Electro-galvanic fuel cell

Electro-galvanic fuel cell as used in a diving rebreather

An electro-galvanic fuel cell is an electrical device, one form of which is commonly used to measure the concentration of oxygen gas in scuba diving and medical equipment.

Operation

A chemical reaction occurs in the fuel cell when the potassium hydroxide in the cell comes into contact with oxygen. This creates an electric current between the lead anode and the gold-plated cathode through a load resistance. The current produced is proportional to the concentration (partial pressure) of oxygen present.

They are used in oxygen analysers in technical diving to display the proportion of oxygen in a nitrox or trimix breathing gas before a dive.[1] They are also used in electronic, closed-circuit rebreathers to monitor the oxygen partial pressure during the dive.[2]

The partial pressure of oxygen in diving chambers and surface supplied breathing gas mixtures can also be monitored using these cells. This can either be done by placing the cell directly in the hyperbaric environment, wired through the hull to the monitor, or indirectly, by bleeding off gas from the hyperbaric environment or diver gas supply and analysing at atmospheric pressure, then calculating the partial pressure in the hyperbaric environment. This is frequently required in saturation diving and surface oriented surface supplied mixed gas commercial diving.[3][4]

Electro-galvanic fuel cells have a limited lifetime which is reduced by exposure to high concentrations of oxygen. The reaction between oxygen and lead at the anode consumes lead, which eventually results in the cell failing to sense high concentrations of oxygen. Typically, a cell used for diving applications will function correctly for 3 years if stored in a sealed bag of air but only for four months if stored in pure oxygen.

Cell limitations

Oxygen cells behave in a similar way to electrical batteries in that they have a finite lifespan which is dependent upon use. The chemical reaction described above causes the cell to create an electrical output that has a predicted voltage which is dependent on the materials used. In theory they should give that voltage from the day they are made until they are exhausted, except that one component of the planned chemical reaction has been left out of the assembly: oxygen.

Oxygen is one of the fuels of the cell so the more oxygen there is, the more electricity is generated. The chemistry sets the voltage and the fuel, the oxygen, sets how much electric current it can give. If you put an electric circuit on the cell that draws current you can draw up to this current but ask for more and the voltage from the cell fades.

Failures in cells can be life-threatening for technical divers and in particular, rebreather divers.[5] The failure modes common to these cells are: failing with a higher than expected output due to electrolyte leaks, current limitation due to exhausted cell life and non linear output across its range. These failures are usually attributable to physical damage, contamination during manufacture or defects in manufacture.

Failing high is invariably a result of a manufacturing fault or mechanical damage. In rebreathers, failing high will result in the rebreather assuming that there is more oxygen in the loop than there actually is which results in hypoxia.

Current limited cells do not give a high enough output in high concentrations of oxygen. The rebreather assumes there is insufficient oxygen in the loop and injects to reach a setpoint the cell will never achieve resulting in hyperoxia.

Non-linear cells do not perform in an expected manner across its range of oxygen partial pressures. Calibration will not pick up this fault which results in inaccurate loop contents of a rebreather. This gives the potential for decompression illness.

Preventing accidents in rebreathers from cell failures is possible in most cases by accurately testing the cells before use. Some divers carry out in-water checks by pushing the oxygen content in the loop to a pressure that is above that of pure oxygen at sea level to indicate if the cell is capable of high outputs. This test is only a spot check and does not accurately assess the quality of prediction of failure of that cell. The only way to accurately test a cell is with a calibrated test chamber which can hold a static pressure without deviation and the ability to log the results and graph them.

Testing

A galvanic oxygen cell checker

The first certified cell checking device that was commercially available was launched in 2005 by Narked at 90 but did not achieve commercial success. A much revised model was released in 2007 and won the "Gordon Smith Award" for Innovation at the Diving Equipment Manufacturers Exhibition in Florida.[6] Narked at 90 Ltd won the Award for Innovation for the Development of Advanced Diving products at Eurotek 2010 for the Cell Checker and its continuing Development. Now used throughout the world by organisations such as Teledyne/Vandegraph National Oceanic and Atmospheric Administration, NURC (NATO Underwater Research Centre) and Diving Diseases Research Centre.

See also

References

  1. ↑ Lang, M.A. (2001). DAN Nitrox Workshop Proceedings. Durham, NC: Divers Alert Network. p. 197. Retrieved 2009-03-20.
  2. ↑ Goble, Steve (2003). "Rebreathers". South Pacific Underwater Medicine Society Journal. 33 (2): 98–102. Retrieved 2009-03-20.
  3. ↑ IMCA D 030 Rev. 1, (August 2005); Surface Supplied Mixed Gas Diving Operations http://www.imca-int.com/documents/divisions/diving/docs/IMCAD030.pdf
  4. ↑ IMCA D 022 (May 2000), The Diving Supervisor’s Manual http://www.imca-int.com/documents/divisions/diving/docs/IMCAD022.pdf
  5. ↑ Vann RD, Pollock NW, Denoble PJ (2007). NW Pollock, JM Godfrey, eds. "Rebreather Fatality Investigation". Proceedings of the American Academy of Underwater Sciences. Diving for Science 2007. Dauphin Island, Ala.: American Academy of Underwater Sciences (Twenty-sixth annual Scientific Diving Symposium). ISBN 0-9800423-1-3. Retrieved 2009-03-20.
  6. ↑ "REBREATHERS - From Twenty Thousand Leagues Under The Sea & Beyond...". Defence & Community International Magazine. Retrieved 2009-03-20.
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