This grouping of ions and water molecules means there are fewer free water molecules for dissolved oxygen to bind to, allowing the gas to diffuse out of solution. As a result, increased salinity leads to reduced dissolved oxygen levels.
Salinity does not change how dissolved oxygen interacts with sensors, so changes in salinity do not affect concentration measurements (mg/L). Any sensors calibrated to measure percent saturation, however, must be properly adjusted to ensure accuracy.
As an example, let’s compare a zero salinity sample to seawater.
A sensor is initially calibrated to 100% saturation in water at 0 salinity, 760 mmHg, and 25°C is 8.26 mg/L. If that same sensor was placed in 100% oxygen-saturated sea water (34 salinity, 760 mmHg, 25°C), then it would read only 82% because the maximum saturation is 6.80 mg/L. A correction factor must be determined by measuring the salinity of the sample and cross referencing with a Salinity Correction Factor Table. This table is provided to the right. An example of how to calculate dissolved oxygen concentration with the correction factor is shown later in the article.
Dissolved oxygen sensors are commonly paired with conductivity sensors for the purpose of estimating salinity. The relationship between conductivity and salinity is defined by the Practical Salinity Scale of 1978, commonly abbreviated to PSS-78. While the scale is not linear or easily programmable into a monitoring system, pHionics has written an article explaining how salinity can be linearized for water quality monitoring systems such as Onset’s HOBO RX2100 or RX3000. A temperature-compensated conductivity sensor is required to linearize the scale. If you are in the market for one, we highly recommend the pHionics STs Series Conductivity for accurate measurements and long-term durability.