Nitrox and Dive Shops at Altitude

[yle]

I was recently asking a shop about how EANx21 is done (if someone wanted to put straight air in a tank and then later go back to nitrox), and they said that getting EANx21 from just the air side of a blending system should be simple just by asking for "oxygen compatible air,"but they also said that a problem with asking for EANx21 is that "At altitude you are less than 21% oxygen and to get EAN21 you would have to mix the gas." Is that right? I thought oxygen makes up 21% of air everywhere, pressures just change. Maybe I misunderstood the answer.

You understood their answer. And it seems they understood your question. But their answer was wrong.

You are correct. Air is approximately 21% oxygen regardless of how far up you go. No doubt the shop people were confused by having heard about some of the same issues that have been presented in this thread... but they apparently didn't bother to take the time to truly understand the situation.

If you go to a scuba shop at 5000' and they fill your tank with "oxygen compatible air" you will have 21% oxygen in your tank. If your analyzer does not read 21%, it needs to be recalibrated.

 

[yle]

You are still at 21% O2, just a different partial pressure.

EX: Tahoe is about 6200ft and my computer reads 0.18ppO2 if I have Air(21%O2).

Partial Pressure = Fraction of Gas x ATA

Do you mean your computer is calculating the O2 pp... i.e. it uses the percentage O2 that you set, multiplied by the pressure the computer actually measures? And it does this while you are diving, right?

If so, this is a separate issue than what the OP is asking for. He's asking about filling tanks in a shop, and how the ambient air affects the O2 percentage; yours is how the ambient pressure affects the pp of O2 that you're breathing while diving.

 

[Asheron]

Do you mean your computer is calculating the O2 pp... i.e. it uses the percentage O2 that you set, multiplied by the pressure the computer actually measures? And it does this while you are diving, right?

If so, this is a separate issue than what the OP is asking for. He's asking about filling tanks in a shop, and how the ambient air affects the O2 percentage; yours is how the ambient pressure affects the pp of O2 that you're breathing while diving.

I set the fraction f the gas in my computer and it then reports he partial pressure depending on the atmospheric pressure on the computer. It does this at all times, so as I drive from the Sacramento valley to Tahoe I can watch the ATM drop(it shows the barometric pressure live) and the ppO2 drop.

I agree that the %O2 will not change, but since a Nitrox analyzer sensor is actually a ppO2 sensor I feel it is important that he know what his analyzer is showing and if just adjusting it to 21% is the appropriate method. After researching a little bit it seems the majority of analyzers can just be calibrated to FO2 because it just shifts the display point of the Linear voltage measurement that the sensor produces.

It is mentioned in the article I posted earlier that this is not the correct method for "All" so that leads me to believe that maybe some very old or homebrew analyzers display ppO2.