AN94: Wide-Range Conductivity Calibration
On request, Sea-Bird provides a conductivity sensor that has been modified to provide conductivity readings over a wider range than our standard conductivity range. The wide-range calibration is indicated by a ‘w’ at the end of the serial number on the conductivity calibration sheet (for example, 4216w); note that the serial number on the instrument label does not include the ‘w’.
The modification is implemented by inserting a precision resistor in series with the conductivity cell. Therefore, the equation used to fit the calibration data is different from our standard equation.
To fit the calibration data, pseudo-conductivity is calculated from true conductivity with the following formula:
σ' = pseudo conductivity = Cσ / ( C + Rσ )
- σ = true conductivity
- σ' = pseudo conductivity
- C = cell constant = 2000.0 (S/m)
- R = series resistance (varies, depending on range; for example, 250 ohms for 70 S/m calibration)
The calibration coefficients are generated at Sea-Bird using pseudo-conductivity, water temperature, and sensor frequency.
Procedures are provided below for processing data from a conductivity sensor with a wide-range calibration, using:
- Your own software
- Sea-Bird software
Processing Data Using your own Software
If processing data using your own software, first calculate pseudo-conductivity using the coefficients on the calibration sheet, then convert pseudo-conductivity to true conductivity using the following formula:
σ = true conductivity = Cσ' / ( C - Rσ' )
Processing Data Using Sea-Bird Software
Sea-Bird’s Seasoft software (Seasave and SBE Data Processing) automatically calculates true conductivity (s) for a sensor with a wide-range calibration. On the conductivity calibration sheet, the serial number for a conductivity sensor with a wide-range calibration has a ‘w’ at the end (for example, 4216w). The 'w' in the sensor serial number in the configuration (.con or .xmlcon) file indicates to Seasoft that this is a wide-range conductivity sensor, leading the software to prompt for the cell constant and the series resistor value. Sea-Bird provides a configuration file when you purchase a new instrument, and when the instrument is recalibrated. If you need to re-create the configuration file, proceed as follows (example shown is for SBE 19).
Note: You can create or modify the configuration file in SBE Data Processing (post-processing software) or Seasave V7 (real-time data acquisition software); both programs use the same configuration file.
SBE Data Processing
- In SBE Data Processing’s Configure menu, select the CTD. The configuration dialog box appears.
- Double click on the conductivity sensor. The Conductivity calibration coefficients dialog box appears. Enter the serial number, making sure to include the ‘w’ at the end of the serial number; enter the remaining calibration coefficients, and click OK.
- The Wide Range Conductivity dialog box appears if the sensor serial number ended with a ‘w’. Enter the cell constant (C = 2000) and series resistance (R; see configuration file and/or documentation provided with instrument), and click OK.
- Make other changes to the configuration as needed, and click Save or Save As.
- Click Configure Inputs in Seasave V7. The Configure Inputs dialog box appears.
- Select Create to make a new configuration file. Select the instrument in the Please Select an Instrument dialog box, and click OK. The configuration dialog box appears.
- Follow Steps 2 through 4 from the SBE Data Processing section above.
Application Note Revision History
|June 2013||Initial release.|