CTD probes contain a range of sensors to measure water conductivity, temperature and depth as the probe travels down through the water column. During its return to the ship, water samples are taken at relevant depths using Niskin bottles. The CTD rosette on the RRS Sir David Attenborough will also be equipped with a fluorometer, transmissometer, pH meter and Lowered Acoustic Doppler Current Profiler (LADCP).
Depending on the material of their housing, CTD probes can be lowered to several thousand metres. The RRS Sir David Attenborough will be able to deploy CTD arrays weighing up to 10 tonnes either over the side of the ship or via the moon pool.