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Issues Archive

March/April 2008 Vol. 12
No. 2
 Figure 1. The SBE 43 DO  
  sensor (without  
  protective plenum  
  housing)  
 Figure 2. SBE 43 per cent  
  oxygen saturation values  
  at a monitoring buoy in  
  Cockburn Sound (Western  
  Australia), a  
  biologically productive  
  coastal lagoon, June 26   
  December 22, 2006. The  
  SBE 43 sensor was  
  recovered on November 5,  
  2006, and replaced with a  
  new calibrated sensor on  
  November 9, 2006. The  
  mean measured difference  
  at the time of the sensor  
  swap indicates less than  
  a 5% change in the  
  initial sensor  
  calibration over 4  
  months. The decline  
  observed in percent  
  oxygen saturation in  
  December, similar to that  
  observed in late  
  September, is a natural  
  trend of oxygen draw-down  
  and rapid storm  
  replacement observed at  
  multiple mooring sites  
  within Cockburn Sound  
  (data courtesy of Water  
  Corporation)  
 Figure 3. External plenum  
  SBE 43 DO sensor after a  
  four-month deployment at  
  Shilshole Marina, March   
  July 2007.  
 Figure 3. SBE 43 DO  
  sensor after a four-month  
  deployment at Shilshole  
  Marina, March July 2007  
  with the plenum removed  
    
 Figure 4. Original  
  calibration of SBE 43  
  sensor SN 1114 used in  
  Cockburn Sound (green  
  dots); post-recovery  
  calibration prior to  
  sensor cleaning (blue  
  triangles);  
  re-calibration after  
  sensor cleaning (red  
  squares). Notice the loss  
  of sensitivity in the  
  post-recovery calibration  
  (blue triangles) is  
  strictly linear,  
  corrected by a simple  
  multiplier to the slope  
  coefficient (SOC) as  
  demonstrated in Table 1 
 Figure 5. SBE 43  
  dissolved oxygen time  
  series plotted in dark  
  blue, Shilshole Bay  
  Marina, Seattle, WA  
  (USA), March 23 July  
  31, 2007. Data with a  
  slope adjustment made  
  after May 13 are  
  co-plotted in cyan.  
  Average Winkler values  
  are shown as open pink  
  circles, and the percent  
  difference between the  
  SBE 43 and Winkler  
  averages are co-plotted  
  along the right y-axis as  
  black solid squares  
  (before the May 13  
  validation and slope  
  adjustment), and as red  
  open squares(from May 13  
  forward following the  
  adjustment). The dashed  
  curved lines are drawn to  
  illustrate how correcting  
  the in situ data can  
  prolong deployment while  
  maintaining accuracy in  
  real-time (or  
  post-processed) data.  
  Mean standard deviation  
  of the Winkler replicates  
  is 0.03 ml/L.  
 Table 1. Reference  
  Winkler water samples and  
  sensor readings at three  
  dissolved oxygen  
  concentrations during the  
  post-recovery calibration  
  of SBE 43 sensor SN 1114  
  (Figure 4). The ratio  
  between the Winkler  
  values and corresponding  
  SBE 43 output can be used  
  to calculate the SOC  
  correction factor. Note  
  the SOC correction factor  
  remains constant at each  
  validation point over the  
  range of oxygen values  
  shown, illustrating that  
  any single validation  
  point alone could be used  
  to correct the slope  
Long-term oxygen measurements  

By Carol Janzen, Nordeen Larson and David Murphy,  
  Sea-Bird Electronics, Inc., USA
  

Accurate long-term O2 measurements using the SBE 43 in harsh bio-fouling environments 

Numerous implementations of oxygen sensor technologies including galvanic, Clark electrode, optical, and others are being used for long-term measurements. Obtaining accurate long-term data from any of them requires a sensor with high initial (calibration) accuracy, inherent sensor stability, and an effective defence against fouling. The user also needs a practicable means of determining how well the initial accuracy was preserved at convenient intervals during deployment or post-recovery. 

Sea-Bird's SBE 43 dissolved oxygen (DO) sensor (Figure 1) was engineered to be integrated with a pumped CTD and provide rapid-response dynamically accurate DO measurements during profiling. The Clark electrode technology was chosen over optical methods for its high accuracy potential and fast response, both critical to profiling applications. However, the SBE 43, being a complete redesign of the Clark electrode sensor, incorporates features that eliminate previous causes of instability, and so is also able to deliver stable moored measurements for months in high fouling environments.

The SBE 43 accuracy and stability is derived from its re-engineering, careful calibration and effective bio-fouling controls. Modern electronics eliminate electronic acquisition error. Every SBE 43 is calibrated at three different oxygen concentrations at each of six temperature points (18 points in total) using Winkler titration, and temperature and salinity standards. The resulting sensor accuracy is unsurpassed by other technologies. Electrochemical drift, a limitation in previous Clark designs, exists somewhere below the calibration uncertainty of 1 µM/kg and has not been observed in several years of factory calibration data. In fact, upper estimates based on ocean deployments are less than 0.5 per cent over 1000 hours of sensor flushing time (Janzen and Larson, 2008). Sea-Bird's unique implementation of flow controls significantly reduces biofouling impact on the sensor. This allows for much longer deployments (months versus weeks) than typically achieved by continuously exposed DO sensors.

THE SBE 43 DEFENCE AGAINST BIO-FOULING 

Moored deployment data from many customers demonstrate that the SBE 43 DO sensor provides high-resolution and low-drift measurements resulting in retained accuracy within a few per cent for periods of three to five months with no servicing. This is observed in a variety of aquatic environments, including productive coastal waters with significant biological fouling pressure (Figure 2). 

The SBE 43 DO sensor is flushed with a flow controlled pump to deliver a fresh sample of ambient water to the sensor. For moored applications, the unique plumbing arrangement serves as a first line of defence against fouling, protecting the sensor from continuous exposure to external biological contamination (Figure 3). Pumping prior to each measurement flushes stagnant water out of the plumbing. Between measurements, anti-foulant placed at each end of the conductivity cell's plumbed path diffuses into the stagnant water to neutralise any biota that enters the system during the previous flush. The flushing itself agitates and removes the neutralised biota from the sensor.

SBE 43 CALIBRATION DRIFT AND ITS CORRECTION

The SBE 43 sensor output is linear with respect to oxygen concentration and maintains a stable output at zero oxygen (Equation 1): 

Oxygen(ml / L) = SOC*[V-Voffset]*[Tcor*Pcor*OXSOL] (1) 

  • SOC is the linear slope scaling coefficient.
  • V is the sensor output voltage; Voffset is a fixed sensor voltage at zero oxygen.
  •  OXSOL is the oxygen solubility function and converts oxygen partial pressure (sensor measurement) to oxygen concentration (Garcia and Gordon, 1992). 

The Tcor and Pcor functions correct for the effects of temperature and pressure. These are lower order terms and remain essentially constant with fouling and sensor age. With negligible electrochemical drift and stable electronics, any loss of sensitivity can be attributed to bio-fouling of the sensor itself. When the sensor does foul, the character of the change in sensor output is a simple loss of sensitivity, as evidenced in Figure 4 and Table 1, and the ratio of measured to true concentration remains constant over the whole range of the sensor. Therefore, adjusting the slope in the calibration equation (SOC) is the appropriate means of maintaining accuracy if fouling occurs.

The sensor's characteristic drift pattern allows for easy data correction using a single in situ reference value to determine the slope (SOC) correction (Figure 4; Table 1). DO data can be corrected either in real-time or during post processing, depending on the availability and quality of an in situ reference value or a post-recovery calibration. This offers a powerful and scientifically defensible way to make residual corrections to data from unattended long-term deployments. 

DATA CORRECTION IN HIGH-FOULING CONDITIONS 

 A SBE 43 dissolved oxygen sensor was deployed at Shilshole Bay Marina north of Seattle, Washington, USA, for four months in 2007 during the biologically active spring and summer seasons. The integrated SBE 43-CTD (conductivity, temperature and depth) was moored at two metres water depth and sampled every 10 minutes following a 30-second flush cycle. Replicate Winkler samples were collected bi-weekly from a 1.2-litre Niskin bottle adjacent to the moored SBE 43 sensor at the time of a sample. The purpose of the test was to monitor how long the sensor could maintain sample accuracy within five per cent of Winkler references. 

The SBE 43 measured dissolved oxygen within five per cent of Winkler reference values for over 107 days (~three months) during high biological fouling conditions (Figure 5). Results from other long-term deployments (e.g. Western Australia data shown in Figure 2) corroborate this performance. This exceeds expectations of other DO sensors, which historically experience impaired coastal DO measurements within four to six weeks (ACT 2004a; ACT 2004b). 

To demonstrate how the correction is made, assume the single validation point made on May 13, (see arrow in Figure 5) is used to correct data after May 13. The average of the replicate Winkler values on that date was 9.737 ml/L, and the SBE 43 reported 9.308 ml/L. To adjust the calibration, a new SOC value is obtained by multiplying the pre-May 13 value of SOC by the ratio [(Winkler value)/(SBE 43 value)] (9.737/9.308 = 1.046) (Equation 2): 

NewSOC = previousSOC * ([Winkler]/[SBE 43]) (2) 1.3866e-04 = 1.3256e-04 *1.046 

The SBE 43 DO data calculated after May 13 using the NewSOC demonstrate how sensor accuracy is maintained near initial calibration accuracy by using a single quality reference sample to correct the calibration slope. Utilising this approach can prolong an instrument deployment between mooring service intervals, and reduce service gaps in moored data streams.

To correct data in post-processing, the simple course is to assume a linear fouling adjustment per day (or week, or month) for the entire period or between any field validation data. This can be programmed into a simple script to calculate calibrated DO data with time. 

SUMMARY

 The SBE 43 DO sensors have high initial calibration accuracy, low (non-detectable) electrochemical drift, an effective anti-fouling approach, and a predictable linear response to bio-fouling. Effective anti-fouling defences maintain a nearly drift-free signal for months, and the predictable character of drift when fouling occurs allows a reliable means of data correction. These characteristics extend the accuracy and useful data life of the sensor, extending the interval between service visits and dramatically lowering the maintenance and data costs of the SBE 43 oxygen sensor. The proven strategy is being used successfully in critical monitoring applications, for example in Cockburn Sound, Western Australia. 

Application notes documenting the SBE 43 performance, calibration and methods for optimising deployments are available at the Sea-Bird Electronics' website (http:// www.seabird.com). 

ACKNOWLEDGEMENTS 

The authors express appreciation to Wayne Farrell of Greenspan Technology, Australia, and the staff and engineers at the Perth Seawater Desalination Plant (Water Corporation, Western Australia) for sharing moored water quality data from the Cockburn Sound real-time monitoring project. 

REFERENCES 

  • ACT (Alliance for Coastal Technologies), Performance Verification Statements for the Aanderaa Instruments Inc. Dissolved Oxygen Optode (ACT VS04-01), Greenspan Technology Dissolved Oxygen Sensor (ACT VS04-02), In-Situ Inc. Dissolved Oxygen RDO Sensor (ACT VS04-03), YSI Inc. Rapid Pulse Dissolved Oxygen Sensor ACT VS04-04 (www.act-us.info/evaluation_reports.php), 2004a.
  • ACT (Alliance for Coastal Technologies), State of Technology in the Development and Application of DO Sensors, Workshop Proceedings Savannah, GA, January 12-14, 2004, UMCES Technical Report Series: TS-444-04-CBL/Ref. No. [UMCES]CBL 04-089, (www.act-us.info/workshops_reports.php), 18p., 2004b.
  • C Janzen, D Murphy and N Larson. Getting more mileage out of dissolved oxygen sensors in long-term moored applications. In: Proceedings of the Oceans 2007 MTS/IEEE VANCOUVER Conference and Exhibition, Vancouver, B.C. Canada, September 29-October 4, 2007. 0-933587-35-1. (PDF available at http:// www.seabird.com), 2007.
  • C Janzen, D Murphy and N Larson. Assessing the calibration stability of oxygen sensor data on ARGO profiling floats using routine WOCE monitoring data from HOT. Session: 182 - Variability and Trends in Oceanic Oxygen: From a Tracer of Biological Production to a Bellwether of Climate Change, Poster Presentation, 2008 Ocean Sciences Meeting, Orlando, FL, USA, March 3-7, 2008.
  • HE Garcia, LI Gordon. "Oxygen solubility in seawater: better fitting equations," Limnology and Oceanography, vol. 37, no. 6, pp. 1307-1312 , 1992.

 

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