Oceanography

Conductinity, Temperature and Depth System(CTDs)

Today’s standard instrument for measuring temperature , salinity and often also oxygen content is the CTD, which stands for conductivity, temperature, depth. It employs the principle of electrical measurement. A platinum thermometer changes its electrical resistance with temperature. If its incorporated in an electrical oscillator, a change in its resistance produces a change of the oscillator frequency, which can be measured. The conductivity of seawater can be measured in a similar way as a frequency change of a second oscillator, and a pressure change produces a frequency change in a third oscillator. The combined signal is sent up through the single conductor cable on which the CTD is lowered. This produced a continuous reading of temepratrue and conductivity as functions of depth at a rate of up to 30 samples per second.

Electrical circuits allow measurements in quick succession but suffer from “instrumental drift”, which means that their calibration changes with time. CTD systems therefore have to be calibrated by comparing their readings regularly against more stable instruments. They are therefore always used in conjunction with reversing thermometers and a multi-sample device.