Last fall I was on the RV Hugh R Sharp for a short research cruise out in the Delaware Bay. We were sharing the Sharp with chief scientist Dr. George Luther, who was doing a mooring deployment that contained a dissolved oxygen sensor (among several other sensors). As part of the calibration check to make sure the readings were correct while we were on station, Dr. Luther did several CTD casts to take some water samples at various depths. I snagged the trusty video camera and got him to explain what he was doing and why.
To verify the accuracy of modern electronic oxygen sensors, oceanographers still verify the dissolved oxygen concentration using what’s called the Winkler test for dissolved oxygen. Dr. Luther showed the process of fixing oxygen into a MnOOH solid, which is then measured by the Winkler titration. This allows scientists to compare the oxygen readings they’re getting now with historical records of oxygen levels going back to the late 1800’s (an important thing to do when you’re trying to determine long-term trends by comparing historical records against more recent observations). It also allows them to verify the readings that they’re getting from modern electronic oxygen sensors.
I’ll sneak down to Dr. Luther’s lab soon and video the second part of the process, where they add the additional chemicals to the mix and determine the actual concentration of dissolved oxygen. Thanks again to Dr. Luther for taking time to explain the process.