Conductivity

RBR offers two types of conductivity sensors; an inductive cell for the marine applications, and an electrode sensor for the freshwater salinity.

The normal oceanographic conductivity range of 0 to 85 mS/cm is measured by an inductive cell to an accuracy of ±0.003 mS/cm. The use a three-coil system with a closed loop feedback results in a superior temperature compensation. The titanium housing provides a depth rating for the cell of up to 6,600m. It also provides shielding and stable cell geometry. The response time of the inductive cell is less than 95 milliseconds when profiling. An axial cell with a fast response thermistor canl be supplied for all profiling applications.

The normal freshwater conductivity range of 0 to 2 mS/cm is measured by a three electrode cell, with an accuracy of ± 0.003 mS/cm and resolution to better than 0.02 µS/cm (0.00002 mS/cm). The maximum depth rating of this conductivity cell is 2,000m.

Salinity and speed of sound are calculated by the logger software. RBR uses the internationally accepted method for deriving salinity from conductivity, temperature and depth. (1978 Practical Salinity Scale).

RBR software gives the user the choice between the Chen and Millero UNESCO algorithm (default) , the older Wilson algorithm, and the more recent Del Grosso algorithm for computing the speed of sound (sound velocity). Download the RBR technical brief (.pdf format) on Seawater Salinity Calculation.

We use a Guildline Autosal 8400B (±0.002), which is standardized using IAPSO Standard Seawater. All calibrations are carried out using eight points, and a cubic polynomial curve fit. All loggers are supplied with a calibration certificate showing the deviation from the curve at each point.

Below is a graph of residuals for the freshwater range of 0 to 2 mS/cm (graph in µS/cm).

Below is a graph of residuals for the marine range of 0 to 85 mS/cm (graph in mS/cm).

Inductive conductivity cell advantages