The second variable in our "What is Affecting Your Dissolved Oxygen Measurements" series that affects DO concentration is the salinity of the water sample. While the % saturation reading is not a function of the salinity (or dissolved solids content) of the water, the mg/L concentration changes significantly with salinity. As the salinity of water increases, its ability to dissolve oxygen decreases. For example, oxygen saturated freshwater with 0 ppt salinity at 25ºC contains 8.26 mg/L of oxygen while oxygen saturated sea water (~36 ppt) at the same pressure and temperature contains only 6.72 mg/L of dissolved oxygen.
Thus, salinity (along with temperature) must be factored into the instrument’s calculation of mg/L. This calculation is based on the % saturation reading, temperature reading, and the measured or entered salinity value using formulae found in Standard Methods for the Examination of Water and Wastewater. The calculation for converting % to mg/L and an example is provided in "What is Affecting Your Dissolved Oxygen Measurements? Part 1 of 4" blog.
Correcting for Salinity
The salinity value used by the instrument in the calculation of mg/L is obtained one of two ways, depending on the instrument being used. For YSI dissolved oxygen instruments that also measure conductivity (Pro2030), the salinity value measured by the conductivity sensor is used for the mg/L calculation. Therefore, it is important to ensure the conductivity sensor is calibrated and reading accurately in order to obtain accurate DO mg/L readings.
For YSI dissolved oxygen instruments that do not have a conductivity sensor, the salinity value of the sample must be manually entered by the end user. See the salinity guide below for a list of typical salinity values for various types of water.
Salinity Guide - Average salinity by water type
|Water Type ||Average Salinity |
|Fresh Water ||<0.5 ppt* |
|Brackish Water ||0.5 to 30 ppt |
|Sea Water ||33 to 37 ppt |
|Saline Water ||30 to 50 ppt |
|Brine ||>50 ppt |
*Salinity is a unitless measurement determined from conductivity and temperature readings according to the Practical Salinity Scale which can be found in Standard Methods for the Examination of Water and Wastewater. Historically, salinity values determined via the Practical Salinity Scale were given the designation “ppt” because these values were very close to those determined by the previously used method where the mass of dissolved salts in a given mass of water (parts per thousand) were reported. Today, ppt is commonly replaced by psu (Practical Salinity Units) as the preferred unit to describe salinity calculated by the Practical Salinity Scale; however, these values are equivalent since they are determined by the same method.
When sampling water of varying salinity, for example in brackish waters such as estuaries or coastal wetlands, it is recommended that you use a dissolved oxygen instrument that also measures conductivity for highest data accuracy. A dissolved oxygen instrument that also has a conductivity sensor will use the real-time salinity readings from the conductivity sensor for every mg/L calculation (such as the Pro2030). This will make sampling easier since it will not be necessary to manually change the correction factor at each sampling new site.
It's important to keep in mind, when calibrating a DO meter, if you have to manually input the salinity value to input the value of the water you will be measuring. If you have a conductivity sensor in conjunction with the DO sensor, make sure the conductivity is calibrated properly in order to compensate for the correct salinity value.If you missed it, "What is Affecting Your Dissolved Oxygen Measurements?
Additional Blog Posts of Interest:
What is Affecting Your Dissolved Oxygen Measurements? Part 4 of 4
What is Affecting Your Dissolved Oxygen Measurements? Part 3 of 4
What is Affecting Your Dissolved Oxygen Measurements? Part 1 of 4