September 2012    Download the Entire Issue (PDF)

Vol. 27, No. 9   What's this?

Improving Rebreather Safety: Part II

oxygen analyzers: the rebreather’s weakest link

from the September, 2012 issue of Undercurrent  

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Last month, I highlighted a number of issues discussed in May at the Rebreather Forum 3 (RF3), sponsored by PADI, Divers Alert Network and the American Academy of Underwater Scientists.

When talking about diver error, it's common knowledge that most sport divers avoid rebreathers because they seem too technical and complex, and it is indeed the technical side that causes problems. Bruce Partridge, CEO of Shearwater Electronics, summed it up this way, "Divers must interpret the readouts from three roaming oxygen sensors, which are known to be unreliable. They dive with no carbon dioxide gauge, and they don't have good data on the risks, or what is most likely to go wrong."

Most experts agree that current oxygen-sensing systems are the weakest, yet most critical, links on a rebreather. If the PO2 (partial pressure of oxygen) in the loop is too low, the diver will suffer hypoxia, go unconscious and drown; too high, and the diver risks hyperoxia, convulsions and drowning.

But what most divers might not appreciate are the limitations of oxygen-sensing systems, which were made clear at RF3 by Arne Sieber, CEO of Seabear Diving Technology. Sieber explained that the galvanic oxygen sensors, made especially for the biomedical industry, were never designed to be used in diving. In fact, the sensors are meant to be calibrated under the same conditions that they will be used, but that's not how it's done in diving.

"Divers do all the wrong things," Sieber explained. "We calibrate the sensors at 0.2 bar (air) and 1.0 bar (oxygen) at ambient pressure and temperature, then use the sensors at up to 1.6 bar at much hotter temperatures." This leads to increased sensor errors, as well as a decreased lifespan.

Sensors can err because of the gradual consumption of their reactive material and aging, and thus fall out of calibration. Worse is that "transient failures" from a loose electrical connection or condensation cause the sensor to generate erroneous data, and then go back to working correctly when the condition abates. Nigel Jones, principal at RMB Consulting, believes that these "transient failures" are likely behind many unexplained rebreather diver fatalities....