No, Sea-Bird does not perform low or narrow range calibrations on our CTDs. However, CTDs are used successfully in many freshwater environments.
Conductivity calibrations performed at Sea-Bird are valid in the range of 0 – 9 S/m (or 0 – 7 S/m, as specified for some instruments), and the calibration coefficients can be applied in freshwater for accurate calculations of conductivity.
Sea-Bird recognizes that calibration using natural seawater and IAPSO standards for ocean conductivity ranges may result in a small offset and possible slope errors near zero conductivity. For example, the estimated magnitude of offset error is < 0.002 S/m, and of slope error is < 0.002 S/m per 1 S/m change. This is an example of a conservative error estimate for initial accuracy of conductivity sensors used in freshwater, which can be challenging to calculate due to lack of a freshwater standard. However, sensor precision will be near the resolution (0.00004 S/m). Sea-Bird CTDs provide high precision and sensor stability, allowing an accurate measure of conductivity gradients (dC/dz) or water sample differences, regardless of ‘true’ conductivity values. For these reasons, Sea-Bird CTDs that have been calibrated in seawater can be used successfully in many freshwater systems.
Lastly, note that the Practical Salinity Scale (PSS-78) is defined as valid for salinity ranges from 2 – 42 PSU. For additional references on freshwater algorithms used in the limnology field, see the following literature:
- Millero, Frank J. 2000. Equation of State for Freshwater. Aquatic Geochemistry, 6: 1 – 17.
- Pawlowicz, R. 2008. Calculating the Conductivity of Natural Waters. L&O: Methods 6, 489 – 501.