Altitude questions

[inquis]

Is there a significant difference between diving 45 feet at 7000 feet vs diving 45 feet at sea level and then taking and elevator up to 7000 feet in a matter of minutes.

Absolutely yes.

If you surfaced with a supersaturation of 85% at sea level (quite acceptable) it would increase to 126% during the elevator ride -- bad news. If you dove for the same bottom time at 7000 ft, your supersaturation upon surfacing would be 94% (on par with the DSAT algorithm that backs the PADI recreational dive planner) -- so pretty safe.

(This is ignoring the bubble expansion present in the elevator case. That makes that case even worse.)

 

[BackAfter30]

It isn’t a function of how much gas you’ve taken on. You are correct that the difference is small. The problem is that when you surface at 7000ft the absolute pressure your body experiences is substantially lower than that at sea level. This allows more gas to come out of solution - bubbles - relative to how much comes out at sea level. The same occurs if you dive at sea level then drive to altitude or fly. You body is exposed to lower pressure before you have fully expired all of the excess gas - more bubbles.

 

[Omb]

cool stuff. Do you have a link to how you calculated the 85% and 94% supersaturation for sea level vs 7k feet?

 

[inquis]

Subsurface is an open source log/planner you may find useful. I just adjusted the dive time to give 85% at sea level. It handles planning dives at altitude as well, and I just used the same time. You will have to increase the GFHigh parameter to avoid mandatory deco stops.

For the elevator dive, you have to use the equivalent depth of 53 ft at an altitude of 7000 ft (i.e., the same pressure as the 45 ft sea level dive = 2.32 atm.) This way the surfacing GF will be relative to 7000 ft ambient. There will be some small differences from the numbers I gave due to descent/ascent rates, salinity, etc. mismatches, but hopefully you will find it illustrative.

 

[kinoons]

I guess my question is: Is there a significant difference between diving 45 feet at 7000 feet vs diving 45 feet at sea level and then taking and elevator up to 7000 feet in a matter of minutes. It seems like the tissue loading from being 45 feet underwater would be pretty close to the same at sea level vs 7k feet. Is that correct? Just trying to reconcile guidance to not drive over passes after diving, yet it’s ok to dive at altitude if you just ascend extra slow to the top

Think of this more simply - your net elevation change in a sea level dive to 75 feet deep and back to sea level is zero. Then you off gas at whatever your sea level rate is.

If you dive at 7000 ft surface elevation to 75 feet and then return to and stay at 7000 feet your net elevation change is again zero. You have to deal with the off gasing (and less ATM pressure) but the net change in elevation is zero. (Assuming prior acclimation at that elevation; grossly simplified example).

What your asking about isn’t a net zero sum. Instead you’re asking about sea level to 75 feet, back to sea level, and the up 7000 feet. It’s the sudden up to 7000 feet (so now a net gain of 7000 ft, not zero, and equivalent loss of atmospheric pressure) that can be the issue if your physiology happens to be more susceptible to decompression sickness (again lots of known and unknown variables in that risk assessment).