As discussed in recent blog-posts, calibration at sea level produces an output from the cells for 1 ATM or 1 bar PO2.
After calibrating your rebreather in pure oxygen, your cells should return to .21 when exposed to air. If they do not, you should re-do the entire calibration and check to see if the cells recover properly. If they don’t, you should replace the offending cell(s).
Many rebreathers allow the diver to read the voltage of their sensors through their computer display. If your rebreather allows this, you should keep good records of mili-volts in air and at maximum oxygen PO2. The voltage should be linear, meaning that if your sensor reads 10.0 mV in air, it should read 47.6 mV in pure oxygen. Prior to confirming your oxygen calibration, you should ensure that your sensors are indeed linear and reading correctly. At sea level, take the mV in air and multiply by 4.76 to come up with an anticipated mV in oxygen.
If you are trying to calibrate with a percentage of oxygen that is less than 100% pure, then you should take the percentage and divide by .21 and use that multiplier instead of 4.76. For example, if your rig allows you to calibrate with 80% and that is what you are using, then you would take .80/.21 = 3.81. If your sensor is 10 mV in air, then your mV in 80% should read 38.1 mV. Diving at a set point greater that 1.0 may leave you vulnerable to hyperoxia unless you can verify the cells linear output at depth or under greater pressures.
By completing an oxygen flush at 20 feet or 6 meters you can watch the cells to ensure they not current limited. When flushing with pure oxygen at this depth, you should read 1.6 or very close to that on your displays.
If you want to confirm linearity at depth prior to hitting the water, manufacturers like Narked at 90 have created Cell Checkers and Mini Cell Checkers that can give you peace of mind before a diving trip. http://narkedat90.com
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