Nitrox and Dive Shops at Altitude

[pauldw]

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.

 

[Asheron]

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

 

[Johnoly]

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

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.

 

[Asheron]

I believe you would still want to calibrated it to the expected Partial Pressure at the altitude you analyzing the gas at.

I'm actually drawing a blank because I pick my tanks up from the Sacramento valley which is really close to sea level.

 

[Foothills]

Depends on the gas analyzer, but most modern analyzers will analyze the fraction of the gas stream and therefore don't need altitude adjustments. Some of the older gas analyzers which needed a very specific flow rate of gas would read low (I live at altitude & often dive at ~4,500'+ elevation and 3/4 of my dives are altitude, and my Open Water instructors did a good job with the supplementary altitude section of the course, and had enough references to send us searching out more info).

This analzer trait also caused some work-place grief a few decades back as one analyzer assigned to the crew I was on was was reading PO2, and was showing our work-space as hypoxic. After a ton of confusion, that episode taught me to always read the instructions that come with your analyzer (or any piece of safety critical/life support equipment).

 

[2airishuman]

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.

 

[loosenit2]

Nitrox and Diving at Altitude – Utah Scuba Divers

 

[Asheron]

Found analox paper on it. Just calibrate at the altitude your at and go for it.

https://www.analoxsensortechnology...._Use_of_instrument_at_different_altitudes.pdf

 

[Johnoly]

Found analox paper on it.....

Thanks,,that paper is very clear what is happening when taking an OE2ii reading whether at sea level or 14,000 feet.

 

[yle]

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

Oh goodness... the ambient pressure at 4600' is NOT 0.6 ATM

The graph you linked to had altitude in METERS. The ambient pressure at 4600 METERS (i.e. about 15,000 feet) is 0.6 ATM.

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,

Yes... that's more like it. Altitude of 14,171 FEET is ambient pressure of about 0.6 atm.