A question from a sea level Florida diver.......If I took my handheld Nitrox analyzer ( analox o2eii ) to the top of Tahoe (or Mt. Everest) and took a reading in the normal outside air it would still read 21% ?
How do you guys analyze nitrox tanks with hand helds ?
I've never done any altitude diving, so clueless on it.
I've dived in the Black Hills in lakes where the surface is at 4600' or so.
The air pressure at that altitude is 0.6 ATM.
Altitude above Sea Level and Air Pressure
I have not used an analyzer there but -- as pointed out upthread -- they in reality read the partial pressure of O2 and convert it to a percentage.
With an oxygen analyzer calibrated to 21% at sea level (or 100% in pure oxygen at sea level) you would expect to read around 18% at 4600 feet. I haven't tried it. But in reality the practice is to
calibrate the analyzer using either air or pure oxygen, or ideally both, which compensates for local pressure. So if you calibrate the analyzer at 4600 feet, for 21% in air, and 100% in pure oxygen, then it will read the % oxygen in your cylinder accurately just as it would at sea level.
The highest peak I've climbed is 14,171 feet (
Kit Carson Peak - Wikipedia) which corresponds to an atmospheric pressure of roughly 0.6 atm, with an oxygen partial pressure equivalent to around 12.5% at sea level. I was fine despite the athletic climb but had taken over a week to acclimate to the altitude.
Anyway the answer to your question is that, if properly calibrated, an O2 analyzer will read correctly at any altitude where diving is feasible. Proper calibration is the key to this. That said, your DC will adjust for altitude, or you will have to adjust the tables yourself if that's what you're using, and the result will be that your NDLs will be shorter than they would be at sea level.