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.