Notice 1: Problem with SBE CTD Pressure Sensors on Argo Floats, Recommendation to Stop Float Deployments and Return CTDs to Sea-Bird for Repair

4 May 2009

Dear Argo colleagues,

We have been working to understand the details of a problem with the Druck pressure sensors used in SBE-41 and SBE-41CP CTDs on Argo floats. The problem is called "Druck micro-leaks" because, in 3 cases over the last two years where sensors were recovered, it was determined that oil was leaking out of a sealed inner sensor chamber through micro-cracks in glass-to-metal seals at the back of the sensor. The oil leak rate is very slow, just a few micro-litres per month. As oil leaks, a flexible titanium diaphragm, that transmits ocean pressure to the oil chamber, deflects into the sensor chamber to make up for the oil loss, and as a consequence, the sensor develops a progressive negative offset in measured pressure. This becomes evident in Argo data as a negative surface pressure. With enough oil loss the diaphragm deflects inward far enough to contact and short out the pressure sensing element. There is evidence that the diaphragm bottoms out and does not result in a further leak of oil or ocean water through the sensor.

In early March 2009 an expanded analysis of Argo surface pressures, done by Dana Swift of University of Washington, revealed an increase in the occurrence rate of floats exhibiting negative surface pressure offsets from units deployed in 2007 and later. The jump in occurrence rate is alarming; from low 3% of floats pre-2007 to about 12% of the floats analyzed from the 2007 deployment. Now more than 500 floats has been analyzed. The best recent statistics are that floats deployed in 2008 are showing an even higher occurrence rate, exceeding 10% and perhaps approaching 30%.

It can take up to 500 days after deployment for the negative surface pressures from sensors with micro-leaks to exceed the normal variation in healthy sensors. So reliable statistics for floats deployed in 2008 and early 2009 make some time to establish.

Regardless of exact statistics, the pressure problem and its consequences are severe. All non-deployed CTDs need to be tested and bad pressure sensors replaced.

• We have stopped shipping Argo CTDs from Sea-Bird until we have a method to identify good reliable pressure sensors.
• We strongly recommend that float manufacturers stop shipping floats to customers, and return CTDs to Sea-Bird for evaluation and repair.
• We strongly recommend that the Argo community stop deploying floats, and consult with the float manufacturers about the logistics for getting CTDs returned and repaired.

The urgency to stop deploying CTDs (floats) and get them repaired is high. However, the urgency to get CTDs returned to Sea-Bird is not as high for two reasons:

• We do not yet have a reliable method to identify the good pressure sensors from Druck to use as replacements. Verifying a reliable screening method and establishing a stock of good sensors may take a month or more.
• Among the screening methods we are testing, there is a possibility that a method could be found that float groups could apply at their location. If this were the case, it could be more time efficient and quickly identify the 70%?? of floats with good pressure sensors that would then be immediately available to deploy.

We are working on 3 solutions:

1. Create a test procedure in the laboratory that can accelerate the microleak failure and consequently separate Druck sensors into the two distinct categories observed in Argo float data: the bad sensors (micro-leakers) and the good sensors that don't drift at all for 5+ years. Despite very focused effort at Druck Ltd, in the UK and at Sea-Bird we have not been able to develop a reliable test yet. We are working on several test methods, and one test that could potentially be done by Argo scientists at their own facility, saving time and allowing good sensors to be identified and those floats available to be deployed without further delay or a return to the US.
2. We have a stock of about 100 pressure transducers from Paine that could be used for programs that must deploy floats soon. The Sea-Bird SBE-41 and 41CP CTDs were equipped with Paine pressure sensors until early 2000. The Paine sensors exhibit a positive drift with time but less than +10 dbars over the life of a float. Paine sensor that drift typically exhibit a span change as well (slope in pressure calibration) that has a magnitude of about 10% of the offset drift.
3. Finally, we have been working with a Swiss company, Kistler, to develop an alternative to the Druck sensor. Five years of development have produced a very promising sensor. That sensor is in the final stages of testing at Sea-Bird, but has passed all the design criteria in testing at the Kistler facility and the first production batch of 100 sensors suitable for Argo floats is in the pipeline. First production sensors should be available within 90 days.

A report with more details of the microleak problem, field units, lab testing, etc is being prepared. We are also developing the details of the warranty relief that Sea-Bird will offer for this situation. The Argo community can expect these two reports very soon.

While we are still in the initial phases of understanding the problem and the potential remedies, we welcome your questions and will do our best to answer them completely.

Sincerely,
Norge Larson, President
Sea-Bird Electronics, Inc
+1 425 643-9866  tel
+1 425 643-9954  fax
norge@seabird.com

• Sea-Bird Notice 2 (2 March 2010): Request for Help from the Argo Data Management Group
• Sea-Bird Notice 3: (coming soon)
• Other links:
  Argo home page on University of California at San Diego website
  International Argo Information Center home page on JCOMMOPS website

Last modified: 26-Apr-2010

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