Impact of Stirring on the Luminescent Dissolved Oxygen Sensor

The impact of stirring on the response of the field YSI ProODO luminescence dissolved oxygen sensor was measured in a 300-mL biological oxygen demand (BOD) test bottle with a magnetic stirrer either on or off. With a water sample from a fish transport experiment, the dissolved oxygen dropped by 2-3 mg/L when the magnetic stirrer was turned off for 11 min. The initial rate of dissolved oxygen decline ranged from -0.39 to -1.04 mg/L per min for this water. The dissolved oxygen returned to a value projected from the oxygen uptake rate when the stirrer was turned on. A much smaller impact was observed for 18.2 M Omega high-purity water (mean: -0.11 mg/L). Acid-washing the BOD bottle/stir bar and cleaning the sensor cap reduced the decrease to -0.08 mg/L. Based on t-tests between pairs of stirring-on and stirring-off results, the lack of stirring had a significant effect on measured dissolved oxygen. The source of this effect and its potential impact on the accuracy of the optical probe under typical operating conditions remain to be determined.