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.

CTD With And Without Sleve


Conductivity0 to 100.000 µS/cm1 µS/cm0.25% ±5 µS/cmMeasured
Density990 to 1035 kg/m30.04 kg/m3± 0.02 kg/m3EOS80
Depth0 to 100 m0.01 m ±0.25% FSEOS80-
Pressure0 to 100 dBar0.01 dBar0.25% of FSMeasured
Salinityup to 420.01±0.1PSS-78
Sound velocity1400 to 1730 m/s0.01 m/s±0.15 m/sChen-Millero
Specific conductivity0 to 250,000 µS/cm1 µS/cm0.25% ±5 µS/cmEOS80
Temperature-5 to +45°C0.01°C±0.05°CMeasured

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