Falling back to tables after a computer failure

[Dr Deco]

Hello Readers:

60-Minute Limiting Halftimes and the RDP

At the time that Dr Rogers was working on his calculations, which later would be sold as the Recreational Dive Planner, most recreational divers were diving only at most a couple of days on the weekend. By the time the program was nearing completion, live aboard boats were becoming popular and the face of diving was undergoing a change.

While most diving of a recreational nature could be governed by the 60-minute compartment, there were some cases where this was not valid. For these instances, the “ad hoc rules” (YZ and so on) were added. As long as one is within these limits, you could always simply stay out of the water for six hours

Dr Deco :doctor:

Readers, please note the next class in Decompression Physiology :1book:
http://wrigley.usc.edu/hyperbaric/advdeco.htm

 

[Diver0001]

....snip....

As a mental exercise, I reverse engineered the PADI RDP and wheel. This GIF in my gallery is the output from a simple Excel spreadsheet. I had to assume H2O water vapor pressure of 1.6fsw, then played around a bit to determine what PADI chose for PGs. My calcs are +/-1 minute of RDP and Wheel. (PG A is 12% of M, PG B is 21%, then is goes up in 3% increments from C=26% to Z=95% of DSAT/Spencer M-values).

I, for one, am impressed.

R..

 

[Tobagoman]

(1)Computers track all compartments. The PADI table tracks ONLY the 60 minute compartment

Depends on your computer and its paradigm model, some use the Spencer limits with the 60 minute washout the same as the PADI RDP. The paradigm model your talking about is the EE washout where all compartments wash out at their underwater halftime rates (I prefer the former).

 

[Dr Deco]

Hello Readers:

Half times

Actually, all algorithms will account for all halftimes up to about 400 minutes. However, for typical recreational diving (air, single tank), one would find that all compartments do not add dissolved nitrogen in any appreciable manner. That is, the longer compartments do not play any role since they never can get enough nitrogen from a few single-tank dives.

It is thus possible, if one were performing calculations by hand (as in the old days with a slide rule), to simply leave them out. With a computer, one calculates them but they play no role. In general, the 60-minute compartment is the longest of significance, therefore it is controlling for all but the most severe recreational dives. In many cases, one finds that a shorter compartment is controlling. That is one reason why a dive computer will yield more bottom time on repetitive dives than a table, since it generates the closest dive to the theoretical limits.

If one were making a table or designing a computer for most recreational dives, you could leave out the longer (i.e. greater than 100 minutes) compartments. With computing power what it is, there is really no reason to do so, and it is left in decompression meters. You do not see it in the output, however, for most rec dives. The same is true for the RDP; you will not see the long compartments play a role for most dives. It is only in those instances on several long dives in one day that the “ancillary rules” come into play.

Tracking

In the original calculations of Dr Rogers, all compartments were tracked, but only the sixty-minute one ever played any significant role. One might say that all compartments are tracked but not all are printed.

Bubble Reservoirs :06:

In actuality, there are probably not any halftimes longer than 400 minutes. Beyond this time, tissue would be undersupplied with blood to be metabolically active. The gas in these compartments is most likely sequestered in microbubbles.


Dr Deco :doctor:

Readers, please note the next class in Decompression Physiology :1book:
http://wrigley.usc.edu/hyperbaric/advdeco.htm

 

[AtomicWalrus]

If your computer failed on a dive and you hadn't previously planned it with a table, wouldn't it be safer and simpler to just bail out and abort the dive as indicated by the computer manual? As far as I know, dive computers don't generally give you enough data on your tissue compartment saturation levels to allow you to determine the appropriate starting & ending pressure groups on your tables. Not much of a problem if you're starting from a completely desaturated state, but in the middle of a multi-dive sequence, I'd think that the residual nitrogen levels would be rather critical. Probably the best approach would be to plan your dive profile using a multilevel planning tool like the wheel, try to stick to your plan, and then use your computer as a track of your actual dive profile & backup to your tables.

 

[BIGSAGE136]

THE WHEEL, AND SLIDE RULES RULE!. Now there's some gadgets that wont rot your brain!

Still got yours?

 

[jviehe]

If your computer failed on a dive and you hadn't previously planned it with a table, wouldn't it be safer and simpler to just bail out and abort the dive as indicated by the computer manual? As far as I know, dive computers don't generally give you enough data on your tissue compartment saturation levels to allow you to determine the appropriate starting & ending pressure groups on your tables. Not much of a problem if you're starting from a completely desaturated state, but in the middle of a multi-dive sequence, I'd think that the residual nitrogen levels would be rather critical. Probably the best approach would be to plan your dive profile using a multilevel planning tool like the wheel, try to stick to your plan, and then use your computer as a track of your actual dive profile & backup to your tables.

Well, the idea would be to at least bring a watch, and if you know your bottom times and depths, then you can plan your dive after the fact and find out your PG. Its no different than planning it before the dives. However, if your dive falls outside of the tables, but you know you didnt exceed NDLs, you would at worst be a Z diver and plan accordingly.

Thats basically the question, though, is that accurate?

 

[Diver0001]

If your computer failed on a dive and you hadn't previously planned it with a table, wouldn't it be safer and simpler to just bail out and abort the dive as indicated by the computer manual? ....snip.....

Yes it would. The point is that it isn't strictly necessary.

R..

 

[O2BBubbleFree]

I don't carry my dive tables on the dives since I don't trust my brain to work the tables under the influence of nitrogen. Instead, before the dive, I write the max times for my planned depth and the next two depths (just in case) on my wrist slate. That way if the computer fails, I can, in effect, fall back on the tables, just by not exceeding the written times.

If I happen to exceed the written depths, I'll abort the dive.

Of course, this requires that I remember to start my stopwatch at the beginning of the dive.