Altitude Theoretical Pressure Group Calculations

[Robert Storrie]

I have a question and someone out there might have an answer. For a altitude dive at Lake Tahoe with an elevation of 7,000', how would you calculate your pressure group when there are none listed in my current tables? Took a deep Diver certification and hit 119' actual feet on dive computer, theoretical depth was 151', what is the equation for finding out the pressure group for the theoretical depth for a dive that was 29 minutes, although most of the dive was in shallower water, not at 119'. Appreciate if someone can shoot me an answer to calculating these dives, or a link. Appreciate it.

 

[KenGordon]

Use tables designed with this in mind, such as the BSAC88 tables. No fudges, just sets of different tables for different altitudes. There is a whole lot of complication, especially while travelling and changing altitude.

Or use planning software, Multideco definitely does this. Either plan for a given surface interval or use a phone/tablet to replan for subsequent dives.

Keep in mind that if you travel afterwards you will add another thousand feet (or three) leaving. To be safe with that you should plan the dive at that higher altitude of give it a good while first.

Try to mitigate all the usual sources of bad deco outcomes, hydration, repetitive diving, poor ascent control, late nights drinking, cold, and exertion when you get out.

 

[DivemasterDennis]

It's not that hard. First, you need altitude tables which you can get on line from Padi. Look at your actual depth, convert it to theoretical depth, then calculate pressure group based on theoretical depth. That's the textbook way. That's what you did- imperfect, have to do some guessing. But do you remember the wheel? Calculate pressure group based on multiple levels in the dive. AH, the good old days of the wheel, and cursive writing, and , s-it, I really am an old fart.
DivemasterDennis

 

[tursiops]

I have a question and someone out there might have an answer. For a altitude dive at Lake Tahoe with an elevation of 7,000', how would you calculate your pressure group when there are none listed in my current tables? Took a deep Diver certification and hit 119' actual feet on dive computer, theoretical depth was 151', what is the equation for finding out the pressure group for the theoretical depth for a dive that was 29 minutes, although most of the dive was in shallower water, not at 119'. Appreciate if someone can shoot me an answer to calculating these dives, or a link. Appreciate it.

There are several issues here.
First, 119 ft actual at 7000 ft is too deep for recreational diving.....your theoretical depth is 150-160 ft (depending on whose correction factors you use), which is beyond the 130 usual limit, and even beyond the 140 extreme limit.
Second, the NDL time for 120 feet actual is just 13 minutes (PADI) or 15 minutes (USN). For 150 ft it is beyond the PADI tables, and is only 5 minutes on the Navy tables. For a 29 minute dive, you are into serious decompression diving; the USN tables would require a 2-minute stop at 30 ft, an11-minute stop at 20 ft and a 25-minute stop at 10 feet, before surfacing.
Third, there is no "equation" for finding a pressure group; the pressure group is based on calculating the tissue pressures in multiple compartments, all with exponential uptake and release of nitrogen.
Fourth, diving to 119 ft actual at 7000 ft is WAY beyond the standards for a Deep diver course. that course should stay below 130 ft, which at 7000 ft altitude is shallower than 100 ft actual.

Now, whether you were actually in jeopardy on this dive depends on your profile. Points 1 and 2 above may be irrelevant for the right kind of multi-level profile. Let's hope.
Point 3 suggests you need some more knowledge.
Point 4 suggests you need a new instructor, and the one you had for this class should be reported and penalized.

 

[Bob DBF]

My computer, which is 10 or so years old and low end, automatically corrects to altitude. I would be supprised if yours did not correct and give you the theoretical depth on the display.

It is easier for me to believe there is some confusion of the facts on your part than an instructor violating standards to that degree.


Bob

 

[tursiops]

My computer, which is 10 or so years old and low end, automatically corrects to altitude. I would be supprised if yours did not correct and give you the theoretical depth on the display.

It is easier for me to believe there is some confusion of the facts on your part than an instructor violating standards to that degree.


Bob

I hope you are right! At 7000 ft, an actual depth of 94 feet would give 119 theoretical. On tables you'd call that 100 ft for a theoretical depth of 129 ft. Bingo! Just inside the Deep diver max of 130.

It is still too much bottom-time for square-profile tables, but likely just fine for a multi-level dive, especially using a computer.

 

[Frontpointer1000]

Robert (OP):

What was the resolution of this issue? It's certainly interesting - I'd like to know what you discovered. Did you dive more shallow than you thought? Also - I am assuming you were diving air. You weren't on nitrox, right? Not that nitrox allows you to dive deeper, but it can change your acceptable NDL profile. if on 32 % EAN with an actual depth of 119 feet and an assumed PO2 of 1.6 you could theoretically go to 132 feet. Just wondering.

Incidentally, Lake Tahoe elevation is 6,224 feet. Based on that, I get a TD of 146 feet if AD was 119 feet.