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A close look at optode performance in the test basin at Ifremer

Hypox 2nd annual meeting – Horw, Switzerland Mai 2011. A close look at optode performance in the test basin at Ifremer. N. Lo Bue*; L. Delauney° A.Khripounoff °; A.Vangreisheim° L. N. Gayet°; L. Le Douaron°. *INGV, Rome- nadia.lobue@ingv.it

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A close look at optode performance in the test basin at Ifremer

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  1. Hypox 2nd annual meeting – Horw, Switzerland Mai 2011 A close look at optode performance in the test basin at Ifremer N. Lo Bue*; L. Delauney° A.Khripounoff °; A.Vangreisheim° L. N. Gayet°; L. Le Douaron° *INGV, Rome- nadia.lobue@ingv.it °IFREMER, Brest – laurent.delauney@ifremer.fr

  2. One step behind...: MEDITERRANEAN SEA Mid-Atlantic Hydrothermal sources Lion Gulf Var Canyon 15 m Aanderaa RCM8 or 11 Current meter + OPTODE 3830 Sediment traps Bottom 1700 m Recent analysis of datasets collected at bottom depths showed unexpected O2 variations. These seem to be inversely correlated to current velocity and in some cases well correlated with abrupt current direction changes (Lo Bue et al. “Anomalies of Oxygen measurements performed with Aanderaa Optodes”, submitted to Journal of Operational Oceanography) 1866 m 2113 m 2260 m 1904 m

  3. Mid-Atlantic Hydrothermal sources Mean Oxygen value ~ 307.5 µmol/l. Abrupt oxygen drops (minor value~ 213 µmol/l) Correlation between all parameters acquired by RCM11 shows that Oxygen drops occurred in correspondence of: • T decrease • Speed decrease Deviation from mean values is too big to reflect natural event T increases are associated to fluid emission episodes highlighting the normal behavior of hydrothermal sources A. Vangriesheim, 2007. Mesure d'oxygène par optode installée sur un courantomètre Aanderaa Rcm11. Cas des données des campagnes Exomar 2005 et Envar 4 et 5 (2006, 2007). Rapport.Interne. DEEP/LEP 07-14

  4. Mid-Atlantic Hydrothermal sources Oxygen drops seems mostly oriented towards direction >250° Scatter O2/Speed allows to define the speed limit (~ 5 cm/s) where oxygen drops occurred Oxygen drops are well correlated with low speed values. Correlation with low temperature values should be a consequence of vent inactivity (NO fluid emissions NO T peaks) lower velocity = lower [O2]

  5. Mediterranean data lower velocity = lower [O2] No evident correspondence between O2 and direction No correspondence with temperature Correspondence with current regime

  6. Why testing the Optode and what did we expect to find? The aim of the test was to check the sensor performance in “controlled” current speed conditions, in order to know the dependence of the Aanderaa Optode from the water motion in front of the sensing foil Creating low hydrodynamic conditions in an artificial tank, the O2 drops should have appeared, as already observed in natural environment . This should allow to better explain strange data recorded in several dataset.

  7. Test site characteristics Length : 50 m Width : 4 m Depth : 3 m Towing carriage of sensors: adjustable speed from 0 to 4,5 m/s (relative uncertainty: 10-³) Mechanical arm to fix and move sensors Control cabin 100% theoretical [O2]: [O2]= 240-250 µM at 35 PSU and 16-17°C Chlorination effect on Winkler analysis =+3.5 µM

  8. Tested sensors 2 Optodes 3830 were tested through independent sensor cable both synchronized through a PC acquisition system (news sensors) 1 Optode 3830 was tested through a current meter RCM8 (the same that   acquired anomalous data in natural environment) in order to check the whole acquisition system MANUAL SYNCHRONISATION between the optode mounted on the RCM8 and the stand alone Optodes

  9. Data acquired Min value recorded 228.4 µM Motion at 1 cm/s The sensors immersed in the channel were left to stabilize  for more than two hours in completely calm water, then the truck was started up to speed of 1 cm/s. Sensor stabilization time (~ 2 hours) Sampling rate: 1 sample/s Motion at 1 cm/s During the stabilization time the optode mounted on the RCM8 appeared rather instable (values floating between 243 and 228.4 µM). Increased O2 values recorded during truck motion: hydrodynamic on sensing foil effect or O2 mixing???

  10. Data acquired Motion at 3 cm/s During stabilization time, all sensors showed a different variability. In calm water condition the sensor 350 reached a minimum value of about 227.9 µM (drop of about 34 µM) . Motion at 3 cm/s Sensor stabilization time (17 hours) Sampling rate: 1 sample/s Inconsistency in the sensor records during the stabilization time Stand alone Optodes The ∆O2 recorded by the three sensors at the start up of the truck was between 12 and 16 µM Optode on RCM8

  11. Data acquired Motion at 5 cm/s Motion at 5 cm/s Motion at 5 cm/s stabilization time (2 hours) stabilization time Sampling rate: 1 sample/s Sensor out of the water An inconsistency in the sensor record during the stabilization time: a drop of about 10 µM was recorded by Optode 500. ∆O2 recorded during truck’s start up are all positive shifts but different absolute values Stand alone Optodes ∆O2 3.5 µM ∆O2 7.5 µM ∆O2 10.5 µM ∆O2 8 µM ∆O2 6.2 µM ∆O2 9 µM Optode on RCM8 ∆O2 10 µM ∆O2 23 µM ∆O2 8.3 µM

  12. Data acquired Motion at 10 cm/s Sampling rate: 1 sample/s Motion at 10 cm/s Sensor stabilization time (16 hours) During stabilization time the O2 values recorded by the Optode on RCM8 gradually decrease of about 27 µM Stand alone Optodes During truck’s start up the ∆O2 recorded was between 13 and 21 µM Optode on RCM8 Min value recorded 201.5 µM

  13. Data acquired 4 days of measurement in stationary water Consistency of signals Sampling rate: 1 sample/s Stand alone Optodes Sampling rate: 6 samples/ hour Optode on RCM8 ∆O2 of about 100 µM

  14. Data acquired Motion at 1 m/s Sampling rate : 1 sample/s ∆O2 < 10 µM for both sensors recorded during truck’s start up Autonomous Optodes Optodes on RCM8 Sampling rate : 10 sample/min ∆O2 of about 25µM

  15. Statistical summary

  16. Remarks • In stationary water conditions sensors showed different trends. Up to now no explanations about that!! • The O2 increases observed during the startup of the trolley are not directly proportional to the increases of speed. The optode mounted on RCM8 seems more affected by hydrodynamism (low hydrodynamism ∆O2 =6 uM, high hydrodynamism ∆O2 =26 uM). • Could the anode effect be responsible of recorded drop? If so, decrease [O2] should have affected all (RCM8) measurements collected during the stabilization time in calm water. • Also, can oscillation of the order of ±30-40 µM be really related to anode • corrosion?

  17. Conclusions In consideration of the performed test, optode sensors seem to be affected by current speed variations (26.7uM). The frequent drops (100uM) observed during the test in stationary condition confirmed that the anomalous data recorded in the past in natural environment can be referred to the sensor instability in stationary waters, explaining the occurrence of unexpected O2drops. Could a pump improve the efficiency of the sensor ensuring a constant flow in low hydrodynamic environments ? Thank you for your attention!!!

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