Altitude Diver

[emoreira]

If I may ask a question: At what altitude above sea level is a dive considered an "altitude dive" such that deco computations should take it into account? The lake in which people dive here in Atlanta is approximately 1000 feet above sea level. I never thought of myself as living in the mountains, but is 1000 feet significant?

The course documentation for Altitude diving says that starting in 1000 ft is considered an altitude dive, though the correction should be minimal.
And it's true, many dive schools inland, where check-out dives are done in quarries or dams, are done well over 1000 ft over sea level and no Altitude Diving card is issued.

 

[knotical]

Changing the Altitude course to classroom-only seems reasonable to me. The main difference in dives at altitude, besides the obvious one of not going as deep, is to do safety stops at slightly shallower depths.

When conducting Open Water classes in Colorado, we introduced the basics of altitude diving, but did not issue Altitude cards because that required more dives, and more classwork. For those who did go on to get the specialty, I added more emphasis on the types of questions that often come up on ScubaBoard – notably traveling to/from various altitudes before and after diving. I also introduced the effects of Nitrox at altitude. Did you know that for certain combinations of altitudes and Nitrox mixes, you can use air tables, unadjusted? ( see my latest blog entry at: http://www.scubaboard.com/forums/blogs/knotical/583-nitrox-altitude-can-cancel.html )

 

[boulderjohn]

As is often the case, I agree with knotical. In his past location, we were not far apart geographically. When I conduct OW dives at altitude, we discuss the corrections as a part of the planning for the dives. Under standards, students are required to participate in the planning, and I included altitude adjustments for this. When my present shop does OW dives locally, we include a page-long summary of altitude considerations and adjustments for all students. I don't see how you can have students participate in the dive planning at altitude without talking about altitude adjustments.

As for the specialty, I don't think dives are necessary, for the same reason they are not considered necessary for nitrox. However, since those are the rules at the present, then those are the rules.

 

[DivemasterDennis]

I agree that the altitude dive specialty is best done with dives. Dive planning is a big part of safety, as is the experience of the impact on altitude in individual respiration, fatigue and exertion levels. It's about diving, not piling up a bunch of C-cards. I for one thought Nitrox should have kept the open water dives for the same reasons- dive planning and execution. Therre is a difference in diving at altitude, both in profile planning and in the impact on the diver. I do not understand those who want to limit the quality of a new dive experience, leading to a new diving competence.
DivemasterDennis

 

[XS-NRG]

I have an interesting question for the altitude experts. Let's say you live in San Fran at sea level and you are flying to the middle of the country to a ficticious high altitude lake called "Altitude Lake" and it's at 5000 FT. The flight it 6 hrs 30 mins. Going from sea level to 5000 FT is obviously a 5000 FT climb and according to PADI that would be 10 pressure groups and you would be a "J" and then the SIT time would start before diving.

Now, here's the twist: most airplanes fly around 30,000 FT but they are pressurized to the equivalent of approx 5000 FT (so you can breathe during the flight of course). Assuming that pressurization occurs in the first 15-30 mins of the flight (or sooner) you would effectively go from sea level to 5000 FT quickly then your SIT time would be the remainder of the flight (6 hours). Would that count as a ground level SIT and bring you off the tables? Would you effectively have zero residual at all? Let's say the lake is at 2000 or 3000 or 4000 you should still have zero residual and if the lake was higher (say 7000 FT) you would go from 5000-7000 upon landing and therefore only be a "D". I'm not saying this is a good rule of thumb or practice but it certainly is a curious thought. What do you think? Yes I know that there could be a problem or a particular plane doesn't pressurize all the way to 5000 but assuming a perfect situation would that not hold true?

 

[boulderjohn]

I have an interesting question for the altitude experts. Let's say you live in San Fran at sea level and you are flying to the middle of the country to a ficticious high altitude lake called "Altitude Lake" and it's at 5000 FT. The flight it 6 hrs 30 mins. Going from sea level to 5000 FT is obviously a 5000 FT climb and according to PADI that would be 10 pressure groups and you would be a "J" and then the SIT time would start before diving.

Now, here's the twist: most airplanes fly around 30,000 FT but they are pressurized to the equivalent of approx 5000 FT (so you can breathe during the flight of course). Assuming that pressurization occurs in the first 15-30 mins of the flight (or sooner) you would effectively go from sea level to 5000 FT quickly then your SIT time would be the remainder of the flight (6 hours). Would that count as a ground level SIT and bring you off the tables? Would you effectively have zero residual at all? Let's say the lake is at 2000 or 3000 or 4000 you should still have zero residual and if the lake was higher (say 7000 FT) you would go from 5000-7000 upon landing and therefore only be a "D". I'm not saying this is a good rule of thumb or practice but it certainly is a curious thought. What do you think? Yes I know that there could be a problem or a particular plane doesn't pressurize all the way to 5000 but assuming a perfect situation would that not hold true?

Actually, I think planes are pressurized to 7,000-8,000 feet. I contacted DAN with a similar question, and since their altitude guidelines are for ascents greater than 2,000 feet, they said it is safe to fly immediately after diving at a lake at, say, 6,000 feet.

The problem with answering your question definitively is that the altitude guidelines are not well studied, and there are conflicting opinions from different organizations. What follows is my take on your question, and I suspect some will disagree. I believe the countdown starts as soon as you reach altitude. In your example, I would indeed count the plane flight. Similarly, I live at 5,400 feet, and when I dive in one of of lakes at 10,000 or more feet, I take into account the elevations I encounter on my drive.

But driving brings into account another factor--unlike flying with its direct ascent, your drive may take you through a series of altitude changes and leveling off--in effect, a series of decompression stops. To my knowledge, this has not been studied at all. When I drove from Kona on the west shore of the big island in Hawai'i to the volcano on the east side, it took a number of hours to get there, with a small ascent followed by a long leveling off, followed by an ascent with a long leveling off, etc. How do you take that into account?

Divers in Colorado frequently dive in Santa Rosa, NM, and they have to drive home over Raton Pass, an elevation a little over 3,000 feet higher than their dives. The drive from Santa Rosa starts with an immediate elevation gain that is within the DAN guidelines, and then is roughly level for a couple of hours (decompression stop?) before the final climb up the pass. Technically, the drive home violates DAN/PADI guidelines, but many thousands of divers do it every year. The only one I know who ever got significantly bent after such a drive was my dive buddy for the weekend. We both drove home together, breathing O2 as we drove, running out of it at the very top of Raton Pass. The next morning I was just fine, and he was on his way to the chamber. The only difference between us was that when we got back to Boulder, I went home at that altitude, and he went to his home at about 8,000 feet. Was that the difference? Who knows?

 

[Panama Jones]

Actually, I think planes are pressurized to 7,000-8,000 feet. I contacted DAN with a similar question, and since their altitude guidelines are for ascents greater than 2,000 feet, they said it is safe to fly immediately after diving at a lake at, say, 6,000 feet.

The problem with answering your question definitively is that the altitude guidelines are not well studied, and there are conflicting opinions from different organizations. What follows is my take on your question, and I suspect some will disagree. I believe the countdown starts as soon as you reach altitude. In your example, I would indeed count the plane flight. Similarly, I live at 5,400 feet, and when I dive in one of of lakes at 10,000 or more feet, I take into account the elevations I encounter on my drive.

But driving brings into account another factor--unlike flying with its direct ascent, your drive may take you through a series of altitude changes and leveling off--in effect, a series of decompression stops. To my knowledge, this has not been studied at all. When I drove from Kona on the west shore of the big island in Hawai'i to the volcano on the east side, it took a number of hours to get there, with a small ascent followed by a long leveling off, followed by an ascent with a long leveling off, etc. How do you take that into account?

Divers in Colorado frequently dive in Santa Rosa, NM, and they have to drive home over Raton Pass, an elevation a little over 3,000 feet higher than their dives. The drive from Santa Rosa starts with an immediate elevation gain that is within the DAN guidelines, and then is roughly level for a couple of hours (decompression stop?) before the final climb up the pass. Technically, the drive home violates DAN/PADI guidelines, but many thousands of divers do it every year. The only one I know who ever got significantly bent after such a drive was my dive buddy for the weekend. We both drove home together, breathing O2 as we drove, running out of it at the very top of Raton Pass. The next morning I was just fine, and he was on his way to the chamber. The only difference between us was that when we got back to Boulder, I went home at that altitude, and he went to his home at about 8,000 feet. Was that the difference? Who knows?

i see all the talk about the huge number of pressure group changes to go up 5000 feet but i don't undersand.

for the purposes of deco, we will talk about depth in terms of ATM. 33 feet = 2 ATM, 66 feet = 3 ATM. 1ATM (sea level) is 14.7 PSI, 2 ATM is double that and so on,

8000 feet above sea level is 10.91 PSI. Sea level is 14.7 The pressure difference in going from sea level to 8000 in ATM is like ascending around 10FSW. And it's taking a miniumum than 15 minutes by almost any method (flying or driving) to do it more if you add the timne it takes to get to your car or plane after getting out of the water. I trust that you have to be very careful because DAN and you say I should. i don't fully get it though.

Lot's of people say that 15 feet is not a magic safety stop number. It can be 25, it can be 10. So the difference between a 25 foot safety stop then to sea level is no different (in ATM) than a 15 foot safety stop then to 8000ft. I know...I'm missing something because as long as you are not pushing the deco limits of you dive to tha very max, I don't get the danger. Not that I don't trust. Any clarifications?

 

[knotical]

i see all the talk about the huge number of pressure group changes to go up 5000 feet but i don't understand.

for the purposes of deco, we will talk about depth in terms of ATM. 33 feet = 2 ATM, 66 feet = 3 ATM. 1ATM (sea level) is 14.7 PSI, 2 ATM is double that and so on,

8000 feet above sea level is 10.91 PSI. Sea level is 14.7 The pressure difference in going from sea level to 8000 in ATM is like ascending around 10FSW. And it's taking a minimum than 15 minutes by almost any method (flying or driving) to do it more if you add the time it takes to get to your car or plane after getting out of the water. I trust that you have to be very careful because DAN and you say I should. i don't fully get it though.

Lot's of people say that 15 feet is not a magic safety stop number. It can be 25, it can be 10. So the difference between a 25 foot safety stop then to sea level is no different (in ATM) than a 15 foot safety stop then to 8000ft. I know...I'm missing something because as long as you are not pushing the deco limits of you dive to that very max, I don't get the danger. Not that I don't trust. Any clarifications?

It’s the ratio that matters, not the absolute pressure change.

Assume you were at sea level, or 1 atmosphere, and ascended enough to reduce the pressure the equivalent of 33 feet of seawater, or one atmosphere. You’d then be at zero atmospheres, and literally explode.

At 8000 feet, the pressure is a bit below 0.75 atm, a 25% drop from where you started (or a quarter of the way toward exploding).
Combine that with having recently dived and carrying a nitrogen load, it’s not surprising that DCS does sometimes occur with a modest increase in elevation.

 

[boulderjohn]

It’s the ratio that matters, not the absolute pressure change.

Assume you were at sea level, or 1 atmosphere, and ascended enough to reduce the pressure the equivalent of 33 feet of seawater, or one atmosphere. You’d then be at zero atmospheres, and literally explode.

At 8000 feet, the pressure is a bit below 0.75 atm, a 25% drop from where you started (or a quarter of the way toward exploding).
Combine that with having recently dived and carrying a nitrogen load, it’s not surprising that DCS does sometimes occur with a modest increase in elevation.

I recently had the hardest time trying to explain this to someone. If you look at any one number, you don't see the point. What you have to look at is the change from one to another. To see why diving at altitude matters, use Boyle's Law to track the increase in volume of a bubble going from 100 feet to the surface at sea level and then track the same bubble at altitude. You will see that for the first 60 feet or so of ascent, the difference isn't much to speak of, but as you get closer and closer to the surface, the bubble at altitude gets bigger and bigger than the same bubble at sea level. In the the last 15-20 feet, it really jumps.

 

[Panama Jones]

I recently had the hardest time trying to explain this to someone. If you look at any one number, you don't see the point. What you have to look at is the change from one to another. To see why diving at altitude matters, use Boyle's Law to track the increase in volume of a bubble going from 100 feet to the surface at sea level and then track the same bubble at altitude. You will see that for the first 60 feet or so of ascent, the difference isn't much to speak of, but as you get closer and closer to the surface, the bubble at altitude gets bigger and bigger than the same bubble at sea level. In the the last 15-20 feet, it really jumps.

Knotical's had me confused. But your quote and his combined...I get it. On top of that, I actually knew that and for some reason did not apply it to this situation. If that makes sense.