Info Diving at Altitude

Please register or login

Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

Benefits of registering include

  • Ability to post and comment on topics and discussions.
  • A Free photo gallery to share your dive photos with the world.
  • You can make this box go away

Joining is quick and easy. Log in or Register now!

Diving at Altitude

Diving at altitude demands adjustments to the procedures used at sea level. With moderate increases in altitude and with shallower dives, the differences can be minor, even minor enough to be ignored. As altitude and depth increase, the need for concern grows. There are three primary reasons for this.
1. In cases where the diver has ascended to altitude, the diver begins the first dive with residual nitrogen from change in altitude, so it is similar to having done a dive already.​
2. Decompression sickness depends largely upon the difference in the body’s tissue pressure upon ascent and (especially) surfacing, and that difference is potentially greater at altitude.​
3. Bubbles that are formed in the body can increase in size upon ascent, and that rate of growth needs to be controlled. The growth is greater at higher altitudes.​
Most of what we know about diving at high altitude comes from work done at what are really moderately high altitudes—usually up to 8,000 feet/2,500 meters. Little study has been done at greater altitudes than that. Most of the truly high altitude studies have been done in relation to astronauts and other high altitude pilots, and some of the scientists working on that kind of high altitude work have also been involved with decompression with diving. They caution that there is much more going on when you get to those higher altitudes, so that kind of diving should not be considered a mere extension of the norms associated with diving at more moderate altitudes.

Diving with nitrox has become increasingly popular, and many divers will not realize that altitude affects nitrox use as well. The most significant difference is in maximum operating depths (MOD), the maximum depth a specific enriched air blend can be used safely.

This article will come in Five Parts:
Part One: Starting with Residual Nitrogen​
Part Two: Tissue Pressure Gradient Upon Surfacing​
Part Three: Bubble Growth at Altitude​
Part Four: Strategies for Altitude, Including Very High Altitudes​
Part Five: Maximum Operating Depths at Altitude​
Part One: Starting with Residual Nitrogen
Beginning open water divers learn that before they begin a second dive, they must have a surface interval after the first dive. That is because when they surfaced, they still had more nitrogen in their system than normal. Because the air they breathe on the surface has a lower partial pressure of nitrogen than their body tissues, their body will slowly lose the excess nitrogen they still have in their tissues after surfacing. Unless they wait a long time, though, they will still have more nitrogen in their tissues when they start the second dive than they did for the first. That extra nitrogen, called residual nitrogen, must be accounted for in dive planning. That accounting can be done through dive tables, but today most people use computers, and the computers will factor the residual nitrogen into the following dives.

When a diver travels from low to high altitude, the diver will have residual nitrogen because of the higher partial pressure being breathed at the lower altitude, just like the higher partial pressure breathed during a previous dive. The diver must therefore plan the first dive as if it were a repetitive dive. PADI teaches divers using their tables to treat every 1,000 feet of ascent as two pressure groups, so a diver leaving sea level and traveling to an altitude of 6,000 feet would be in the L pressure group already, so an appropriate surface interval is required. With the PADI tables, a diver in the L pressure group would be back at the A pressure group in 2:10 hours, and that diver could be at a first dive level after 5:10 hours. Because the PADI tables wash out at 6 hours, divers who have been at a site for longer than that need not consider the effects of residual nitrogen. The US Navy tables and other tables that follow them wash out at 12 hours, so they require 12 hours at altitude before residual nitrogen is no longer a factor.

Of course, no one is teleported to a dive site, like crew members being beamed up to the StarShip Enterprise. A diver driving to that altitude would be off-gassing all the way up to the dive site and would arrive well on the way to first dive status. A diver flying in a commercial aircraft would be at an even higher altitude for that time, because commercial aircraft are pressurized to an altitude greater than 6,000 feet. That means that most divers will have already completed much or all of the full surface interval by the time they have set up their gear for the dive.

For divers using computers, most will adjust to altitude automatically but some will have to be adjusted manually. That computer will then know you are at altitude, but it will not know how long you have been there. In most cases, this will not matter. However, in the rare case of a diver preparing to dive with a significant load of residual nitrogen, it should be considered when deciding how close to dive to decompression limits. For technical divers diving with software generated tables, most decompression software programs will ask divers to input their current altitude, their previous altitude, and their time at the present altitude.

Because most divers will have had enough time at that higher altitude before they begin their dives to have gotten rid of most residual nitrogen even without trying to do so, this is the least important of the factors involved with altitude diving. Pressure difference upon surfacing and bubble growth are far more important factors, because they have the same impact no matter how long the diver has remained at that altitude.

Summary: Divers who arrive at altitude from a lower altitude have residual nitrogen in their tissues, as if they had already done a dive. A first dive must therefore be treated as if it were a second dive. Many and perhaps most divers, however, will be at that altitude long enough before the dive to have had enough of a surface interval to eliminate that that problem.

Continued in the next post.
 
Last edited by a moderator:
I have all my notes from the NAUI High Altitude Diving Conference in the fall of 1974. I am going to put them into PDF format and post them here.

Better yet, please post it in a separate thread in Basic, Advanced, or even the History forum and add a link to it here. That will allow use to promote that thread to the ScubaBoard Download Library. Just report the thread and request moving it to the Download Library. There's a review process but it is the perfect content for it.

The link to it that you add here will follow it to the Library (won't break it) and we leave a permanent redirect in the original forum it is posted.
 
Theoretical depth tables like this one are not hard to find, and I assume they are used in all recreational level altitude diving classes today.

What I have noticed at all levels of diving, including technical diving, is that people who know about these tables don't understand why they exist. When I was a student in an agency that said that divers do not need to adjust for altitude, when I argued that they did, even my fellow students who were in science fields argued that there was nothing to be concerned about, since at any depth, ambient pressure at altitude is less than at sea level, so therefore diving at altitude is safer.

These people did not understand that the key factor was not the pressure at any one depth, it was the gradient between the pressure at depth and the pressure at the surface that mattered, and since water weighs the same at altitude as at sea level but the air pressure is less, that gradient is greater at altitude. That is the gradient that gets you into DCS trouble, and that is why theoretical tables like this one were created.
According to the original theory by Haldane, what matters for causing DCS is the pressure RATIO, not pressure GRADIENT...
The pressure gradient is not affected by altitude: ascending from 10 m to surface always provides a pressure difference of 1 bar, hence the gradient is 1/10=0.1 bar/m. This both at sea level and at an alpine lake, where the atmospheric pressure is, say, 0.5 bar instead of 1.0 bar.
Instead the pressure ratio is 2/1=2 at sea level, and 1.5/0.5=3 at the alpine lake.
So, if the diver spends at least 24h at altitude before diving, he could simply use normal deco tables NDL limits, entering depth values multiplied by 1.5.
But instead if she/he ascends to the lake just before diving, the body is significantly already over-saturated, so diving becomes even more dangerous.
 
Accotding to the original theory by Haldane, what matters is the pressure RATIO, not pressure GRADIENT...
The pressure gradient is not affected by altitude: ascending from 10 m to surface always provides a pressure difference of 1 bar, hence the gradient is 1/10=0.1 bar/m. This both at sea level and at an alpine lake, where the atmospheric pressure is, say, 0.5 bar instead of 1.0 bar.
Instead the pressure ratio is 2/1=2 at sea level, and 2/0.5=4 at the alpine lake.
So, if the diver spends at least 24h at altitude before diving, he could simply use normal deco tables, entering depth values doubled.
But instead if she/he ascends to the lake just before diving, the body is significantly already over-saturated, so diving becomes even more dangerous.
In my research and experience, I have not encountered anything like what I understand you to be saying. Perhaps you could point me to a reference.
 
Accotding to the original theory by Haldane, what matters is the pressure RATIO, not pressure GRADIENT...
The pressure gradient is not affected by altitude: ascending from 10 m to surface always provides a pressure difference of 1 bar, hence the gradient is 1/10=0.1 bar/m. This both at sea level and at an alpine lake, where the atmospheric pressure is, say, 0.5 bar instead of 1.0 bar.
Instead the pressure ratio is 2/1=2 at sea level, and 2/0.5=4 at the alpine lake.
So, if the diver spends at least 24h at altitude before diving, he could simply use normal deco tables, entering depth values doubled.
But instead if she/he ascends to the lake just before diving, the body is significantly already over-saturated, so diving becomes even more dangerous.
Hm, this I have not seen. Where did you get this information?
 
Accotding to the original theory by Haldane, what matters is the pressure RATIO, not pressure GRADIENT...
The pressure gradient is not affected by altitude: ascending from 10 m to surface always provides a pressure difference of 1 bar, hence the gradient is 1/10=0.1 bar/m. This both at sea level and at an alpine lake, where the atmospheric pressure is, say, 0.5 bar instead of 1.0 bar.
Instead the pressure ratio is 2/1=2 at sea level, and 2/0.5=4 at the alpine lake.
So, if the diver spends at least 24h at altitude before diving, he could simply use normal deco tables, entering depth values doubled.
But instead if she/he ascends to the lake just before diving, the body is significantly already over-saturated, so diving becomes even more dangerous.

I too would like to see a source for this.
 
Accotding to the original theory by Haldane, what matters is the pressure RATIO, not pressure GRADIENT
For some parts of deco theory, that is true. That's why it's more dangerous ascending from 100 ft at 10,000 ft altitude than at sea level. The atmospheric pressure at the surface of a 10k ft lake is about 0.68 atm, so the pressure at depth is 0.68 + 100/34 = 3.62 atm. An air bubble increases by a factor given by the ratio: 3.62 / 0.68 = 5.33x. At sea level (but still fresh water), pressure at depth is 1 + 100/34 = 3.94 atm, so expansion is 3.94 / 1 = 3.94. Relative to the ocean expansion, the altitude expansion is over 35% greater.

However, gas moves because of a pressure gradient (i.e., difference). In fact, almost everything in nature works this way from the air conditioner in your house (thermal energy moving, a.k.a., a temperature drop) to you trying to fill a pony cylinder off of your main tank. I'm sure you know that trying to get gas to move from a 200 bar cylinder to a 200 bar pony would be a futile endeavor. Moving gas from tissue into blood is no different, and it just won't move without a gradient.
 
In my research and experience, I have not encountered anything like what I understand you to be saying. Perhaps you could point me to a reference.
Wikipedia should be enough:
See the concept of "critical supersaturation ratio".
According to original Haldane's theory, no DCS occurs if the diver halves the pressure at which it was saturated.
Further theories, such as the one by Buhlmann, refined further the concept, assigning tissue-dependent limit values of the critical supersaturation ratio, instead of keeping a factor of 2 for all tissues. All this is widely explained in diving manuals, it was in my first manual at the OW course in 1975...
My point, however, was not negating the importance of pressure gradients in determining the speed of gas transfer. My point was that the pressure gradients in an alpine lake are substantially the same as at the sea level, and the gradient does not depend on the elevation of the lake. Instead, the pressure ratio (for what it does matter, in this case the insurgence of DCS) changes significantly, depending on the lake's elevation. Here an online calculator which provides the atmospheric pressure at a given elevation:
The pressure gradient underwater is governed by the Stevin's law:
grad(p)=rho*g
where rho is water density (roughly 1000 kg/m3 in SI units in fresh water, 1033 in sea water) and g is the gravity acceleration (9.81 m/s2, often rounded to 10).
Hence the pressure gradient, in fresh water, is 1000*9.81=9810 Pa/m, or roughly 0.1 bar/m, if you prefer.
The only difference between sea and an alpine lake is fresh water instead of salt water. The gradient in salt water is slightly larger, due to the higher density of salt water. @boulderjohn seemed to suggest that the pressure gradient in an alpine like is different than at sea level, which is not...
So the pressure gradient does not explain the problems with decompression and DCS in alpine lakes.
Instead, it is the Haldane's critical supersaturation ratio which explains the problem.
The deco formulas need to be revised, taking into account the modification of critical supersaturation ratios caused by the fact that the atmospheric pressure at surface is smaller than at sea level, and possibly also the fact that the diver did reach the lake just a short time before diving, so he is still carrying an amount of Nitrogen which is in supersaturation with regard to the new ambient pressure.
Actually, according to the Haldane's theory, one should experience DCS if ascending quickly to an elevation where the atmospheric pressure is less than half the normal atmospheric pressure of 1 bar at sea level. This occurs at an elevation of 5500 m (18000 ft). That's the reason for which the cockpit of airplanes flying higher need to be pressurized.
 
That's the reason for which the cockpit of airplanes flying higher need to be pressurized.
Partial pressure of oxygen is another reason, which kicks in sooner at 12,500 ft / 3810 m. Pressurizing the plane handles both issues.
 
The deco formulas need to be revised
I believe that was one of Buhlmann's contributions, whose ideas were tested in Lake Zurich at altitude. However, I would expect Haldane used the concept of surface pressure. It was merely that that variable evaluated to 1 in the application he cared about and thus in his formulas.
 
https://www.shearwater.com/products/swift/

Back
Top Bottom