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Dispelling the Myth of the Simple Oxygen Flush Test

A conventional wisdom within the rebreather community is that oxygen sensors can be evaluated for current limitation with what we'll call the "Simple Oxygen Flush Test". The process of the SOFT is usually described as a pause during initial descent at { 20 fsw | 6 msw }, performing an pure oxygen flush to raise the loop PO2 to 1.6 ATA, and then observing the sensors all do in fact indicate correctly. The SOFT has become so widely discussed (although, we suspect, rarely performed) that it is sometimes incorporated into the instruction of new rebreather divers, reinforcing this flawed idea. We have talked to customers who erroneously believe sensors that can be calibrated at 1.0 ATA and validated at 1.6 ATA with the SOFT may continue to be used past their recommended replacement dates.

The simple oxygen flush test is NOT RELIABLE and can produce misleading results. Data from a variety of sources presented at the Rebreather Forum 3 conference in 2012, OZtek conference in 2013, and again at the TekDiveUSA conference in 2014, all demonstrate the dangers of relying upon the SOFT to evaluate oxygen sensors for linearity and current limit as the dive begins. The explanation for the underlying fallacy of the SOFT is complex and well beyond the scope of this TekTip, but is based in the fact that aging oxygen sensors have "recovery" and "fatigue" characteristics. These behaviors will likely allow a failing sensor that has been "resting" in a low PO2 (relative to the typical diving setpoint) to calibrate and later pass the brief response test of an oxygen flush during early portions of the dive (perhaps the first 20 to 30 minutes or so.) However over longer dives, as the sensor is "exercised" at a high setpoint, the PO2 indicated by the rebreather can still become dangerously inaccurate.

The better practice to assure properly functioning sensors is to meticulously follow the replacement and calibration procedures recommended by the manufacturer of your rebreather. Never use oxygen sensors that have been in service longer than 12 months or have a total age more than 18 months. By following these best practices, the SOFT becomes an unnecessary risk.

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