D-2 Inc Hybrid CTD Ocean Data and Laboratory Results

Besides deep ocean observations, assessment of response characteristics of the D-2 Hybrid CTD by lowering sensors at various rates in WHOI’s response tank facility. They stratified the tank with cooler, fresher water overlying warmer, saltier water–double diffusive convection maintains a sharp interface between the layers. They lowered hybrid CTDs through the interface at several speeds to characterize the transient response of the instrument. 

The above figure shows vertical profiles of temperature (T), conductivity (C) and salinity (S) at profile speeds of 10 (black), 25 (blue) and 50 (red) cm/s.  ~2 m have successively offset the 25 and 50 cm/s curves for clarity. From these data, they judge the T probe to have a response time of around 0.5 s, which is quite typical for standard CTD sensors. The conductivity data shows an abrupt jump at the top of the interface with a more rounded drift into the lower layer conductivity. The latter may manifest reduced flow in the fluid boundary layer within the cell and/or a small thermal mass effect. After response correction, the resulting salinity data shows a rapid transition through the interface with no spiking.