altitude diving pressure groups

[Thalassamania]

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.

The location is, I think, fine ... if not, a MOD will move it.

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".

So the diver leaves 300m above sea level and ascends to 750m above sea level. I take it that you are saying that upon arrival at 750m above sea level the diver is in Repetitive Group F. That may be fine, I can't tell you, most tables use some sort of group designator, but they each represent different amounts of gas and regress to a different single tissue so they can not be cross compared.

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)

Again, that makes sense, since the percentage change is likely somewhat higher.

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.

The "safety reasons" (lousy explanation) is that it is the percentage change in pressure that is important, not the linear change in meters of altitude.

I would like to know the background for this recommendation to understand it.

I hope that was clear enough.

BTW: you can use an old capillary gauge to measure your depth though it will not be accurate in terms of depth, but it will permit you to use sea level tables without correction factors because it references the atmospheric pressure where you entered the water and measures percent change (e.g., it will show 10m when you are at at twice the air pressure at the surface of the lake).

 

[DiveNav]

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?)........

Actually, this shouldn't be that difficult to simulate and in fact we are planning to add altitude diving to divePAL.
From a model point of view, the trip from home to the dive site is "just another dive" (or more correctly - it's a slow and long ascent - with few dips here and there - from a shallow dive)

......... they probably have no data to show how the other compartments interact at altitude ......

I don't know about availability of experimental data ..... but one could assume that the Buhlmann model is still applicable.

 

[Thalassamania]

Actually, this shouldn't be that difficult to simulate and in fact we are planning to add altitude diving to divePAL.
From a model point of view, the trip from home to the dive site is "just another dive" (or more correctly - it's a slow and long ascent - with few dips here and there - from a shallow dive)...

It's an ascent from a saturation dive.

 

[DiveNav]

It's an ascent from a saturation dive.

What about this: It's an ascent from a shallow saturation dive

 

[Thalassamania]

Clearly a shallow saturation dive, even less than, say: Jules Undersea Lodge.

 

[Tigerman]

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.

Rather serious typo in there unless you even worse have your math all wrong..
1000ft is NOT 3048m, its 304,8 meters. 10 000ft is 3048 meters and most places called skydiving rather than scuba diving

 

[Dive-aholic]

Rather serious typo in there unless you even worse have your math all wrong..
1000ft is NOT 3048m, its 304,8 meters. 10 000ft is 3048 meters and most places called skydiving rather than scuba diving

Looks like a typo. PADI tables are intended for use less than 10,000 feet elevation. There are areas that can be dived in the 8,000+ feet above sea level range. Not all places would consider that sky diving.

 

[boulderjohn]

Rather serious typo in there unless you even worse have your math all wrong..
1000ft is NOT 3048m, its 304,8 meters. 10 000ft is 3048 meters and most places called skydiving rather than scuba diving

My highest altitude dive so far was at 10,600 feet. My last one was at 9,700 feet. It happens.

 

[Tigerman]

The standard PADI RDP tables are actually intended for use <1000 feet (metric ones say 300m - not sure if they where made metric or imperial and which one is rounded down) before altitude adjustment - I live in the border of altitude diving myself.
And I did say MOST places 10000ft would be skydiving, not everywhere - But I guess the fact that it wasn't a very serious statement didn't get picked up...

 

[boulderjohn]

And I did say MOST places 10000ft would be skydiving, not everywhere - But I guess the fact that it wasn't a very serious statement didn't get picked up...

Relax. We're just yanking your chain.