Pressure group question

[**pYgmY**]

Hi all,

I have a theory question about calculating the post-dive pressure group for a specific dive profile: max. depth = 28.5m, bottom time = 45min, descending to max. depth at the beginning of the dive and then slowly but steadily ascending to the surface (along a shipwreck), including a 3min safety stop at 5m.

As far as I understand it, the times and pressure groups shown on PADI dive tables always assume a square dive profile. This dive would be multi-leveled, but really is more like a slow ascent from max. depth.. How can I calculate the correct post-dive pressure group for a profile like this? And if there is a way, could someone with more experience help me out by providing the correct result (so I know where to continue in the PADI tables for a repetitive dive calculation)?

Thanks!

 

[Wibble]

Thank you for your quick responses! I'm aware that in reality most dives are multi-leveled and that dive computers can calculate more complex profiles automatically. I was just wondering if there is a "manual" method that can be used for profiles like this that I'm not aware of. Thanks for clarifying

It's a good and valid question you asked which gets to the gist of table limitations. This question is also one of the more interesting aspects of scuba diving, if you're interested in the theory behind decompression limits.

Tables are great for square profiles; that's what they're designed for. In essence they have "run the calculation" (the decompression obligation algorithm) for a specific depth and all the times at that depth. The simplicity of a table means they're not designed to be modular, e.g. add 5 mins of this depth to 10 mins of that depth to 3 mins of the other depth.

Nowadays there's loads of different decompression calculators available, such as MultiDeco (many others are available), which runs the decompression algorithm for your custom depth+time (and gas). MultiDeco allows you to set up a dive profile of the 5 mins at x depth, 10 mins at y depth and 3 mins of z depth then works out the total decompression obligation -- which, for recreational dives, means "no decompression limits" (NDL).

For planning the dive, you'd use the planning tool to calculate your timings and some contingency.

Now we have good dive computers, you'd run the dive roughly against your plan and the dive computer would take into account any additional time spent below or above your target depth. This would show you how close you're getting to your NDL (no-decompression limit) and you'd then adjust the rest of the dive to suit -- obviously factoring in your remaining gas, your minimum gas, your maximum dive time (boat skippers do get angry if you're late!), and that of your buddy.

 

[tursiops]

There is a manual way for multi-level. See @arew+4 's post #3:

The PADI Wheel​

​

All dive profile methodologies make assumptions. Stay within the assumptions. For the tables: treat non-square dives as square at max depth.

The Wheel and the eRDPml are not really applicable. He described a approximately linear ascent from the bottom to the SS over the course of the dive. Those devices do not allows that calculation; they are for diving at one level, then another, then another. The levels must be a required distance apart; the Wheel manual says:

If deepest level is:	Next level must be no deeper than:
120-130 ft	80 ft
95-110 ft	70 ft
80-90 ft	60 ft
65-75 ft	50 ft
50-60 ft	40 ft

 

[Kendall Raine]

Thank you for your quick responses! I'm aware that in reality most dives are multi-leveled and that dive computers can calculate more complex profiles automatically. I was just wondering if there is a "manual" method that can be used for profiles like this that I'm not aware of. Thanks for clarifying

There have been a number of methods (e.g. PADI wheel/weighted average depth averaging/ratio deco) to attempt to mock dive computer results using dept/time parameters in integral fashion (e.g. X depth for A minutes + Y depth for B minutes + Z depth for C minutes...etc.) using dive tables. Part of this was necessity (desire to increase bottom time/avoid excessive deco) in the days before reliable dive computers (air=>nitrox=>trimix) and part was polemics from subgroups of the dive community even after reliable computers were mainstream.

Since the tables were developed and tested (including real dive informed probablistic determination of likelihood of DCS for any given NDL/ascent curve) based on a square exposure (x depth for a minutes) trying to extend the validity of the tables with these methods puts you into the experimental realm-beyond the table assumptions. For example, what does averaging do about saw tooth profiles or profiles where you get progressively deeper during the dive and then ascend versus the other way around (your scenario). Weighted average depth would give you the same profile in each case which is potentially not right. The difference may be inconsequential from a DCS standpoint (depth averaging has been used successfully by many divers for decades), but it may not-experimental realm. Using that method takes skill, practice and mental bandwidth for virtually no advantage and potentially significant cost.

Furthermore, the inherent imprecision of getting the weighted average depth correct just using a bottom timer, or following a precomputed multi-level dive profile in real time, adds to the experimental nature of what you're doing.

With so many good computers on the market, the question is why bother? That's not to suggest blindly following a dive computer is a good idea. Not a all. Computers can fail and divers can fail to use them correctly. Some may use excessively conservative algorithms, others the opposite. It makes sense to do your homework prior to purchase/use. Understanding the basics of decompression theory and how schedules are derived is valuable for many reasons. Being honest with yourself (and your partners) about your comfort zone is key. What the computer (or buddy) says you can do and what is "smart" to do aren't always the same.

This brings up another issue, which is dive planning.

For effective planning purposes one needs to think through gas management (e.g. rock bottom), deco, contingency planning (OOG, aborted dives, overstaying, etc.) beforehand. One really good way to do that is with desk (phone) top software. These are cheap, reliable and allow you to use multi-level (integral) profiles to answer those planning questions. They are customizable for conservatism (e.g. gradient factors). Having a couple of precomputed schedules in your pocket affords a degree of redundancy in case your computer fails during the dive or gives weird results (wrong gas assumption).

Sorry to blather on, but I though expanding on a couple of themes brought up by others might be helpful.

 

[L13]

The Wheel and the eRDPml are not really applicable. He described a approximately linear ascent from the bottom to the SS over the course of the dive. Those devices do not allows that calculation; they are for diving at one level, then another, then another. The levels must be a required distance apart; the Wheel manual says:

If deepest level is:	Next level must be no deeper than:
120-130 ft	80 ft
95-110 ft	70 ft
80-90 ft	60 ft
65-75 ft	50 ft
50-60 ft	40 ft

It can be approximated as square using the tables, better with a stair step using the wheel, or much better just use a computer.

 

[tursiops]

It can be approximated as square using the tables, better with a stair step using the wheel, or better just use a computer.

Work out an example or two; let me know what you think.
So long as the approximations err on the side of more conservative than the "real' answer, no problem.
How are you going to get the "real" answer to comprare to?

 

[L13]

Work out an example or two; let me know what you think.

I don't have a wheel, and have no intention to get one since I have a PC, a phone, and a dive computer.

So long as the approximations err on the side of more conservative than the "real' answer, no problem.

Covered that in my previous post: "stay within the assumptions."

How are you going to get the "real" answer to comprare to?

Why would I need a "real" answer to compare to if I stay withing the stair step requirements of the wheel? I would get a longer NDL than the square Table approximation, but shorter than the DC.

Or better yet, I'll use SubSurface on my PC with the same settings as my DC to plan, and not bother with either the table or the wheel.

 

[tursiops]

I don't have a wheel, and have no intention to get one since I have a PC, a phone, and a dive computer.


Covered that in my previous post: "stay within the assumptions."


Why would I need a "real" answer to compare to if I stay withing the stair step requirements of the wheel? I would get a longer NDL than the square Table approximation, but shorter than the DC.

Or better yet, I'll use SubSurface on my PC with the same settings as my DC to plan, and not bother with either the table or the wheel.

I guess i misunderstood. I thought you were going to approximate his linear ascent to the surface.

 

[L13]

I guess i misunderstood. I thought you were going to approximate his linear ascent to the surface.

The linear ascent can be approximated with a square, a stair step, or a computer(which isn't actually linear either, but damn close). I'm too lazy to do any but the computer.

 

[Wibble]

(This may be OT or too advanced for basic)

Would be interesting to see how Ratio Deco*** would work this problem as it averages the time spent at specific depths.

Obviously Ratio Deco would assume other gasses and that the dive is a decompression dive.




*** Ratio Decompression is a manual mental arithmetic method of working out decompression schedules without a computer. Needs a lot of training and practice. It assumes certain gasses are being used.

 

[tursiops]

OK, here's the challenge. Air dive to 120 ft. Compare square profile table, stair-step multi-level, Ratio Deco, whatever else...How much time-in water (including Safety stop) do you get for a non-deco dive?

Worst case: square profile table (PADI RDP) gives 13 mins NDL plud 3 min SS. 16 min in the water.

eRDPml: multilevel. 120 ft for 2 mins, ascend to next level at 80 ft;
multi level NDL is 24 mins, spend 2 mins, ascend to 40 ft;
multi-level NDL is 111 min, spend 111 mins, ascend to SS. 118 min in the water (mostly at 40 ft), but dive is surely OOG.

Same as above but ascend from 80 ft to 60 ft;
multi-level NDL is 40 mins, spend 20 mins, ascend to 40 ft;
multi-level NDL is 83 mins; spend 83 mins, ascend to SS. 110 min in the water (much at 60 ft), but dive is surely OOG.

Bounce dive to 120 ft, spend max time at 60 ft:
NDL at 120 ft is 13 mins, spend 5 min, ascend to 60 ft;
multi-level NDL is 35 mins; spend 35 mins, ascend to SS. 43 min in the water (mosty at 60 ft)

Your turn.

P.S. If you just dive to 60 ft in the first place, you get 55 mins NDL, plus SS gives 58 min in the water. So it costs you 15 mins of in-water time to spend 5 min at 120 ft.