Evaluation of the Aanderaa Oxygen Optode in continuous use in the NOC Portsmouth Bilbao FerryBox system 2005, 2006, with an assessment of the likely errors in the estimation of oxygen concentration anomalies

An Aanderaa Oxygen Optode has been used as part of the “FerryBox” system on the P&O Ferries vessel the MV Pride of Bilbao since the beginning of 2005. This report covers data collected through 2005 and 2006. The accuracy, precision and stability of the Optode have been compared with (1) measurements of oxygen in water samples collected and processed on board the ship by Winkler titration and (2) estimates of the oxygen anomaly. The comparison of the Winkler data with the output from the Optode suggests that the accuracy of the Optode in both years was close to 98% of the true value when compared to the Winkler measurements. The data suggest that the precision based on the comparison of the two data sets was equivalent to a standard deviation of 5 μM or 2% of the average value in 2005 and 2.9 or 1% of the average value in 2006. The resolution of the recorded a data is better than 1 μM. Two sources of error in measurement of temperature which are significant for the calculation of the oxygen anomaly have been identified. Temperatures measured using sensors in board the ship are higher than “true” temperatures. The error changes through the year as the difference in temperature between inside the ship and the external water temperature changes. This error may be as large as 0.5 °C. The anomaly has to be calculated using temperature measurement in the water flowing past the Optode. This is because of delays and mixing in the water being pumped into the ship have smoothing effect on the data, and can enhance the difference between the internal and external temperature to being as high as 1.0 °C. Consequently to avoid this large error due to the timing of the measurements the oxygen anomaly should be calculated from the temperature measured in the pumped water stream adjusted by an offset to the “true” outside temperature.