A CTD (Conductivity Temperature Depth) probe is used for oceanographic physics to measure conductivity, temperature and depth.
It is used by Geopolynesie for bathymetric and hydrologic measures.

 

Parameters	Interval	Resolution	Precision	Method
Conductivity	0 to 100.000 µS/cm	1 µS/cm	0.25% ±5 µS/cm	Measured
Density	990 to 1035 kg/m3	0.04 kg/m3	± 0.02 kg/m3	EOS80
Depth	0 to 100 m	0.01 m ±0.25% FS	EOS80	-
GPS	-	-	10m	-
Pressure	0 to 100 dBar	0.01 dBar	0.25% of FS	Measured
Salinity	up to 42	0.01	±0.1	PSS-78
Sound velocity	1400 to 1730 m/s	0.01 m/s	±0.15 m/s	Chen-Millero
Specific conductivity	0 to 250,000 µS/cm	1 µS/cm	0.25% ±5 µS/cm	EOS80
Temperature	-5 to +45°C	0.01°C	±0.05°C	Measured

source : http://www.sontek.com/productsdetail.php?CastAway-CTD-11