Why don't dive tables....

[jbd]

start at a depth of twenty feet instead of 35 feet(PADI) and 40 feet(NAUI)? I don't know what other agencies start their tables at.

It would seem that tables should start at the depth where nitrogen loading creates limits. It seems kind of strange to do repetitive dives in the 20 to 22 foot range and have nitrogen residual times greater than the time underwater.

 

[jeffsterinsf]

Wienke, Technical Diving In Depth, 2001, pg. 43, gives theoretical NDL limits at 30 fsw of 200 min for RGBM and 290 min for ZHL. The limits (approximately) follow a square-root law (quadrupuling with halving depth), meaning they are "insanely" long for 10 or 20 fsw.

If you take the constant to be 465 fsw min^(1/2) (Weinke, ibid, pg. 40), that gives 2162 min NDL for 10' and 540 min NDL for 20'

Now compare that to an 80 cuft tank. Assume about 1/2 cuft/min SAC -- that's almost 1 cuft/min at 30 fsw. Assuming you leave 750 psi in the tank at the end of the dive, that's only 60 min of air. At 15' you might get 90 minutes out of the tank.

SSI tables do cover from 10' on downward. At 10' you are a group A after an hour, group B after two. At 20' you are group C after an hour. Remember that you aren't supposed to do deeper dives after shallower ones in recreational diving. So where am I going to go from 20' anyways? Even if you decide to stay at 20', the RNT is small compared to the NDL. For example, 90 min at 20' makes me group D. At such a low level of saturation, it takes over an hour to drop a group, so let's stay with group D. RNT is given as 88 min at 20', with "N/L" indicated for the adjusted NDL time. This makes sense, compared to a NDL that would be somewhere around 9 hours of bottom time...

 

[BRW]

Everybody knows the square root law
breaks down for shallow exposures -- that goes
back 50 yrs. The law is illustrative, but
in the real diving zones. Nobody worries
much about 10 -20 fsw, but tables designers
do tack on NDLs at 20 fsw for completeness.

For all intents and purposes, you can
splash around in the 10 -20 fsw zone until
boredom sets in, and until you tire of loading
new tanks at the surface on repets, and probably
not hit 200 minutes. RGBM risk analysis drops
the NDL to 200 min at the 1% hit rate. That is
what counts here. USN Tables (and all Haldane
derivatives) are "passe" and out of date. Read
on in TDID. Also see new released NAUI RGBM
Tables.

 

[jeffsterinsf]

...listen to BRW, he knows the material a bit better than I, having written the book!

 

[Dr Deco]

Dear jbd:

Dive Limits

As has already been indicated, shallow limits for DCS really exceed what can be done with scuba. At these minimal depths, you would not get the “bends,” but you could die of boredom. One needs therefore only to track the gas loads, minimal as they might be, for these exposures with regard to repetitive dives.

Doppler Bubbles

Apparently there do exist tissue micronuclei large enough to grow, that is, they exceed the Laplace cutoff. This relates the critical radius to the pressure change and the surface tension.

Reference

Eckenhoff RG, Olstad CS, Carrod G. Human dose-response relationship for decompression and endogenous bubble formation. J Appl Physiol 1990; 69 (3): 914-8

The dose-response relationship for decompression and venous gas emboli (VGE) formation in humans was examined. Pressure exposures of 12, 16, and 20.5 fsw were conducted in an underwater habitat for 48 h. The 111 human male volunteer subjects then ascended directly to the surface (< 5 min) and were monitored for VGE with a Doppler device (precordium or subclavian veins) for a 24-h period. No signs or symptoms consistent with DCS occurred. However, a large incidence of VGE were noted. Fifty 50% of humans can be expected to generate bubbles after decompression from a saturation pressure exposure of only 11 FSW. These results also imply that endogenous bubbles arise from preexisting nuclei.

Dr Deco :doctor:

 

[blacknet]

jeffsterinsf,

You may want to recheck your bottom time figures, at 15' I have used a single al80 tank (3k psig start, 500psig end) for at least 3 hours underwater.

USN dive manual:

10' NDL unlimited,
60 mins is pressure group A
797 mins is pressure group E
15' NDL unlimited,
35 mins is pressure group A
452 mins is pressure group G
20' NDL unlimited,
25 mins is pressure group A
917 mins is pressure group J



Ed

 

[Tillomar]

BRW once bubbled...
Read on in TDID. Also see new released NAUI RGBM
Tables.

I don't know what TDID means. Please explain where I can read on...
Is there an open source RGBM model software to learn more?

Tillomar

 

[Rick Murchison]

BRW once bubbled...
USN Tables (and all Haldane
derivatives) are "passe" and out of date.

They may be old; they may even be based on theory that's "not quite reality" - but there is a huge amount of work that's been done with 'em under it, and tables galore that refer to 'em still in use - successfully. I just read an article in Alert Diver where some surprising real results indicate a little more attention to Haldanean thinking is likely in order...
And while a move to rework the compendium of current tables, policies and procedures that are underpinned by Haldanean derivatives may indeed be in order, we must realize that they're still current as of today, and to understand the body of work surrounding Scuba diving one must understand at least the basics of Haldanean theory. I can think of lots of terms that characterize 'em, but they likely won't be "passé" for at least another decade.
Appreciate your work, by the way, BW... but it ain't time to throw the baby out with the bathwater - yet.
Rick

 

[SDAnderson]

TDID = Technical Diving in Depth, written by Bruce R Weinke and published by Best Publishing Company. Definitely worth the money.

Best Publishing

I don't know about open source software. People who invest the time and energy into developing deco programs typically have two major concerns: they want to get paid for it, and; they don't want to get sued if it kills someone.

GUE has one for sale.
GAP is shareware.

Bon chance, be careful.

Steven

 

[rmediver2002]

The US Navy dive manual way around having a residual nitrogen time greater than your actual bottom time is the following rule:

For any dive to the same or greater depth than a previous dive where the time from the RNT table is greater than the actual bottom time of the previous dive, use the actual bottom time of the previous dive for your residual nitrogen time.

Basically if the dive is as deep or deeper than the previous dive you could not have absorbed an amount of nitrogen greater than your actual bottom time, so use that time in computing your repetative dive.

Jeff lane