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The CTD instrument is designed to measure conductivity, temperature, and depth as well as other water characteristics such as salinity, pressure, and density.  The CTD has been used extensively since the 1960s and 1970s to measure water qualities. 

In the past many instruments, such as salinity meters were inserted into the water or kits were used.  However, the CTD changes this by actually being lowered into the water column.  As it is lowered it makes continuous measurements which are sent to a computer on board the ship. Also, water samples are collected at pre-determined depths.

When conductivity is measured so is salinity because electric currents pass through water of a higher salinity more easily. 

Temperature is measured by using a thermistor, a platinum thermometer, or a combination of both to an accuracy of .005 degrees Celsius. 

Pressure is measured with a strain gauge pressure monitor or a quartz-based digital pressure gauge.  Once pressure is measured in decibars a conversion can be done to measure depth. 

Density is calculated from measurements of conductivity (salinity), temperature, and depth. 

Other sensors can be placed on CTD instruments so that dissolved oxygen, pH, turbidity, and flourescence of the water can be measured.

Thus, the CTD allows scientists to make many acurate measurements of the physical characteristics of the water in the water column.

more ...

how to use a CTD
some useful links about CTDs
see a video about using CTDs to hunt for hydrothermal vents

Click here for a video about how CTD's work made summer 2007.