Salinity and miniDOT


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:


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.