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Can I use SUNA V2 that is calibrated for freshwater, in salt water applications and vice versa?

To remove baseline water absorbance spectra, SUNAs are calibrated with nitrate-spiked water, as well as nitrate-free water. The default class-based calibration uses coefficients generated from a library of historical calibrations. For greater accuracy, SUNA sensors are individually calibrated at Sea-Bird Scientific for use in fresh or seawater. Due to difference in absorptivity characteristics of seawater and freshwater, the calibration data for each of these applications is distinct.

Calibrations for instruments to be used in freshwater are determined from aqueous nitrate standards of deionized water with 40 and 4000 µmolar nitrate concentrations and pure deionized water. The second calibration at 4000 µmolar is performed for accuracy in terrestrial applications where nitrate concentrations are large. It is important to note that SUNA units configured for use in freshwater do not give reliable nitrate readings in seawater, as the interference from bromide will not be accounted for without seawater absorption data.

Instruments calibrated for use in seawater use nitrate standards of seawater with 40 µmolar nitrate concentrations and nitrate-free seawater to determine the baseline seawater absorbance spectrum. The use of seawater enables the correction for the UV absorption of bromide and other sea salt components. Instruments calibrated for use in seawater can be used in freshwater applications, but the accuracy will be decreased in regions of high nitrate concentration because the instrument is calibrated for the smaller range of nitrate concentrations typically found in seawater.