Eric Sedletzky
Contributor
Thanks for being honest.
I’ll research it and see if there’s any science behind that.
Do the dive tables have a limit on the number of dives per day?
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I’ll research it and see if there’s any science behind that.
Fascinating! I just played around with some hypothetical dives on the tables, and it really does make a difference if you do the deep one first. But that really just pushes the question back one level--why do the tables treat the same two dives differently depending on the order in which they're done? And do computers do this too?
I won't even try to guess at the real reason, but I can think of a defensible one. Back in college when we rationalized binge drinking as normal, everyone had their theories about how to avoid puking or being destroyed by the hangover without actually drinking less. One popular theory was "beer before liquor, you've never been sicker; liquor before beer, you're in the clear." I never put much stock in it; surely what mattered was the total amount you drank and the time it took you, not the order in which you had those drinks. But in time, I came to see a certain logic to it. Obviously you get drunk faster doing shots than drinking beer. But because of the delay between when any given sip passes your lips and when it hits your bloodstream, you get drunk even...faster-er?... if you switch to shots just as your first few drinks are hitting you. And that makes it harder to titrate to the precise level of dude-I-was-so-wasted you're aiming for without overshooting.
When diving deep, your NDL time goes fast. When doing subsequent dives with residual nitrogen, your NDL time goes fast. Doing a relatively shallow dive that still leaves you in a pressure group near the middle of the alphabet, and then doing a deep dive, is a bit like switching to liquor when you're already buzzed--you may not even see your limit as you blow past it.
Anyway, to the OP's question, while there's no limit specified on the number of dives, you can play around with some numbers and find the practical limit on number of dives for any given depth and time scenario. I only have the PADI tables in front of me, but it looks like if you do one-hour surface intervals and make each dive 10 feet shallower than the last, you can do an exhausting number of (short) dives in one day. But if you do it in reverse, going deeper each time, you may hit a limit on the number of dives you can do in a day, as your surface intervals will soon have to be longer than an hour before you can even make the next dive. It's a fun exercise if you're a nerd like me.
I don’t understand why you’d end up with longer intervals if you do a shallow dive first.
Are you using an example with only two dives? There may be a longer interval after the second dive but if you do two dives a day then you’d not care about the second interval since you are done for the day?
Or is it because you are targeting a certain dive time for the deep dive?
Where I go, usually the deep dive is the wreck dive and the second dive is usually a shallow drift. Most people would want to max their time on the deeper dive as it is usually more interesting. Is that something similar here?
Are you using an example with only two dives? There may be a longer interval after the second dive but if you do two dives a day then you’d not care about the second interval since you are done for the day?
Or is it because you are targeting a certain dive time for the deep dive?
Where I go, usually the deep dive is the wreck dive and the second dive is usually a shallow drift. Most people would want to max their time on the deeper dive as it is usually more interesting. Is that something similar here?
All dive tables have a defined set of rules. They are not the same. There are tables designed by the Navy (e.g. US Navy or Royal Navy), tables designed for recreational use, PADI, BSAC88, Buhlmann, etc, commercial tables developed by the likes of Comex (which are generally commercially secret).
Understanding the 'design criteria' of each table, i.e. the rules is fundamental to safe use of the table.
e.g. BSAC88 define a maximum number of repetitions in any 24hours, and a maximum allowable amount of decompression in any 24 hours.
In truth, modern dive computers have changed the dynamic. Resulting in divers diving 'off the edge' of the original hard table. i.e. a dive with a depth / time combination that requires more decompression than the original published hard table had printed.
In addition, our dive practices have changed considerably, the use of Nitrox, Trimix, accelerated decompression and CCR's where in the vast majority of cases not part of the original design criteria of the table. Not just the design criteria, but the tests that where carried out on live subjects!
The assumption that inert (Helium, Nitrogen etc) gas on gassing and off gassing are just part of the original table algorithm is somewhat flawed. All tables (or algorithms) are based on theory and assumption, they are not perfect representations of what is happening. If they where, all tables would have the same results!
[1]As an example, Buhlmann tables have a number of versions. From Memory Buhlmann 'A' is not used - it is unsafe, i.e. too many bends! So Buhlmann 'B' was developed, this was the standard hard table, and heavily used, but it is not what was fitted to dive computers. Buhlmann 'C', which is more conservative is used in dive computers for realtime calculation. Since then, there are a number of "modified" Buhlmann tables in various dive computers. In addition, it is now common practice to use 'Gradient Factors' to make the original Buhlmann 'C' (or its variant), even more conservative. (Especially for 'Technical Dives')
When Buhlmann was developing the tables, accelerated decompression was not a normal practice, so not part of the trial. Similarly, there are no Nitrox Buhlmann tables - from the original development. Those that exist take the original Algorithm, and modify the inert gas value. i.e. changing the 79% Nitrogen for 64% or 68% etc.
In the early days of Nitrox diving, we used to use EAD (Equivalent Air Depth) with Buhlmann (B) tables, to determine the decompression time. i.e. work on the PN number of the gas used, not the actual depth of the dive.
Dive tables are not the same, because each table makes certain assumptions. Dive tables are not 100% safe because they are not what actually happens to the body, but what someone though might be happening, and then attempted to make a mathematical model of their theory. For the model to work, there are a set of assumptions or rules, break the rules, break the table!
Gareth
[1] I posted a bit more accurately about this some time ago. I am also not an expert!
Understanding the 'design criteria' of each table, i.e. the rules is fundamental to safe use of the table.
e.g. BSAC88 define a maximum number of repetitions in any 24hours, and a maximum allowable amount of decompression in any 24 hours.
In truth, modern dive computers have changed the dynamic. Resulting in divers diving 'off the edge' of the original hard table. i.e. a dive with a depth / time combination that requires more decompression than the original published hard table had printed.
In addition, our dive practices have changed considerably, the use of Nitrox, Trimix, accelerated decompression and CCR's where in the vast majority of cases not part of the original design criteria of the table. Not just the design criteria, but the tests that where carried out on live subjects!
The assumption that inert (Helium, Nitrogen etc) gas on gassing and off gassing are just part of the original table algorithm is somewhat flawed. All tables (or algorithms) are based on theory and assumption, they are not perfect representations of what is happening. If they where, all tables would have the same results!
[1]As an example, Buhlmann tables have a number of versions. From Memory Buhlmann 'A' is not used - it is unsafe, i.e. too many bends! So Buhlmann 'B' was developed, this was the standard hard table, and heavily used, but it is not what was fitted to dive computers. Buhlmann 'C', which is more conservative is used in dive computers for realtime calculation. Since then, there are a number of "modified" Buhlmann tables in various dive computers. In addition, it is now common practice to use 'Gradient Factors' to make the original Buhlmann 'C' (or its variant), even more conservative. (Especially for 'Technical Dives')
When Buhlmann was developing the tables, accelerated decompression was not a normal practice, so not part of the trial. Similarly, there are no Nitrox Buhlmann tables - from the original development. Those that exist take the original Algorithm, and modify the inert gas value. i.e. changing the 79% Nitrogen for 64% or 68% etc.
In the early days of Nitrox diving, we used to use EAD (Equivalent Air Depth) with Buhlmann (B) tables, to determine the decompression time. i.e. work on the PN number of the gas used, not the actual depth of the dive.
Dive tables are not the same, because each table makes certain assumptions. Dive tables are not 100% safe because they are not what actually happens to the body, but what someone though might be happening, and then attempted to make a mathematical model of their theory. For the model to work, there are a set of assumptions or rules, break the rules, break the table!
Gareth
[1] I posted a bit more accurately about this some time ago. I am also not an expert!
Many experiments were done over more than a century on the effects of different ascent strategies on living beings. More than a century ago, John Haldane used goats and other animals. The PADI tables are based in large part on doppler bubble imaging of real divers doing real dives. Buhlmann used data from real dives as well.
Absolutely true. Haldane even did tests on himself when he was developing his first tables.
But no decompression model perfectly represents the human body. They are all models based on best current scientific understanding, and various scientific theories on how best to model the effects of gases under changing pressures on the human body. These scientific theories and models change with time.
At one time Pyle stops where heavily encourage, not so much now. VPM was the thing, now it isn't.
Not knocking any of these, but it shows as much as we do know and understand, there is much we don't.
It also underlines that all models make certain assumptions, these are the rules by which the table is applied.
Gareth
VPM is an example of an algorithm derived from theory and without experimental data. So is RGBM. Pyle derived his theory in part from his own personal experience and his reading of RGBM and VPM.
In the famed Deep Stops conference that first challenged all that theory, Wienke referred to data in support of RGBM, but it was not published data, and he was challenged to tell where he got it. He did not respond to the challenge.
There are significant issues over using human subjects to provide experimental data, it is a mine field of ethical problems. What was acceptable practice in the 50, 60, and 70's is no longer acceptable.
It does however highlight my point, dive models are different, and are not perfect! (Not a perfect representation of how the human body responds.)
What is consider the best practice today, may well change next year (or in a months time)!
All dive models make assumptions, these assumptions are generally the 'rules or restrictions' recommended by the table. Not following the 'rules' of the table will invalidate some (if not all) of the information presented by the table.
To take it to its extreme, a BSAC88 36% Nitrox table assumes you are breathing a gas with 36% O2 (or higher), if you use the Nitrox36 table breathing 27% O2 (i.e. the BSAC88 27% table), then you risk getting DCI!
If you could get data from all divers in a database maybe you could probably adjust the algos with the extra data, could be age/weight/depths which may have been lacking in the original data set etc
You would not have to experiment on humans if they experiment on themselves and give you the data
You would not have to experiment on humans if they experiment on themselves and give you the data
Eric Sedletzky
Contributor
I also suspect that algorithms change and decompression theory changes as people change.
Back in the day divers tended to be more fit, younger, and in general better shape. The theories back then worked because those subjects were in better shape so even if there were flaws it didn’t show. Run those algorithms now and it would be different with an aging diver population, obesity, heart conditions, diving on vacation where a lot alcohol happens, less ability to offgas, etc.
So the algorithms change as the population weakens towards exposure. Plus you have computers which allow divers to run to the maximum edge of NDL’s. Combine this with the above and you have to reel it in quite a bit for the average recreational diving public.
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