altitude diving pressure groups

[Agility]

I am not sure this belongs into "advanced" it does seem a rather basic question, on the other hand it might not really hold interest for too many users.

In the altitude speciality course I did this weekend we talked about pressure groups on arrival at the dive site.
Coming from under 300m above sealevel 2 pressure groups per 300m ascent are added.
So a diver living at 280m who went to 750m would use pressure group F at the beginning of the "surface interval".
But a diver acclimatized at 1480m and ascending to 1950m would have to add 4 pressure groups per 300m placing him in pressure group L. (we calculated this example)
Can anyone explain the rationale behind this rule? After all the ascent is 470m in either case.
My diving instructor said it was for safety reasons.
I would like to know the background for this recommendation to understand it.

[Crush]

The reason that the "rule is for safety" is a poor explanation.

Water is essentially incompressible. Because of this fact, its density, and the mass of the earth the net result is that the pressure exerted on a diver increases by 1 atm for every 30 feet you descend. Put another way, a plot of pressure vs depth is linear.

On the other hand, air is compressible. For this reason a plot of atmospheric pressure vs altitude is slightly non-linear:



I am not sure that this explains the large difference in in the tables you described....

[Agility]

The PADI tables are only intended for use <1000ft (3048m) I presume because of rather few dives at higher sites.
If the compressibility of air is the reason I wonder whether the linear "2 pressure groups per 300m" really is the right approach.

[Thalassamania]

I assume that is is a "rule of thumb" that provides a linear answer that always stays on the "safe" side of the curve.

[boulderjohn]

To provide a true and accurate way of calculating your pressure group upon ascent would require both complexity and the results of detailed scientific experiments that have never been performed. To give you a sense of the complexity, an accurate measure would need to know at what altitude you started your ascent, how long you were at that altitude prior to starting, how long it took you to make that ascent, and what your ascent profile was like. Did you ascent 1,000 feet immediately, stay at that altitude for a half hour, go up another 500 feet, etc. There has really never been any study of this that I know of.

I do not want to present myself as a real expert on this, but I have studied what we do know about diving and altitude enough to believe that the process described here is essentially a guess. Understand that if you are going to ascend to altitude prior to diving, you do indeed have some residual nitrogen in your body. You might as well follow the guidelines you were given because nothing better is available.

[boulderjohn]

To give you an idea of the difficulty in making an accurate calculation, let me describe a dive I did about a month or so ago.

I left my home at about 5,440 feet and drove 40 minutes to a place where I met two other friends for the purpose of carpooling. That was at about 8,230 feet. It took us over a half hour to get together and get started. (What is my pressure group now?) We drove through the foothills, ranging between 8,000-9,000 feet and began climbing the real mountains. It took us about a half hour to reach 11,000 feet. (What's my pressure group now?) We then descended fairly quickly before going up and down over mountainous terrain for an hour or so until we reached our destination at 9,700 feet. (What's my pressure group now?) There we met up with the other divers we were diving with. We chatted for a while while we got our gear out of the vehicles and began setting up. By the time we were ready to get in the water, at least an hour had passed, probably much more. (What's my pressure group now?)

We weren't using standard tables for our dives so we did not even try to make a calculation. If I had taken a helicopter from sea level to that altitude, I would have been an S-Diver on the PADI tables. That's the worst case scenario. Even if I was a PADI S-diver when I had reached the approximate altitude of my dive while still driving, given the final amount of travel and setup time, I was an A diver by the time I got in the water. Given the fact that I started at 5,440 and did a series of deco stops along the way, there was no way it was a factor. I didn't give it a second thought.

I do believe that altitude needs to be considered in diving, but I think it is a really rare situation where someone ascends rapidly enough and then gets in the water quickly enough to make it an important consideration prior to the first dive.

[DivemasterDennis]

As a Colorado diver who dives and trains other divers here, diving at elevations from 5000 to 9500 feet above sea level, altitude tables are essential. In calculating a dive, the tables add theoretical depth depending on altitude at the surface and depth to which you dive. A dive to 50 feet at a site that is 6000 feet at the surface is a "theoretical depth" of 62 feet. Doing that same dive at 9500 feet (rounding up) makes it a theoretical depth of 73 feet. Get the theoretical depth and us it using rdp tables in planning each dive and setting ndl limits and you are safer than any "rule of thumb." Many (most/) computers will recognize or can be programmed for altitude diving variations, but I am in the habit of doing the dive planning from tables as well. The reason for the "theoretical depth" is to compensate for the the less than 1 atm pressure at the surface. Slow ascents are also all the more important when diving at altitude for that same reason, and 30 feet per minute is a maximum for safe ascents. The altitude at which you live and your acclimation to altitude has nothing to do with the need to use the altitude adjustments. As a result, I think Agility's instruction was in error. Pressure changes above the surface, even when driving over the highest mountain passes (and they are all here in Colorado) are so gradual that they have negligible, in fact, undiscernable impact on gasses enter the blood stream. Get your altitude tables, follow them in all dive planning for diving sites over 1000 feet in elevation, and you will ALWAYS be safe. Much better than any misinformed persons "rule of thumb."
DivemasterDennis scubasnobs.complanning over

[knotical]

Agility, I think the heart of your question is "why does the rule of thumb penalize us more at higher elevations for the same absolute change in altitude?"

A small part of the answer lies in the fact that nitrogen absorption and off-gassing are dependent on relative, not absolute pressure differences.

When we climb from sea-level to 1000m, the proportional pressure drop is not quite as great as it is from 2000m to 3000m (as Crush's graph shows).

But since these proportional changes are only slightly different, it doesn't seem nearly enough to double the penalty. I speculate that it is added conservatism arising from a fear of altitude sickness.

As others have pointed out, there are a lot of unknowns and uncertainties, so some extra conservatism probably doesn't hurt.

[Bob DBF]

As I understand, the reason is to give time for the nitrogen to to offgas from your body at the lower pressure at altitude (see the graph Crush has provided), just as you use the tables to give time for the nitrogen to to offgas from your body after diving. Since the PADI tables are based on the 60min tissue compartment controling, they probably have no data to show how the other compartments interact at altitude and use the 2 pressure group/1000' as a safety buffer.

My trip would be from sea level over the pass and down to the water at 6225', I start my time at the pass since I will never dip below water level. Since I could be in the water within 2 hours of starting the trip, it makes a difference.

The "theoretical depth" DivemasterDennis discusses is more important overall but acclimating to altitude should not be overlooked.

Use a vintage capillary depth gauge for accurate depth.




Bob
-----------------------------
I may be old, but I’m not dead yet.

[Dive-aholic]

Can anyone explain the rationale behind this rule? After all the ascent is 470m in either case.
My diving instructor said it was for safety reasons.
I would like to know the background for this recommendation to understand it.

Your instructor was partially correct. Although, I doubt he knows why he was correct if that's all he told you. The main reason is because there is very limited testing done in regards to altitude diving. There's a lot of theory. There's even a book written by Bruce Wienke (caution - while it's a very informative book, it is very dry and could put you to sleep). "Approved" dive practices have been tested on actual divers before they are recommended by the various agencies. There has been very limited testing done in the area of altitude diving, so for safety reasons, the recommendations are very conservative.