its way back to the surface (the upcast) to collect the water samples using the attached bottles.

The CTD system was developed by Neil Brown, of the Woods Hole Oceanographic Institution, to overcome the limitations of an earlier similar system also developed by Brown, called an STD. The improvement was made possible thanks to increased reliability and reduced cost of computer technology.

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