Certified at altitude in South Africa

[Dr Deco]

Hello readers:

One of the readers asked if I might comment on this thread. There are many aspects and I am not sure that I will get all of them.

Ascent Rate vs. Waiting

Just as it is possible to have a linear ascent in diving [especially true in saturation diving], it is possible to have a slow ascent in travel to altitude. The simple, table format does not allow this, however. Clearly, it would just be too complicated, since terrain varies so much that no one course is suitable for all parts of the world. Thus the table is calculated such that one remains at sea level (or whatever the final stop might be) and begins the ascent after a given duration as prescribed by the table.

If one were to suggest a linear ascent for the return trip, one would need to ask, “Is the ascent rate linear, or is there possibly a large ascent at the beginning of the trip and a level drive for the remainder.” This is not a “slow ascent” then.

Inexact Science

Actually decompression is an exact science if all variables are accounted for. This is much easier in altitude depressurization since many variables are more closely controlled.

In diving:

- Gas loads and exercise level at depth very considerably from dive to dive. Gas upatake and elimination is not constant.

- There is a considerable variability amongst divers with respect to the generation of decompression bubbles. This cannot currently be accounted for in dive tables. Many divers have a sort surface interval preceding flying. They do this with impunity since they have found [at their own risk] that they are resistant to bubble formation. How do you find out if you are “resistant?” You fly and take your chances; this is not necessarily a good idea.

- Stress-assisted nucleation plays a role in decompression bubble formation. The post-dive exercise level is not a part of any dive table.

- Physical fitness plays a role in dissolved nitrogen elimination. Fitness is not a part of any dive table.

Altitude Decompression

Altitude decompression is similar to deep-sea decompression. There are physiological and operation differences however.

- Metabolic gases (e.g., oxygen, water vapor, carbon dioxide) play an increasing role at low ambient pressures;

- Individuals breathe oxygen at altitude and risk falls more rapidly because of this;

- DCS is time dependent and sojourns at altitude are often short, and a return to pressure follows.

Dr Deco :doctor:

 

[Charlie99]

A lot of talk here, but not much science, and I, for one, would like to learn something.

At an altitude of 18,000 feet, the pressure is about one-half as that at the surface. That seems to me to be the same change in pressure as ascending form 15.33' deep to the surface. So, climbing in altitude from 0' to 5000', is less than a third of that, or, roughly, the same as 4' under water to the surface.

I can't be doing the math right here. Can someone actually give us some date on this?

Your problem with the math is that you are adding rather than dividing

While there are a bunch of other factors such as H2O vapor pressure, and surface tension of bubbles, the first crude approximation is the same one that JS Haldane came up with. He was looking at RATIOS of pressure.

In a crude sense, 0 to 18,000 foot altitude is a 2 to 1 ratio, the same way an ascent from 33' to the surface is a 2 to 1 ratio. In both cases, DCS is possible, but not likely.