Computer or Not

[CompuDude]

Darkpup, it's interesting to see that there apparently are good solid methods to do multilevel dives without tables. Thanks for bringing this to our attention.

That said, I prefer to avoid that much mental math whenever possible, and plan to continue letting my computer do the thinking for me. I won't risk any unusually deep or complex dive plans, however, until I have a backup computer, just in case Murphy strikes. If/when I get into serious tech diving, I may want to revisit the issue, but I do think that for MOST recreational divers computers are a better choice than trusting their math skills at depth... or on a boat.

 

[Mothball]

Darkpup, you seem to know your stuff well. I've learned a few things and you've provoked some good things to ponder here. It's interesting to read how different divers in different areas and groups think. I've only been on the board for about two hours now and I'm amazed at how many trains of thought there are out there. The few places I dive regularly in my area, pretty much all the divers have computers unless they're students. I'm a couple of light years behind you but it seems we have read much of the same material, so it was kiind of a shock to me when you stated you didn't dive w/ a computer and stated and explained exactly WHY you don't NEED one - and that's really cool. I'm gonna impliment some of what I've learned here into my pre-dive to post-dive homework (which I do religiously anyway).

But to answer up to the poll in question, I dive a VR3 and an Oceanic Versa Pro although I still pre-plan with my PADI Recreational Dive Planner (tables) and my PADI Wheel (multi-level dive planner) and recheck all my homework with what I actually dove post-dive. Semper Fi Dave

 

[TSandM]

I now have a license to learn, but am light years away from knowing.

Love that!

 

[rjack321]

I think some of the "computers give you more (multilevel) bottom time" logic comes from the fact that the Navy planned their tables dives as square profiles. Go down, remove those bolts, come up. When DSAT (padi) made up their tables (which are based on a different dataset), they used the same assuptions. When agencies teach tables, same assumptions...

But, as Jason has amply explained, you don't have to dive a square profile with the tables. By backing off from each table depth-time a few minutes, they work pretty well for multilevel dives.

None of this is quite the exact science its made out to be.

 

[darkpup]

FYI, I was a little bit light on details for the RGBM / deep stop tables we use. The concept behind it is similar to the 120 rule, but it changes as the depth gets shallower.

Here's a complete run down on the Minimum Decompression Rules we use (i.e. NDL table):

130' - 5 min
120' - 10 min
110' - 15 min
100' - 20 min
90' - 25 min
80' - 30 min
70' - 35 min
60' - 50 min
50' - 60 min
40' - 170 min

We typically use 32% Nitrox that at 100' gives us an Equivalent Air Depth of 80' or 20%. Thus 100' becomes 80', and 80' is approximately 60'. You decide how far you want to go on the Nitrox calculations to stay within your level of comfort (i.e. conservative vs. aggressive).

The reason for the 170 min time at 40' on air is because of how Bulhman and Haldane calculate the depth at which a person will start off gassing. The Buhlman model wants you to get to the shallowest depth as safely and quickly as possible to help the off gassing process. Those depths for recreational dives are usually in the 30' to 10' range, and is the basis for safety stops in those ranges. Because 40' is so shallow, the surface becomes your first safety stop, thus the time at that depth before reaching a level of saturation that requires a safety stop in water is quite large.

I'll be working on a mapping project this weekend where the total BT will be more than 90 minutes at an average depth between 40' to 50' (max depth maybe 60') on 32%. We could easily extend that bottom time by carrying bottles of 100% Oxygen for deco at 20', but the level of complexity and danger with on gassing slow tissues and the use of 100% O2 without the proper training / experience becomes more trouble than it's worth.

As for the math involved, I'll leave you with this. There's a big difference between Math and Scuba Math. It all boils down to Accuracy vs. Precision vs. Practicality. The decompression models aren't that precise to begin with, and are different for each person. Thus the Scuba Math we use is a bit lax on the Precision because the level of Accuracy is broad, and the Practical implementation of the math (i.e. being able to hold your safety stop in adverse conditions) is more important.

Hope this helps to continue the conversation.

~ Jason

 

[rjack321]

Haldane would say you can stay at 33' indefinately. His premise was that you could safely halve your ATAs without getting bent.

 

[darkpup]

Haldane would say you can stay at 33' indefinately. His premise was that you could safely halve your ATAs without getting bent.

Richard,

Do you remember what it was for Bulhman, the Navy tables, or RGBM? I assume it's similar to Haldane, but can't remember off the top of my head.

~ Jason

 

[TSandM]

Navy Tables (according to Lippman, Deeper into Diving) 30 feet - 405 minutes. Same source, Buehlmann 400 minutes. DCIEM 30' - 300 minutes. RDP - 360 minutes. NAUI RGBM - 150 minutes. Same source, computer limits: Vytec 204 minutes, Aladdin Pro 334, Cochran Commander 599 minutes.

 

[rjack321]

Do you remember what it was for Bulhman, the Navy tables, or RGBM? I assume it's similar to Haldane, but can't remember off the top of my head.

Unfortunately, I don't exactly. But I have a nagging feeling that all these latter decompression investigations didn't try and come up with Haldanean-like "1/2 ATA" rules. The 1/2 thing is partly a historical artifact of his methods, he was working with goats in diving bells. Remarkably cutting edge deco science for 1900.

Buhlman M-values, would actually push you shallower than 1/2 your ATAs.

(Lynn beat my post) I now realize I might not have understood the question. Are you thinking the depth at which you can stay indefinately or about the 1/2 ATAs part?

 

[darkpup]

Navy Tables (according to Lippman, Deeper into Diving) 30 feet - 405 minutes. Same source, Buehlmann 400 minutes. DCIEM 30' - 300 minutes. RDP - 360 minutes. NAUI RGBM - 150 minutes. Same source, computer limits: Vytec 204 minutes, Aladdin Pro 334, Cochran Commander 599 minutes.

Wow! If nothing else, it clearly shows how broad the differences are from model to computer to table. I'm also hoping some will see how depth averaging doesn't have to be an exact science (i.e. Precision).

Lynne, do you have similar numbers for a dive with an average depth of 60'?

I'd be curious to see how each source differs from one to the next.

~ Jason