Living at Altitude Diving at Sea Level

[Charlie99]

DeepTechScuba once bubbled...
Gentlemen, if you do not have access to a NOAA DIVING MANUAL then send me privately your fax number and I will fax you a copy of the NOAA re-ascent to altitude tables. It is a one page table that answers your question as to how long you must wait at lower altitude before you can return to XXXX ft.

Or download it here .

The air, EAN32 and EAN36 tables to use with it are also on the Dive planning section of NOAA NDC site.

 

[Charlie99]

jbichsel once bubbled...
Am I missing something here?

I was alway taught that as you ascend in elevation the pressure decreases, not increases.

To say that "4000' is the same pressure as 4 feet in sea water", kind of upsets the whole nature of physics, eh?

Maybe I'm not understanding.

Here's referring to the fact that the pressure _change_ is the same as 4' of water ----- the same as going from sea level to a complete vacuum is only a 33' change in pressure.

FAA maximum allowed normal cabin altitude of 8,000' is about 3/4 of normal atmospheric pressure.

The old goat bender Haldane figured that a reduction in pressure by 1/2 is where significant numbers of goats and people start getting bent. Sure enough, take a NON-diver from sea level up to altitude such that cabin pressure is about 1/2 atmosphere and you start seeing people get bent.

That pressure difference is ONLY about the same as 17' of water. It's the pressure RATIOS that hurt.

 

[Charlie99]

cnidae once bubbled...
Come on. These guys did'nt get bent from the alttitude, they were already bent when they came out of the water. 4000' is the same pressure as 4 feet in sea water.

There is more than a grain of truth here, but it's not the whole story.

Bent vs non-bent is a continuum. They may very well have been asymptomatic and remained asymptomatic had they remained at sea level. More recent decompression models such as VPM and RGBM assume that bubbles in your tissues exist. Ascent both increases the size of bubbles and at the same time decreases the critical radius above which bubbles will continue to grow.

Where a short time to ascent makes a big difference is after a dive with a marginal profile where the diver is not perfectly clean. A short time to ascent can turn the sub-clinical mild fatigue after such a dive into full blown clinical DCS.

 

[Genesis]

Grab yourself a copy of VPM-B if you'd like and play with it.

You can set altitude in that deco program, and then run a few NDL profiles on it.

You'll find that if you do this and add 10' or so to your bottom depth (to accomodate the higher surface pressure) you have fairly radical changes in allowed profiles (in the "downward" direction!) before mandatory stops become part of the equation.

As an alternative to sitting at sea level, you could make the dives as if your "surface" was at altitude, and shorten the times and make the stops, if any, required for diving with a surface altitude as set that way. Some computers can be manually set for this (the Suuntos can), or you could use a table program like VPM to generate custom tables for this situation.

IANAD (I am not a doctor!) but the physics of this would appear to line up correctly, as your "final ascent to the 'surface'" at altitude would be MUCH slower than any such program would likely require, and the "depth" from which you would make your ascent would be only 10' fsw or so - extending the 10' stop indefinitely is perfectly fine with these meters and programs.

If I lived at 4000' altitude and wanted to dive at sea level on a regular basis, that's how I'd approach the problem rather than sit around for 12 hours.

Treat the dive as if it took place at 4000' and adjust your profile and deco schedule accordingly.