Russell Cuhel prepares the Sea Bird to do a profile.

Preparation includes removing the protective covers from pH and oxygen probes, and attaching an upside-down Niskin bottle for near-bottom sampling. At the last minute, the shroud is removed from the light sensor and the power switch flipped to "on".The CTD Electronic Water Analysis Device (SeaBird) Basic physical and chemical aspects of water are now usually measured with a "CTD". Originally designed for Oceanographers, CTD stands for Conductivity-Temperature-Depth, the three major ingredients required to understand ocean circulation (physics). Conductivity is a measure of dissolved salts (related to salinity), temperature is the heat content of water, and depth is where in vertical space the instrument is. A typical CTD-style instrument is pictured to the left. It weighs about 50 pounds and costs about $25,000.
 
Our unit measures these things and much more. For most users, the more interesting additional factors include transmission ( a measure of cloudiness), fluorescence (an approximation for algae), visible light (sensor shown at left), and dissolved oxygen. Many of the measurements are made at eight times per second. If we lower the CTD at 5m/minute, we get a sample every 10cm. Each of these tell us one part of the character of the water under study.

Related web links:
Sea-Bird Electronics SBE 25