fine print in diving table, or can you do >3 dives on tables

[el-ninio]

Dr Deco once bubbled...

It is correct that as the dives, and gas loads, progress throughout the day, the “limiting compartment” will shift to a slower one.

[/B]

my dive computer may be able to track this shift, but the table can not.

take for example the case of a deep dive that is limited by a compartment that is slower then the tracked one. say I dive this to the limits of the table, I.e. I leave the water with the slow compartment at 100%.
The table now registers the dive acording to the load lever lf the traced compartment, which is not yet 100% full. I stay out of the water for some time (the slow compartment loses less of it's load thent he traced one), and repeat the same dive to the limit of the table minus the resudual nitregen time (which is based of the traced compartment ). the way I see it, such a dive will overload the slow compartment with this dive, and get me bent.

now, you may be able to overcome this problem by penelizing such a dive's repetative group so that this setup will yeald a remainding bottom time which is safe (this will probably mean that any dive shalower then the first dive is OK). But if the secound dive is deaper the the first, then such a hack, as described above, will no longer do the trick! the loading of the slow compartmnet will be faster the the loading of the fast (tracked) one, and you will probably get bent.

From my understanding, the means that reverse profiled repated dives, as well as long seareas of dives, will bring the standard tables to a dangures zone.

 

[Charlie99]

el-ninio once bubbled...
take for example the case of a deep dive that is limited by a compartment that is slower then the tracked one.

I snipped the rest of your post, since it is based on the erroneous statement above.

Deep dives are limited by compartments that are FASTER than the table's controlling compartment.

The only dives that are limited by compartments slower than the 60 min HT used by the PADI RDP are extremely long, shallow dives. The W,X,Y, Z rules that force a minimum SI time keeps the average depth (including SI) to a low enough value such that the slower than 60HT compartments never reach their their M-values.

:banana: Slumber alert!!! Don't read past this point unless you are a geek like me that wouldn't trust PADI tables until you had figured out their internal workings. :banana:


The PADI RDP uses a 60 minute HT as the controlling compartment. The US Navy and tables derived from it (pre-RGBM NAUI, SSI, etc) use a 120 minute compartment. At least for the PADI RDP, a simple excel spreadsheet with a single formula will regenerate all of the PADI table +/-1 minute roundoff errors.

Using Spencer's/DSAT/PADI M-values, the 60 minute compartment is the controlling one only for 35' and 40'.
Depth -- controlling DSAT compartment HT
50' 40/30 min HT
60' 30 min HT
70' 20
80' 20
90' 20
100' 10
100+ either 5 or 10 minute compartment.

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Now, let's look at what happens on an 80' dive, followed by another 80' repetitive dive.

80' 29 minutes will run the 20 min HT compartment to about 58fsw, 95% of it's M-value. The 60 min compartment goes to about 43fsw, 68% of it's M-value, corresponding to pressure group Q on the RDP.

After a typical 1 hour SI, the PADI table has you in PG F, which is around 34fsw (halfway between ending loading of 43fsw and the 25fsw N2 loading of sea level). Meanwhile, the 20 minute compartment has decayed by 3 halvings, and is at only 29FSW -- equivalent to a residual nitrogen time (for 20HT only!) of less than 2 minutes. So on the 2nd dive, you will have to be at 80' for 28 minutes before exceeding 20min HT compartment limit. Meanwhile, group F, 80' on PADI RDP only allows 16 minutes. Clearly, in this case the PADI table adds conservatism by tracking only the slower compartment.

(For the 40 minute compartment, 80', 29 min loads to 49.5fsw, 80% of M-value. 1 hr SI goes to 34fsw, which corresponds to an 80' residual nitrogen time of 9 to 10 minutes in the 40 min compartment. So the adjusted NDL for 40 min compartment is 20 minutes ---- still longer than the 16 minutes calculated by PADI RDP, based on 60 minute compartment)

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I won't bother dredging up the details, but I did look at a few reverse profile dives and the PADI RDP still works nicely for them also.

 

[el-ninio]

Charlie99 once bubbled...
quote:
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el-ninio once bubbled...
take for example the case of a deep dive that is limited by a compartment that is slower then the tracked one.
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I snipped the rest of your post, since it is based on the erroneous statement above.

OOPS....

I'll go over my math again...

Thanks for the clerification and example, i've been meaning to run some via matlab myself, but had a problem with repreducing the repeatativ dive tables. this post may give me the key to understan how to do this..

 

[Dr Deco]

Dear SCUBA SOURCE Readers:

Nice response Charlie99. Yes, compartments faster than the one used for the repet designators always limit the dive. That compartment will be the last to saturate with the diving equipment available.

In the case of recreational SCUBA, Dr Rogers determined that the 60-minute compartment would limit. If you are a hardhat, surface supplied diver, then the limiting compartment will be longer. It is 120 minutes in the case of US Navy tables since they were designed for decompression work.

Dr Deco :doctor: