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.
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:
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.
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 NitrogenBeginning 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.
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