Quiz - Skills & Environment - Diving at Altitude

[BRT]

I believe the survey question assumes the diver is saturated at the surface higher altitude. Increasing water pressure with increasing depth does not change regardless of how high (or low) the body of water is relative to sea level, assuming the density of water doesn't change relative to the same type of water at lower altitudes. The amount of uptake of inert gas in a tissue partly depends on the pressure difference between the existing pressure (which is lower than sea level at the surface) and the new pressure at depth. This pressure difference is greater at altitude than it would be at sea level so more inert gas is absorbed hence more nitrogen narcosis at shallower depths.

So by being able to raise the PP of N2 more, in spite of the PP of N2 being lower, you cause more narcosis? This would mean that you would get more narcosis by going from the surface to 100' than you get by going from 100 feet to 130 feet.

 

[EFX]

I think the answer implies that if you start at the surface at a higher altitude and descend to some depth and stop there, the same level of narcosis will be felt at a shallower depth than at starting at sea level with all other things being equal.

 

[tursiops]

I think the answer implies that if you start at the surface at a higher altitude and descend to some depth and stop there, the same level of narcosis will be felt at a shallower depth than at starting at sea level with all other things being equal.

Do you think narcosis depends on more than PPN2 (and CO2 buildup)?

 

[EFX]

I'm not sure what you are asking here. The ppN2 and CO2 buildup are included as those things remaining equal. I'm assuming that the CO2 buildup is the same for a diver diving from a lake at sea level to a high mountain lake, and the ppN2 is also the same: 0.79 atm for air at 34 ft fresh water.

 

[boulderjohn]

I'm not sure what you are asking here. The ppN2 and CO2 buildup are included as those things remaining equal. I'm assuming that the CO2 buildup is the same for a diver diving from a lake at sea level to a high mountain lake, and the ppN2 is also the same: 0.79 atm for air at 34 ft fresh water.

Ambient pressure is a combination of the weight of the water and the weight of the atmosphere. At 6,000 feet elevation, the atmospheric pressure is about 0.8 ATA. At 34 FFW, ambient pressure is 1.8 ATA. ppN2 at 34 feet is 1.58 at sea level. It is 1.4 at 6,000 feet.

You are right that water weighs the same at any altitude, but ambient pressure always includes atmospheric pressure, and that is the real danger in diving at altitude. When you are diving at 100 FFW at 6,000 feet elevation, ambient pressure is not much less than than it would be at sea level, because the ambient pressure is created primarily by the weight of the water. Thus, at that point in a dive, a diver would have nearly the same amount of gas loading as at sea level. As the diver begins to ascend, it is still not that much different, so in the initial stages of ascent, off-gassing is similar to sea level. As the diver gets closer and closer to the surface, though, the portion of total pressure created by the atmosphere becomes more and more important, and the pressure gradient becomes greater and greater than it would be at sea level. The last 34 feet of ascent are significantly different in the change in pressure. When the diver surfaces, ambient pressure is 20% less than at sea level, so the diver's tissue pressure must be less as well.

If you go to a program like multi-deco and compare deco dives at altitude to the same dives at sea level, you will see the practical effect of what I just described. The ascent profiles will be almost identical until you reach the last shallow stops, at which point the altitude decompression stops are extended to get the body's tissue pressure lower relative to surface pressure.

Going back to narcosis, there is no reason that it would be more significant at shallower depths at altitude--for the same reason.

 

[EFX]

Going back to narcosis, there is no reason that it would be more significant at shallower depths at altitude--for the same reason.

If that's the case then why is the answer d. all of the above, which includes the choice:

a. nitrogen (sic) narcosis may occur at shallower depths.

 

[tursiops]

If that's the case then why is the answer d. all of the above, which includes the choice:

a. nitrogen (sic) narcosis may occur at shallower depths.

Because the answer is wrong.
I don't have a copy of the most recent PADI IDC Dive Theory exams, but I'm sure someone on here does.

 

[John C. Ratliff]

I tried to vote for both B and C, but the program wouldn't let me choose two answers. I chose C because I still use a Bourden tube depth gauge, and corrections need to be made to that type of depth gauge when you get to altitude. So I had to choose D, even though I had never heard of nitrogen narcosis being different at altitude. At 4000 feet, the Bourden tube depth gauge (oil-filled depth gauge) reading was shallow by 4.7 feet.

I went to the NAUI High Altitude Diving Conference, took extensive notes, and sent them to William L. Burner, III, Major, USAF and our Staff Surgeon. I was at the time a Staff Sargent in USAF pararescue, and was sent by our unit, the 304th Aerospace Rescue and Recovery Squadron (Reserve) to get this information. Our rescue mission could involve diving at altitude. I recently re-discovered my notes from that conference, in which we heard details from Dr. Al Behnke, Dr. Stuart Mackay, and Dr. Richard Bell. It occurred on August 24-25, 1974. This was at Lake Tahoe, as I recall. Bob Tolar was Conference Representative from NAUI, according to my notes. From my notes, during which I was quoting Dr, Behnke:

The effects of narcotic substances are additive. Two substances combined will produce twice the effect. This is not a problem with nitrogen narcosis while diving at altitude however. Since the pressure is actually less, diving on air at altitude won't increase the likelihood of nitrogen narcosis.

I have just discovered this part of my notes while reading through them. Here is another jewel of a saying from Dr. Behnke:

In making and modifying recompression tables, Dr. Behnke stated the Golden Rule for this operation. You don't try out anything on anyone which you don't try on yourself. In this way, mistakes aren't repeated!

SeaRat

 

[TMHeimer]

Because the answer is wrong.
I don't have a copy of the most recent PADI IDC Dive Theory exams, but I'm sure someone on here does.

Think I agree. I was sure about B & C, but doubted A, same as John C. did. That left only voting D to get it marked correct, whether D was in fact correct or not.

 

[NAM001]

Deep Adventure Scuba Colorado Technical Diving - For Those Looking to Take Diving to a New Level
The article on my resource page should answer a lot of the questions, but I will answer some of them quickly here:
1. The 2 pressure groups per 1,000 feet of altitude only works with PADI tables.
2. It starts when you reach that altitude. If you arrive at 6,000 feet from sea level, you are an L diver, and a 2:10 surface interval later you are an A diver.
3. Since you spent some time ascending while getting there, you were off-gassing on the way, but there is no way to calculate that.
4. Putting 2 and 3 together, the odds are you won't have to worry about it.
5. If you got there from a higher altitude, you are already off the table, much to the surprise of the local dive shop here that tells the students coming from our mile-high altitude that they will be J divers when they descend to 4,600 feet to dive in New Mexico.
6. I don't know of any computer that calculates your change in altitude before you have done your first dive. Some software programs do that (like multi-deco).
7. The first Shearwater computers did not calculate altitude at all. A friend of mine bought one and contacted the company because the depths seemed wrong. As they talked to him, they realized he was diving at altitude and made that adjustment in their computers. Years later, during a weekend of diving at altitude, our group noticed that different divers were getting different depth readings on our Shearwaters. I contacted them, and they figured out that the difference was in how the computers turned on. If we turned the computer on before diving, it knew we were at altitude. If the computer turned on by itself after the dive started, it thought we were at sea level. They then corrected that so that now Shearwater computers that turn on after being submerged will go back to their last altitude reading.
8. Suunto RGBM computers used to (maybe still do) require manual settings for altitude. They gave you the choice of 3 ranges.
9. When you start getting over 10,000 feet, things get serious, and you should do some real investigation before doing anything but shallow dives.

tHANK YOU SO MUCH for your response. Hopefully there will be many that can use your answers to the issues I presented to stop and think a bit before just doing things as normal. Further more see just how their computer handles this situation.

In regard to the shearwaters my old predator has a set up parameter to input the pressure so I could enter 843mbar if I wanted to. later firmware updates removed that ability and used the computer pressure sensor to auto enter the current pressure. one fix generated issues only seen in altitude changes. Now the pressure info is taken formteh depth sensor locked and used in various computations but there are differences whether you are in tech or rec mode. I see those same differences in my petrels also.

Very important to turn on the shearwater prior to hitting the water. YOU need to have it sense teh altitude pressure and lock it in prior to the auto power up gets activated. It dont make a hoot to speak of unless you have changed altitudes.

I seriously meant I had legit questions and you answered them Thanks again.