Diving at Altitude, Compensate or not?

[amascuba]

Take your example of treating 80' dive at 4,000 feet as a 92' dive. At sea level, 80fsw is 2.4atm. If you reduce the atmospheric pressure by 16%, then that same 80fsw would become about 2.8 of the local atmospheres. One way to account for that would be to use whatever depth is 2.8atm on a sea level table --- about 92'. This perhaps is an oversimplistic way of looking at things, but it is a pretty reasonable approximation of what one needs to do in order to dive at the same risk levels whether at sea level or altitude.
Charlie Allen

Charlie,

If you decrease the atmospheric pressure by going to altitude, how does that increase the pressure of a dive at altitude? I can understand compensating for altitude if you had a quick change in atmospheric pressure without letting the body acclimate to the new pressures, but as this thread started out it assumes that the body is or will be acclimated to the atmospheric pressure over a period of time.

80 foot dive at 1ATA (sea-level) = 80/33 (water pressure in ATA) + 1 (atmospheric pressure in ATA) = 3.42ATA (ambient pressure in ATA)

80 foot dive at .8ATA (altitude) = 80/33 (water pressure in ATA) + .8 (atmospheric pressure in ATA) = 3.22ATA (ambient pressure in ATA)

Partial Pressures at 1ATA = .209ATA PP02, .782ATA PPN2, .09ATA PPx

Partial Pressures at .8ATA = .168ATA PP02, .623ATA PPN2, .072ATA PPx

 

[Charlie99]

Charlie,

If you decrease the atmospheric pressure by going to altitude, how does that increase the pressure of a dive at altitude? I can understand compensating for altitude if you had a quick change in atmospheric pressure without letting the body acclimate to the new pressures, but as this thread started out it assumes that the body is or will be acclimated to the atmospheric pressure over a period of time.

80 foot dive at 1ATA (sea-level) = 80/33 (water pressure in ATA) + 1 (atmospheric pressure in ATA) = 3.42ATA (ambient pressure in ATA)

80 foot dive at .8ATA (altitude) = 80/33 (water pressure in ATA) + .8 (atmospheric pressure in ATA) = 3.22ATA (ambient pressure in ATA)

Partial Pressures at 1ATA = .209ATA PP02, .782ATA PPN2, .09ATA PPx

Partial Pressures at .8ATA = .168ATA PP02, .623ATA PPN2, .072ATA PPx

It doesn't increase the absolute pressure. It increases the pressure RATIOS.

Using your numbers from above -- ascending from an 80' dive at sea level you are going from 3.42ata to 1 ata, for a RATIO of 3.42.

Ascending from an 80' dive at 0.8ata altitude, you are going from 3.22 to 0.8ata, a RATIO of 4.025.

Only for a dive to 100' at sea level will you have a ratio of pressure at depth to surfacing pressure of 4. So in terms of relative ratios of pressures, the 80' dive at 0.8ata is equivalent to a 100' dive at sea level.

Detailed calculations will come up with a slightly different result, but just looking at pressure RATIOs is a pretty good approximation. If you think about bubble size growth it is pretty obvious that pressure ratio is what counts (ignore the fixed surface tension numbers). Many decompression studies, dating all the way back to Haldane have shown that pressure ratios is a good first cut approximation for doing decompression from deep dives. If you go look at the various models, whether Workmann/USN, Buehlmann, RGBM, or VPM you will find that the ratio of pressures is the primary driving force. There are some numbers that are related to absolute pressure differences, but the ratios play the most important part.


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Details -- not essential to basic understanding, but might help ......
Go find the "Understanding M-Values" article by Erik Baker. You will find that the M-values -- maximum allowable compartment pressures -- have a slope factor. Normally this is used in determining ceilings on a deco dive, but they can also be used to for altitude calculations. See my previous post for a general description of why ceilings and altitude calculations are related.
On an 80' dive, the controlling compartment is in the 10 or 15 minute range and whether or not one is acclimated to the altitude won't have too much effect. That is less true for long, shallow dives, where the controlling compartments are slower ones.

 

[amascuba]

I understand that for every 1000ft above sea-level there is approximately a 4% decrease in atmospheric pressure. I understand what you are saying about ratio's, but none of that takes into account the body being acclimated to the altitude. It doesn't take into account the bodies ability to adapt to it's environment, which is what happens when the body goes from sea-level to altitude. My thinking would be that the bodies ability to adapt makes the ratio's useless.

I'll do a search for the article though. Thanks.

 

[Charlie99]

I understand that for every 1000ft above sea-level there is approximately a 4% decrease in atmospheric pressure. I understand what you are saying about ratio's, but none of that takes into account the body being acclimated to the altitude. It doesn't take into account the bodies ability to adapt to it's environment, which is what happens when the body goes from sea-level to altitude. My thinking would be that the bodies ability to adapt makes the ratio's useless.

I'll do a search for the article though. Thanks.

I see from your profile that you have or soon will take Advanced Nitrox and a TDI Decompresion Course. Searching out and reading some of Erik Baker's articles will help you a lot in understanding deco. Even though Haldanian/dissolved gas models have been superceded by dual phase/bubble models, a good intuitive understanding of the Haldanian models makes many things in decompression, including the bubble models, much easier to understand.

http://www.decompression.org/baker/home.htm is a link to several of his papers.

Understanding M-Values is a short but very informative paper. When I first read it, the numerical values were somewhat confusing, because the table of M-values is in feet-saltwater ABSOLUTE, rather than depths.

Calculating No-Stop Times is a good paper to work through to see if you really have the numbers from the M-value paper down correctly. Remember that the M-values are in fsw-absolute, that you need to subtract out the H2O + CO2 correction of 1.6 to 2fsw from the depth before then apply the fraction N2 in order to get the inspired N2 pressure.

At this point, you have the tools if you desire, to run your own decompression calculations using any of the various dissolved gas models, making whatever assumptions you desire about acclimatization to altitude. The same slope factors for the M-values that are used to adjust them for depth to generate deco ceilings can also be used to adjust the M-values to the lower pressures found at altitude (within reason of course..... the models aren't tested or valid at less than 0.5ata, but are still reasonable at the 0.75 or 0.8ata more typical max altitudes).

At this point the Deep Stops article should make sense, and reinforce your understanding of both the dissolved gas models and how deep stops are a natural result of minimizing overpressure gradients in the fast compartments.

----------------

That link also has some relatively readable articles on VPM. Personally I find Erik Baker's style of writing much, much easier to understand that that of Mr. RGBM, Bruce Wienke. Erik Baker goes to the trouble of explaining what definitions and notation he is using whereas RGBM articles are typically full of undefined terms and symbols that I have to guess at.

 

[amascuba]

I see from your profile that you have or soon will take Advanced Nitrox and a TDI Decompresion Course. Searching out and reading some of Erik Baker's articles will help you a lot in understanding deco. Even though Haldanian/dissolved gas models have been superceded by dual phase/bubble models, a good intuitive understanding of the Haldanian models makes many things in decompression, including the bubble models, much easier to understand.

http://www.decompression.org/baker/home.htm is a link to several of his papers.

Understanding M-Values is a short but very informative paper. When I first read it, the numerical values were somewhat confusing, because the table of M-values is in feet-saltwater ABSOLUTE, rather than depths.

Calculating No-Stop Times is a good paper to work through to see if you really have the numbers from the M-value paper down correctly. Remember that the M-values are in fsw-absolute, that you need to subtract out the H2O + CO2 correction of 1.6 to 2fsw from the depth before then apply the fraction N2 in order to get the inspired N2 pressure.

At this point, you have the tools if you desire, to run your own decompression calculations using any of the various dissolved gas models, making whatever assumptions you desire about acclimatization to altitude. The same slope factors for the M-values that are used to adjust them for depth to generate deco ceilings can also be used to adjust the M-values to the lower pressures found at altitude (within reason of course..... the models aren't tested or valid at less than 0.5ata, but are still reasonable at the 0.75 or 0.8ata more typical max altitudes).

At this point the Deep Stops article should make sense, and reinforce your understanding of both the dissolved gas models and how deep stops are a natural result of minimizing overpressure gradients in the fast compartments.

----------------

That link also has some relatively readable articles on VPM. Personally I find Erik Baker's style of writing much, much easier to understand that that of Mr. RGBM, Bruce Wienke. Erik Baker goes to the trouble of explaining what definitions and notation he is using whereas RGBM articles are typically full of undefined terms and symbols that I have to guess at.

Excellent! Thanks for the information Charlie. I'll read up and try to gain a better understanding.

 

[Dr Deco]

Hello amascuba :

Charlie99 has given some good answers, and my thanks to him on this.

The basic point to remember about diving at altitude is the FINAL pressure that you reach upon surfacing. To a first approximation, altitude diving has nothing to do with off gassing. Yes, the body will offgas a little faster since there is less nitrogen at altitude, but that is minor for an initial explanation. It also has little to do with uptake of nitrogen while diving since that is primarily determined by the water pressure [again, to a first approximation].

Diving tables are based on the difference in nitrogen dissolved in your body, from the dive, and the external pressure when you reach the surface. There must be a certain pressure difference of bubbles will form [to a first approximation]. If this pressure difference is small because the ambient pressure is reduced, then you must have less dissolved tissue nitrogen [from the dive] to have a safe, DCS-free outcome.

If you are an aviator or astronaut, simply going to high altitude [low external pressure] will result in DCS, and you can omit any diving at all.

Dr Deco :doctor:

The next class in Decompression Physiology for 2006 is September 16 – 17. :1book: http://wrigley.usc.edu/hyperbaric/advdeco.htm

 

[NJMike]

Okay, I'm still trying to digest all of this....

Here's a potential scenario for me, right after Christmas. I live in NJ at an elevation of approx 1000'. After Christmas (with all my new gear!) I drive down to Mt. Storm Lake in West Virginia, at an elevation of 3244' (if I can find a buddy).

I do 2 dives (water should be above 60 F) to a max of 60', then pack up and drive home.....downhill to 1000'.

Since I am not ascending after the dive, is this a problem?

 

[rjack321]

Using two examples of people, who were probably already subclinically bent before, ascending to altitude probably shouldn't dictate the need or lack thereof to compensate for altitude. (I know many people diving in Lake Tahoe (6000ft) who do not compensate on deco and non-deco dives and are just fine.)

Most reasonable altitude increases equate with a few feet of water pressure that I believe we can physiologically compensate for if they are not very fast.

I would like to see more data on people's profiles paired with altitude induced DCS symptoms/incidents before reaching judgements on cause and effect.

 

[O2BBubbleFree]

Okay, I'm still trying to digest all of this....

Here's a potential scenario for me, right after Christmas. I live in NJ at an elevation of approx 1000'. After Christmas (with all my new gear!) I drive down to Mt. Storm Lake in West Virginia, at an elevation of 3244' (if I can find a buddy).

I do 2 dives (water should be above 60 F) to a max of 60', then pack up and drive home.....downhill to 1000'.

Since I am not ascending after the dive, is this a problem?

Here's a simple answer: not if you do it right (and that has nothing to do with DIR )

As Amascuba pointed out in his first post, most of the agencies have altitude certs. I'm in no way a proponent of card collecting, but I think this one is a must if you dive at altitude.

And as Charlie99 and Dr. Deco explained, there are legitimate reasons why the tables have to be adjusted, and other considerations.

Rjack321 pointed out that there are a lot of people diving Lake Tahoe without taking any extra precautions. Good for them. I took the altitude cert specifically so I could dive Tahoe, and am glad i did.

In the cert class you learn how to adjust the tables, how to adjust your safety stops, how to adjust your diving if you have been 'at altitude' for less than 24 hrs., and a few other things.

Again, I'm not a card collector. I have OW and alt., and will probably get a nitrox card 'cause it's difficult to get the gas without it. 99% of the cards I think are a waste of time. Altitude, however, is about the only one I recomment. And, of course, from a good instructor.

The agency I got mine from (and I assume the other agencies as well) considers anything 1000' or more above sea level to be an altitude dive.

Just my opinion, of course.

 

[TheAvatar]

FIRST: Altitude diving is much more about physics than physiology.

8000ft you have about .75ATM ambient. If you are at 33ft you are experiencing about 1.75ATM. If you surface you went from 1.75ATM to .75ATM or about 57% less pressure. At sea level you would have gone from 2ATM to 1ATM, only a 50% change. The lower the ambient surface pressure, more specifically the greater the percentage change in pressure, the less likely the dissolved gasses will stay happily in solution.

From my understanding, it is overall really that simple (relatively, when simplified).

As you ascend in altitude, the pressure ratio between the surface and a given depth grows... as though you were diving deeper the higher you went in altitude.

SECOND: ACCLIMITIZATION

Human ability to acclimate to altitude is about dealing with the effect secondary and tertiary to lower ambient ppO2 and almost nothing else. We compensate for this physiologically with increased heart rate, blood pressure, and increased respiratory rate and depth.

Over time we compensate further:

Days: You will see increased 2-3BPG presence in hemoglobin to affect the oxygen disassociation curve
Weeks: You will see increased hematopoisis: you will have a higher crit
Months: You will develop higher relative vascular density in new muscle

Once you are under the water the low ppO2 issues is more than gone. I imagine almost all altitude compensation mechanisms are superfluous other than the effects of having a higher crit or problems and/or issues with recent ascents to altitude because there is some dehydration caused by diuresis induced by respiratory alkalosis experienced at altitude in the course of compensating for lower ppO2 with increased RR.

To reiterate, we are dealing with absolute pressure changes and ratios when worrying about DCS, not sub .21atm ppO2 on the surface.

Okay, I'm still trying to digest all of this....

Here's a potential scenario for me, right after Christmas. I live in NJ at an elevation of approx 1000'. After Christmas (with all my new gear!) I drive down to Mt. Storm Lake in West Virginia, at an elevation of 3244' (if I can find a buddy).

I do 2 dives (water should be above 60 F) to a max of 60', then pack up and drive home.....downhill to 1000'.

Since I am not ascending after the dive, is this a problem?

That is a wonderful question!

If you are diving within the limits for your altitude adjusted depth, there is NO PROBLEM WITH PURELY DESCENDING. Ambient pressure increases which inhibits bubble formation in you.

This brings up the third point:
THIRD: THERE IS OFTEN MORE THAN ONE EXTRA TABLE REQUIRED FOR ALTITUDE DIVING

If you make a single dive at sea level, you need one table. If you make multiple dives at sea level, you need two tables.

If you drive up to altitude and make a SINGLE dive and then drive home you need FIVE TABLES.

1. You use a pre dive ascent to altitude table which accounts for your nitrogen load from being at a lower altitude and thus breathing higher ppN2 and having an initial equivalent N2 load
2. You use an altitude-depth adjustment table which accounts for the surface/depth pressure ratios
3. You use that data to determine your residual nitrogen from the surface interval table you normally use for multiple dives
4. Then you use your normal dive table with your residual nitrogen and adjusted depth to plan your dive.
5. After your dive, you use the post dive ascent to altitude table to see if/when it is safe to ascend to a given altitude in the course of your return home (normal reality of travel in the mountains).

-TheAvatar
9,300ft above sea level