What to do if . . .

[Jason Ooi]

I agree what you say

The conventional advice would be stay on the surface, breathe 100% O2 if available and monitor yourself for signs of DCS.

 

[paolov]

I agree what you say

how about this

....i'd suggest a minimum of 20 mins S.I. on Oxygen ....

)


.

 

[Lake Mead Tony]

Question, Does everybody dive the tables or dive the computer using the tables as a reference. Also does everybody dive PADI tables or Navy Standard Air Deco Tables. There are a lot of differences between the two. Until I took EAN I used the Navy tables, until I got my computer.

 

[lowwall]

I think the consensus here is getting back in the water will do nothing for you.

True, but...how do I put this delicately? I think posters have done an admirable job of tailoring their responses based on the implications of the original post. However, it's likely that the same basic situation (OOA on a deco stop with more gas 1/2atm. away) couched in different terms might yield a little less unanimity.

 

[Diver Dennis]

Different terms as in...?

 

[Lake Mead Tony]

I am not trying to change the thread. Maybe I should have just started a new thread or not.

 

[Diver Dennis]

Actually it might be a good idea for another thread Tony. There will be a lot of different answers for that question.

 

[boulderjohn]

Question, Does everybody dive the tables or dive the computer using the tables as a reference. Also does everybody dive PADI tables or Navy Standard Air Deco Tables. There are a lot of differences between the two. Until I took EAN I used the Navy tables, until I got my computer.

Here's a quick description of the difference.

Any table is a crude estimate of how your body is absorbing/releasing nitrogen. The tissues in your body absorb nitrogen and release nitrogen at the same rate, but they do it at vastly different rates from each other. Within a single tissue, that rate varies in relation to the difference in partial pressure of nitrogen between the tissue and its surroundings--the greater the difference, the faster it absorbs/release nitrogen. In theory, if the pressure difference is too great on release, DCS becomes a possibility. (As in a shaken bottle of soda being opened.)

Because of this, theoretical tissues (or compartments) were established to represent the difference in the many different tissues. Since no table can predict all the tissues, a table uses one theoretical tissue (or compartment) that they think will govern the dives, and they assume (with intended conservative inaccuracy) that all the faster tissues will release nitrogen at that slower rate.

The Navy tables assume that divers will dive deep and long, going into deco. They decided that what is called the 120 minute compartment governs their dives. That compartment loses half its absorbed nitrogen in 120 minutes. It loses half of what is left in another 120 minutes. It loses all (98.4%) in 6 X 120 = 12 hours. (Remember, though, that it absorbs at the same rate, so it was not 98.4% saturated with nitrogen to begin with unless you were at depth for 12 hours, or were in a lot of repeated dives.)

The PADI table are based on the fact that recreational divers do not make their first dive (or other dives) that deep or that long. They used Doppler technology to observe bubble formation and determined that for recreational divers, the 40 minute compartment was the controlling compartment for all but a few special cases. In order to be more conservative, they backed off and used the 60 minute compartment. That means that that compartment (and all the faster ones) is 98.4% clear in 6 hours.

[Edit: in those 6 hours, the 120 minute compartment is 87.5% clear. it takes the next 6 hours to go to 98.4%]

In both tables, the change in pressure groups reflects the theoretical change in that compartment.

The Navy tables only have a relative few pressure groups, so a lot of rounding off has to be done. PADI made more pressure groups so that less rounding would be involved.

In summary,

• The PADI tables require a more conservative (less deep and shorter) first dive.
• The PADI tables have more pressure groups to make a more accurate decision with less rounding.
• The PADI tables use the 60 minute compartment instead of the 120 minute compartment in the belief that recreational dives, with a few exceptions, are governed by that compartment (actually, by the 40 minute compartment).
• The exceptions noted above are accounted for by the WX and YZ rules in the PADI tables.

If you are going to be doing a lot of deep dives with deco stops, you should prefer the Navy tables, because they are more likely to pertain to you. The average recreational diver will do just fine with the PADI tables. That was the intent from the start.

 

[Lake Mead Tony]

That is why I used the Navy tables to a tee. I will post a new thread to discuss this further!

 

[MikeFerrara]

What's so hard about planning your dive so you have enough gas to do the decompression time that you want to do?

The PADI table is a rather narrow view of decompression and if you're going to be diving them that close to the edge you might want to look at some other tables/models to see what kind of ascent they call for on the same dive. In a nutshell, any diver where the PADI table calls for emergency decompression of 8 or 15 minutes at 15 feet is going to be one that I would rather be starting my decompression deeper...so, if you're diving the PADI table, stay away from those "NDLs".

There are procedures for handling missed decompression stops that are tuaght in any technical class that deals with staged decompression but it's probably beyond the scope of a discussion centered around the PADI tables.