Can I deploy a Sea-Bird CTD in freshwater, such as a river or lake?
Yes, CTDs are successfully used in many freshwater systems, such as to examine water conditions and vertical density gradients.
It is important to check that the pump on your CTD is enabled for freshwater use. For example, for the SBE 19plus V2, you should change the minimum conductivity frequency setting to enable the pump to turn on at low conductivities. The minimum conductivity frequency should be set to 1 – 5 Hz above the in-air zero raw conductivity frequency for your instrument (see the instrument calibration sheet). For extremely low conductivity systems, such as alpine lakes, you may consider setting the minimum conductivity frequency to 0. It is also important to increase the pump delay to 1 – 2 minutes, to allow enough time for the CTD to reach the water and purge all the air from the plumbing before the pump turns on. See the CTD manual for more details on configuring the pump.
In addition, for profiling CTD applications, note that the cell thermal mass corrections that Sea-Bird uses are intended for seawater. These corrections should not be applied to freshwater data; they can give bad results, due to non-linearity and the way that the derivative dC/dT is calculated in areas where conductivity changes are very small.