Is there a Dive computer that gives you an ending pressure group?

[Charlie99]

Computer vs. Ending Pressure Group: My understanding is that computer manufacturers have different algorithms than the paper tables contemplated by the OP.

The Pelagic/Oceanic/Aeris/Sherwood computer are based upon the DSAT/PADI model, as are the PADI RDP and PADI Wheel.

The computers have a full implementation of the model, while the RDP is a strictly limited version. The Wheel is more complete, but still very much a partial implementation.

The pressure groups of the RDP and Wheel are based solely upon the 60 minute compartment, although NDLs and MDLs (wheel limits for multilevel dive) are based upon all compartments. That's why the maximum allowable pressure group for an NDL dive is less for a deep dive than for a shallow dive --- for any dive deeper than 40', the fast compartments reach their limit before the 60 minute compartment reaches its limit (which is shown on the table as pressure group Z).

The above mentioned computers show a tissue loading bargraph which is analogous to pressure groups, except that the computers calculate for ALL of the compartments of the model and then display the bargraph for just the one that is closest to the limit. Except for extraordinarily long dives or dives with long required decompression, the compartment that is closest to the limit will always be faster than the 60 minute compartment.

Charlie Allen

 

[Peter Guy]

Arctic D -- of course there is no such thing as a "real" pressure group. I only used "real" in the sense that it would account for the multi-level nature of a dive as opposed to the RDP's assumed square profile. However, TSandM pointed out that in many "real" multi-level dives, a lot of time is spent above 35 feet and so the "average depth" shown at the end of the dive would be/could be significantly shallower than the RDP/Wheel depths -- which would lead to an "artificially" higher Ending Pressure Group (if such a thing actually existed).

This led us to a discussion of whether the RDP/Wheel takes into account deco stops (as far as I can tell they do not) in their determination of Pressure Groups.

I suppose this is partly why I've never paid much attention to "the tables" except as part of a class. Me, I'll go ahead and use minimum deco/ratio deco and not contemplate "ending pressure groups."

 

[Charlie99]

This led us to a discussion of whether the RDP/Wheel takes into account deco stops (as far as I can tell they do not) in their determination of Pressure Groups.

The RDP and Wheel definitely do NOT take into account any offgassing at safety stop/deco stop.

I suppose this is partly why I've never paid much attention to "the tables" except as part of a class. Me, I'll go ahead and use minimum deco/ratio deco and not contemplate "ending pressure groups."

Pressure groups are for planning repetitive dives. Repetitive diving is the weak spot in the ratio deco method. Ratio deco handles repetitive diving by 1) additional lowering of the max times for shallow depths, 2) minimum surface interval times, and 3) allowing repetitive dives to have mandatory decompression required above and beyond the normal minimum deco stops.

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There is nothing that prevents a diver from simultaneously using multiple methods for planning and tracking his dive profile. The various planning an deco tracking methods all have their advantages and disadvantages.


Quite obviously, the weak spot of any square profile table is multilevel diving. Average depth/ratio deco handles multilevel well, with good realtime flexibility. Ratio deco requires effort and practice, particularly to avoid losing track in emergencies or in the excitement of when the whale swims by.

Effectively, a square profile table "penalizes" you for making a slow ascent with deep stops, since all time until starting a direct ascent to the safety stop is counted as bottom time. Both ratio deco and computers avoid this problem.

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The PADI Wheel handles multilevel dive pre-planning well, but is somewhat complicated and it is difficult to change the plan once the dive is underway.

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The PADI table handles repetitive diving well, but at the expense of 1) artificially lower the 40 and 35' NDLs, and 2) having to add in some ad hoc rules about minumum SI on dive sequences of 3 or more dives where any one of the dives hits pressure group W or higher.

The USN and USN-derived tables are excessively conservative on repetitive dives because repetitive dive calculations are based upon the 120 minute compartment. These tables are optimal for planning and tracking very long, shallow dives.

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Computers handle realtime variations in the dive profile well, but some divers blindly rely upon them to the point that they would fail to notice if the computer started feeding them bogus data.

 

[ArcticDiver]

Thanks Charlie for going to a greater depth in some of the ways the algorithms are different. But you only covered one family of computers. Other computer manufacturers use other algorithms that often produce quite different readings. These may, or may not, be closer to or further deviate from what a chosen set of tables will produce.

Another important issue is situational awareness. If a person chooses a particular algorithm after awhile they get a feel for about where they are on the dive. The situational awareness adds to the safety and fun of the dive.

Frankly, since all algorithms are someone's SWAG I don't think it is important, or even useful, to try to convert data between them. What is important is to pick one and follow it religiously. If used intelligently they all work most of the time.

 

[Charlie99]

But you only covered one family of computers. Other computer manufacturers use other algorithms that often produce quite different readings. .......Frankly, since all algorithms are someone's SWAG I don't think it is important, or even useful, to try to convert data between them.

/slight hijack ---

1. Small changes in the model limits result in big changes in allowed NDL when shallower than 60' or so because of the exponential nature of ongassing. In other words, one computer have calculated loadings that are 90% of the limit while another one has you right at the limit. At 40' or so, it will take a very long time, to load up that compartment the last 10%. That's one reason I like computers that have a N2 loading bargraph --- in many ways that is more meaningful than an NDL time.
I track deco status in my head, but only really to about 5 levels --- clean, lightly loaded, medium, heavy, and loaded to the gills. Depending upon that crude loading level (and confirmed by computer bargraph) I will choose to vary my ascent and stops from virtually none, up through 12 to 15 minutes. Usually in the range of 7 to 10 minutes total time for ascent, deep stops, and shallow stops.

2. The change of DCS risk with loading levels is much, much more gradual than most divers think. Both Bruce Wienke and the US Navy have crunched the data of thousands of dives and come up with risk statistics vs. time at depth. For example, at 80', extending bottom time from 15 minutes all the way up to 60 minutes will increase the DCS risk by only a factor of 5!!!!
(For reference, the PADI 80' NDL is 30 minutes with 4 min ascent, the USN NDL is 40 minutes with 2 minute ascent, ratio deco is 30 minutes with 6 minute ascent).
Similarly going from 8 minutes at 100' all the way up to 50 minutes at 100' also results in only an increase of DCS risk of a factor of 5.

DCS risk is low enough, that a single diver is not going to know if he has increased his DCS risk by even a factor of 10 ---- in other words, one can get very sloppy in decompression planning and never know it from the results.

3. A slow ascent and some extra time shallow can fix just about any error in decompression planning, provided you have the time and gas. If you run various profiles through a decompression program such as V-planner or GAP, one can see that worries about changing over to tables after a computer failure can be taken care of simply by extending the 20-10' stop.

 

[darry_99]

Have you seen what I did?

http://www.scubaboard.com/forums/do-yourself-diy/193341-tim-s-recreational-dive-log.html

Tim - this looks awesome! I just downloaded it and am going to play around with it. Thank you!!

 

[TheGoast]

Tim - this looks awesome! I just downloaded it and am going to play around with it. Thank you!!

Just send me an email for a serial number