Filling tanks at altitude

[Bob from California]

I have a compressor and trying to find the best way to proceed

I live in the mountains at 5,777′ what should I consider when filling tanks for personal use

Will I need to adjust my dive computer while diving at sea level to a lower p02?

Comparison:

PPO2 at 5,777 feet: ≈ 2.49 psia

PPO₂ at Sea Level: ≈ 3.09 psia (rounded)

Difference:

The partial pressure of oxygen at 5,777 feet is 3.087 psia-2.4948 psia ≈ 0.5922 psia lower than at sea level.

 

[gmerick]

It's still air. 20.9% o2. An absolute pressure gauge will read the same at 5777 as it does at 0'.
Ambient pressure of O2 @ 5777 = 2.49 psia, Partial pressure of O2 @ 5777 =20.9%.
Once the air is in the tank, PPO2 @ 5777' = PPO2 @ 0' = 20.9%. No adjustment needed. If your computer does altitude compensation, no change needed in that either.

 

[Bob from California]

Thanks, I wasn't sure if I was just overthinking it.

It's still air. 20.9% o2. An absolute pressure gauge will read the same at 5777 as it does at 0'.
Ambient pressure of O2 @ 5777 = 2.49 psia, Partial pressure of O2 @ 5777 =20.9%.
Once the air is in the tank, PPO2 @ 5777' = PPO2 @ 0' = 20.9%. No adjustment needed. If your computer does altitude compensation, no change needed in that either.

 

[lostsheep]

You need to RTFM.

No one online knows your setup but even if they did you shouldn’t trust them.

 

[boulderjohn]

If you are going to dive at that altitude and are going to fill your tanks with nitrox for that dive, you should be aware that the MOD will be different. If you don't make any adjustments, though, there is no danger, because, contrary to what many people believe, the MOD will be deeper at altitude. That means using the sea level norm is safer at altitude than it is at sea level.

Your computer will adjust for that, too, assuming it adjusts for altitude. Your PPO2 warnings will be deeper.

 

[Wallowa]

If you are going to dive at that altitude and are going to fill your tanks with nitrox for that dive, you should be aware that the MOD will be different. If you don't make any adjustments, though, there is no danger, because, contrary to what many people believe, the MOD will be deeper at altitude. That means using the sea level norm is safer at altitude than it is at sea level.

Your computer will adjust for that, too, assuming it adjusts for altitude. Your PPO2 warnings will be deeper.

OK...

OP was asking about his compressor use at altitude and Pp oxygen....and RTFM is cute, but stupid.....one item Bob is that your compressor will take a little longer to pump up a bottle due reduced ambient gas density...at 3,989 ABSL it takes perhaps 5 mins more for me to reach 3K psi starting at 500 psi with a 4+cfm compressor....no bigee and as stated it will be 21%/78%...could be incorrect but I believe Dalton's Law is applicable...

 

[Bob from California]

Really appreciate the insight. Currently Don't have nitrox filling capabilities. So mainly was thinking about how bad I could screw things up if I'm filling normal air 21% at altitude and bringing it down to sea level to dive If I would need to make adjustments.

If you are going to dive at that altitude and are going to fill your tanks with nitrox for that dive, you should be aware that the MOD will be different. If you don't make any adjustments, though, there is no danger, because, contrary to what many people believe, the MOD will be deeper at altitude. That means using the sea level norm is safer at altitude than it is at sea level.

Your computer will adjust for that, too, assuming it adjusts for altitude. Your PPO2 warnings will be deeper.

 

[Bob from California]

OK...

OP was asking about his compressor use at altitude and Pp oxygen....and RTFM is cute, but stupid.....one item Bob is that your compressor will take a little longer to pump up a bottle due reduced ambient gas density...at 3,989 ABSL it takes perhaps 5 mins more for me to reach 3K psi starting at 500 psi with a 4+cfm compressor....no bigee and as stated it will be 21%/78%...could be incorrect but I believe Dalton's Law is applicable...

Thanks for this. I have noticed it taking a little longer to fill.

So then if the air is still 21% but less dense would then I need more of it at sea level.

Asking ai it came up with this.

Total Air Quantity: A scuba cylinder filled to the
same gauge pressure (e.g., 3000 psig) at 5777 feet contains significantly less total air (measured in mass or Standard Cubic Feet) compared to a cylinder filled to the same gauge pressure at sea level. The reduction at 5777 ft is approximately 18.4%.

Underlying Cause: This difference arises because gauge pressure measures pressure relative to the local ambient atmospheric pressure. Since ambient pressure is lower at altitude, the absolute pressure achieved inside the cylinder for a given gauge reading is also lower. Furthermore, the compressor intakes less dense air at altitude, resulting in fewer air molecules being packed into the cylinder. The reduction in SCF is closely approximated by the ratio of ambient pressure at altitude to standard sea-level pressure.

So my thinking was it was less o2 but it's just less dense

 

[Wallowa]

Thanks for this. I have noticed it taking a little longer to fill.

So then if the air is still 21% but less dense would then I need more of it at sea level.

Asking ai it came up with this.

Total Air Quantity: A scuba cylinder filled to the
same gauge pressure (e.g., 3000 psig) at 5777 feet contains significantly less total air (measured in mass or Standard Cubic Feet) compared to a cylinder filled to the same gauge pressure at sea level. The reduction at 5777 ft is approximately 18.4%.

Underlying Cause: This difference arises because gauge pressure measures pressure relative to the local ambient atmospheric pressure. Since ambient pressure is lower at altitude, the absolute pressure achieved inside the cylinder for a given gauge reading is also lower. Furthermore, the compressor intakes less dense air at altitude, resulting in fewer air molecules being packed into the cylinder. The reduction in SCF is closely approximated by the ratio of ambient pressure at altitude to standard sea-level pressure.

So my thinking was it was less o2 but it's just less dense

Bob,

I have to laugh, not at you, but with you....I also suffer from "overthinking" ....when your compress air into your bottle with your compressor it does just that compress....for example when your aluminum 80cf bottle has 3,000psi it has 80cf [Luxfer lists it at 77.4 cf]....you could take that to 10,000 ft absl or to 500 feet+ underwater and it still would have 80cf at 3,000psi; rigid container with compressed gas at rated pressure and volume...the only difference compressing gas into to a bottle at altitude is the additional time it takes for the compressor to draw in the less dense air at altitude and then compress it to your targeted 3,000psi [in example above]....

The amount of air you "need' [use] at sea level or for that matter at any elevation once underwater on a dive is dependent on your consumption rate and depth....the reg will deliver at the correct ATA pressure and deeper you go the more pressure and volume is provided by the tank and reg to equalize your air spaces....freshwater vs salt water density differences vs pressure at depth is insignificant....

"So my thinking was it was less o2 but it's just less dense"....compressed air in a scuba tank is at 21% oxygen and at the density that it was compressed to...not less dense.

I probaly was overthinking your question but hope that helps.

 

[Bob from California]

Bob,

I have to laugh, not at you, but with you....I also suffer from "overthinking" ....when your compress air into your bottle with your compressor it does just that compress....for example when your aluminum 80cf bottle has 3,000psi it has 80cf [Luxfer lists it at 77.4 cf]....you could take that to 10,000 ft absl or to 500 feet+ underwater and it still would have 80cf at 3,000psi; rigid container with compressed gas at rated pressure and volume...the only difference compressing gas into to a bottle at altitude is the additional time it takes for the compressor to draw in the less dense air at altitude and then compress it to your targeted 3,000psi [in example above]....

The amount of air you "need' [use] at sea level or for that matter at any elevation once underwater on a dive is dependent on your consumption rate and depth....the reg will deliver at the correct ATA pressure and deeper you go the more pressure and volume is provided by the tank and reg to equalize your air spaces....freshwater vs salt water density differences vs pressure at depth is insignificant....

"So my thinking was it was less o2 but it's just less dense"....compressed air in a scuba tank is at 21% oxygen and at the density that it was compressed to...not less dense.

I probaly was overthinking your question but hope that helps.

Hahah I really appreciate your response. And the community here. It has been great asking all kinds of questions thanks for sharing your knowledge