what is "altitude diving" and max # of dives

[segraves1]

The NEDU forum

the what?

 

[bleeb]

Don't have the link handy but the US Navy manual has a good section on altitude diving including travel to and from altitude.

SEA 00C3 Diving Publications and Technical Documentation

The NEDU forum here should have a link.

The US Navy's Navy Experimental Diving Unit has it's own forum here on SB that they're good enough to post in every now and then.

 

[Diver0001]

During my Advances OW course, it talked about altitude as being "anything over 1000feet" and to "limit daily diving to 2 dives at altitude".

Phoenix (lake Pleasant) is 1500+ feet and I KNOW people do more than "2 dives a day" around here

So what do people really consider "altitude" and when does a person say "this is the most dives I should do in a day"?

The definition of "altitude" depends on what your tables were designed to do. The PADI tables were designed for use at sea level to 500ft (or was it 100oft?). Above that and you need to adjust for the reduced atmospheric pressure.

Many modern computers automatically adjust for altitude up to about 10,000ft IIRC

As for the limitation of the number of dives, I can only guess that the reasoning behind it is that the "adjustment" tables haven't been formally tested in combination with the RDP and the agency is covering it's legal butt.

To my way of thinking there shouldn't be any difference as long as you're making the correct adjustment for the reduced atmospheric pressure.

This is an interesting topic for the Dr. Deco forum. I would suggest doing a search in that forum to see if Dr. Deco has given us his opinion on this topic before.

R..

 

[knotical]

I wonder if this is another good application for Nitrox.

http://www.scubaboard.com/forums/2198370-post79.html

 

[Deefstes]

I'm curious, is there any other difference to altitude diving than just your on-gassing and off-gassing happening at different rates?

In other words, if you're already using tables adjusted for altitude, is there anything else that you need to do differently just because you are at altitude? If not, then surely you can dive as much as you want provided you stay within the limits imposed by the tables much the same way as you would had you been diving at sea level.

Right? No?

 

[AzAtty]

I'm curious, is there any other difference to altitude diving than just your on-gassing and off-gassing happening at different rates?

In other words, if you're already using tables adjusted for altitude, is there anything else that you need to do differently just because you are at altitude? If not, then surely you can dive as much as you want provided you stay within the limits imposed by the tables much the same way as you would had you been diving at sea level.

Right? No?

It really isn't a question of question of faster ongassing or offgassing. The rate at which tissues absorb and release inert gas is independent of pressure (it's a logarithmic equation based upon the half time of the tissue, which is really a question of gas solubility and tissue permeability). Changes in pressure affect whether a tissue is in a state of equilibrium and what we call the "critical tension" of a particular tissue.

The models behind tables and computers are based upon sea level atmospheric pressure. You remember from math class that changing one variable changes the equation, right? Say you decrease 1 ATM to 1/2 ATM. The result of your equation (bottom time at a given depth) changes, and now the bottom time appearing on the table is invalid. In order to "balance" the equation, the concept of "theoretical depth" is used to offset the changes caused by decreased atmospheric pressure at altitude. Thus, you are able to continue to use a table (which is a fixed medium). This is a oversimplification the concept (ratio constancy of critical tension over ambient pressure--at least in the Haldanean world), but I think you'll get the point--change one variable, and you have to change something else to offset it or you'll get a wrong answer.

Recommendations for altitude dives are (1) use theoretical depths; (2) wait at least six hours after arriving at altitude to dive (PADI table, Navy and others differ in recommendations); (3) limit depth to 100 feet; and (4) conduct no more than two dives in a day.

In Phoenix, where the OP dives, we routinely exceed 100 feet and dive more than twice daily. However, I would suggest not pushing the NDLs on more aggressive dives at altitude if you're using a table (unless its a Buhlmann that has been adjusted for depth).

 

[Diver0001]

It really isn't a question of question of faster ongassing or offgassing. The rate at which tissues absorb and release inert gas is independent of pressure

I have my doubts about that.

In order to offgass you need to have some kind of over-pressure in the tissues (positive gradient)

Henry's law would seem to suggest that the higher the gradient, the faster the offgassing. (just like the higher the negative gradient the faster the on-gassing.... like during a deep dive vs. a shallow dive)

At least that's the basis of Haldanian deco theory. I know some of the bubble models work a little differently because they walk the fine line just "this side" of boyle's law but the basis is still the same....

R..

 

[DA Aquamaster]

That's pretty much the point.

A theoretical depth is based on the lower atmosheric pressure at altitude at the surface. So for example if instead of 15 psi you only have 13 psi atmospheric pressure, you would reach 2 ATM of pressure at a shallower depth.

At higher altitudes, it can get pretty extreme with an actual depth of 130' having for example a 170' theoretical depth. Now, the actual pressure you are under at 130 actual feet is a lot lower than would be the case if you were really under 170 ft of water but the number of atmospheres of pressure relative to the surface atmospheric pressure is identical. The greater theoretical depth adds conservatism needed to compensate for the lower pressure you will encounter at the surface once the dive is done as you will have a higher pressure gradient after you surface due to the higher altitude and lower atmospheric pressure.

So the rate of on-gassing at 130' actual water depth at high alttitude is for all intents and purposes the same it would be at seas level on a 130' dive (and the same applies to PPO2, narcosis issues, etc) but you assume and use the greater theoretical depth to compensate for the lower atmospheric pressure and related off-gassing issues.

 

[AzAtty]

I have my doubts about that.

In order to offgass you need to have some kind of over-pressure in the tissues (positive gradient)

Henry's law would seem to suggest that the higher the gradient, the faster the offgassing. (just like the higher the negative gradient the faster the on-gassing.... like during a deep dive vs. a shallow dive)

At least that's the basis of Haldanian deco theory. I know some of the bubble models work a little differently because they walk the fine line just "this side" of boyle's law but the basis is still the same....

R..

I knew I'd draw questions on that statement. Perhaps I can settle the issue with a statement from Bruce Wienke: "halftimes are independent of pressure." The tissue half time determines the rate at which a tissue ongasses or offgasses. Ambient pressure does not affect the tissue half time. The half time is fixed per tissue per gas (e.g. helium has a different half time than nitrogen). For example, in perfusion controlled inert gas transport, the perfusion constant is DEFINED by a logarithmic function that does not consider pressure and the key variable which is the tissue's half time.

Henry's law merely states that a gas will diffuse into a liquid when gas tension outside the liquid is greater than the gas tension inside the liquid. The law states also that the reverse is true. Henry's law says NOTHING about how fast or slow that diffusion will occur. You're right--there must be a "driving gradient" to ongas or offgas, and that's what Henry's law says. But that's also where people get confused because the math behind the theory is not explained very well (or at all). Analogies are given to explain the concept, but the analogies do not accurately reflect the equations behind the theory.

Ambient pressure merely determines when a tissue starts to ongas/offgas/reach equilbrium. It does not affect the rate of change of gas tensions in tissues. That event is controlled by the tissue's half time, which is independent of pressure. Ambient pressure is also a consideration when determining whether a critical tension ("M-value") has been reached or exceeded, and thus determines when stops are necessary or unnecessary.

 

[Diver0001]

Analogies are given to explain the concept, but the analogies do not accurately reflect the equations behind the theory.

I hear what you're saying and I'm not going to say you're wrong but intuitively... the harder you blow.... (at least before boyle's law takes over) the more air you can push through a sheet of silk.

I personally have my reservations about the things Dr. Wienke says because (a) I suspect his motives are to make profit and not to be "right" and (b) his theory only seems to hold for a narrow bandwidth of dives. At extreme depths all the masters of our sport are still using Haldane. There's a reason for that.

I'd like to have Dr. Powell's opinion on this before we go further.

With your permission I'd like to have this thread split and the last bit kicked into the Dr. Deco forum. I think it's important for us to have an expert opinion on this.

R..