Salinity and miniDOT

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The miniDOT and other optical dissolved oxygen sensors measure oxygen within a special plastic material that has the property of changing its fluorescence in response to the amount of oxygen present. Oxygen arrives within this material by diffusion from the surrounding environment, which is usually water. 

The miniDOT determines oxygen concentration in water dissolved oxygen  by optically measuring the fluorescence of the plastic material. This measurement gives the oxygen concentration within the plastic material. Dissolved oxygen is computed based on this measurement, on the assumption that the oxygen concentration in the plastic material is in equilibrium with dissolved oxygen, and on the known equilibrium relation between dissolved oxygen and oxygen (for fresh water) in the plastic.

The presence of salt ions in water changes the equilibrium relation between dissolved oxygen and oxygen in the plastic. miniDOT has no salinity sensor and so it is unable to sense if the surrounding environment is fresh or salt water. Software computes dissolved oxygen based on the assumption that the surrounding environment is fresh water. This is the dissolved oxygen recorded on files within the miniDOT.

Software Operation

The miniDOT is supplied with software that can implement a correction in the case where the environment is salt water. This correction is accomplished in miniDOT Concatenate.

The operator is allowed to enter one salinity from which software corrects all the miniDOT dissolved oxygen files being processed. The correction is based on the ion group found in sea water and is useful from 0 to 42 ppt.

External Resources

If salinity varies significantly during the recording of dissolved oxygen then miniDOTControl makes an error when applying just one salinity value to all measurements. The significance of variation can be judged from the calculator found here:

http://water.usgs.gov/software/DOTABLES/

The miniDOT dissolved oxygen measurements can be compensated individually if salinity measurements are available from some other instrument.

The miniDOT Control software can compensate miniDOT dissolved oxygen over the salinity range of 0 to 42 ppt. Compensation is available only for sea water. Dissolved oxygen measured in water containing other ions or sea water ions but in different composition ratios will require different compensation.

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