For most dissolved oxygen testing, it is not necessary to compensate for salinity. The salinity of a sample can have an affect on the dissolved oxygen (DO) concentration in highly saline water (>1 part per thousand salinity). In this case an adjustment should be made for the DO concentration values. 

If using the Go Direct^® Optical Dissolved Oxygen Probe (GDX-ODO) in a high saline environment, you can turn on the DO Salinity channel after connecting the sensor.  Salinity is not actually measured by the probe, but it can be manually entered once this channel has been selected, by selecting Calibration on Sensor Actions of the DO Salinity reading. If no value is entered, a default salinity concentration of 35 ppt, the typical salinity of ocean water, is used to make the correction. 

If using either the Dissolved Oxygen Probe (DO-BTA) or Vernier Optical DO Probe (ODO-BTA) the DO concentration value will need to be corrected for after gathering the readings. 

To determine the adjusted value, use the formula:

DO(salt) = DO – (k*S)

Where:
– DO(salt) is the dissolved oxygen concentration (in mg/L) of air-saturated salt water.
– DO is the dissolved oxygen concentration (in mg/L) of air-saturated distilled water as determined from standard tables (found in the sensor’s user manual).
– S is the salinity value (in parts per thousand) of the sample.
– k is a constant. The value of k varies according to the sample temperature, and can be determined from the table below.

Salinity correction constant values

With the [Product DO-BTA], the calibration value entered into the software for air-saturated water (from standard tables) should be adjusted for salinity using the equation and values above as well.