General notes#
Salinity is a key oceanographic parameter, and obtaining high-quality measurements of salinity is a key reason why we do CTD casts at sea. Salinity is calculated from measurements of the electric conductivity of the seawater along with temperature and pressure (which are also measured by the standard CTD).
Compared to temperature and pressure sensors, conductivity measurements are prone to drift and outliers. The main reason for this is that the conductivity reading is proportional to the volume of water in the “conductivity cell” where the measurement is made[1]. Any contamination of the cell can therefore disturb the accurate reading of conductivity. The typical culprit in oceanographic measurements is growth of biological organisms within the cell, but other contaminants like oil can also disturb the measurement. Issues can also arise due to problems with the mechanical pump pumping water across the sensor, or problems with the sensor itself.
As a result, it is very important to thoroughly clean and rinse the conductivity cells during the cruise[1]. In addition, CTD salinity measurements should be compared with independent laboratory measurements of salinity based on in-situ water samples collected along with the ship CTD data in order to check the measurements and ensure that the sensors are performing well. The main objective of this comparison is to validate the CTD-based measurements, but if major discrepancies are discovered, measures may be taken to fix the conductivity sensors, or corrections may be applied to the CTD salinity data to better align them with the “true” values measured in the laboratory.
Whenever possible, we aim to do these laboratory salinity measurements on board. With some planning (and if the salinometer works correctly), the calibrated CTD dataset can be ready at the end of the cruise.
Underway salinity comparisons can also help identify issues with conductivity while there is still time to take measures ad solve problems..
Personnel hours are reuired for training, collecting water samples, setting up the salinometer, performing the laboratory analysis, and comparing CTD and laboratory data. Make sure to acccount for this when planning the cruise!