Mar 27

Salinity Calculation for CTD-Divers

Diver-Office offers the option to export salinity data to Excel. The salinity data can only be exported for CTD-Divers. This post outlines how the conductivity data from the CTD-Diver is converted to salinity data. 

 

Introduction

Salinity is the saltiness or dissolved salt content of a body of water. Salinity is an important factor in determining many aspects of the chemistry of natural waters and of biological processes within it [wiki]. Salinity cannot not be measured directly, but needs to be calculated from conductivity measurements. Salinity is most commonly reported using the Practical Salinity Scale 1978, a scale developed relative to a standard potassium-chloride solution and based on conductivity, temperature and barometric pressure measurements. Salinity expressed in the PSS is a dimensionless value, although by convention, it is reported as practical salinity units (psu). Salinity in practical salinity units is nearly equivalent to salinity in parts per thousand [usgs].

Diver-Office (version 2016 and higher) offers the option to export salinity data to Excel. Figure 1 shows the Default Export Settings with the salinity export data option highlighted. The salinity data can only be exported for CTD-Divers. Salinity data cannot be shown in Diver-Office.

Figure 1 - File export settings as used in Diver-Office 2016.

Computation of Salinity

If specific conductivity values have been compensated to 25 °C and water depths are sufficiently shallow that pressure corrections are not necessary, salinity can be calculated using the conductivity measured by the CTD-Diver. The salinity is expressed in practical salinity units (psu). The relationship between specific conductivity and salinity is shown in Figure 2. The underlying data set is listed in Table 1 and obtained from [usgs].

In case the CTD-Diver is set to measure conductivity instead of specific conductivity at 25 °C the conductivity will be converted

Where Cs is the specific conductivity at 25 °Cm and Tm are the measured conductivity and temperature, respectively.

Figure 2 - Conversion of specific conductivity, in millisiemens per centimeter, to salinity, in practical salinity units [wiki].


Table 1 - Rating table for conversion of specific conductivity, in millisiemens per centimeter, to salinity, in practical salinity units [usgs].

Specific conductivity in mS/cm Salinity in psu Specific conductivity in mS/cm Salinity in psu Specific conductivity in mS/cm Salinity in psu
0.100 0.046 11.000 6.233 38.000 24.099
0.300 0.142 13.000 7.464 41.000 26.220
0.500 0.240 15.000 8.714 44.000 28.364
0.700 0.340 17.000 9.981 47.000 30.532
1.000 0.492 20.000 11.911 50.000 32.722
2.000 1.016 23.000 13.873 53.000 34.935
3.800 2.001 26.000 15.865 56.000 37.172
5.000 2.679 29.000 17.885 59.000 39.430
7.000 3.836 32.000 19.931 62.000 41.712
9.000 5.022 35.000 22.003 65.000 44.016


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Last modified on Monday, 27 March 2017 15:49