Dive Tables

[Blackwood]

Charlie -

Ascent schedule is figured with a pause at 75%Dmax and a 1 minute for each 10 feet from 50%Dmax on up.

A 90 minute SI applies in all cases. Otherwise, stops times are doubled.

 

[Charlie99]

Ascent schedule is figured with a pause at 75%Dmax and a 1 minute for each 10 feet from 50%Dmax on up.

A 90 minute SI applies in all cases. Otherwise, stops times are doubled.

Any other rules that you haven't listed yet?

For example, does "A 90 minute SI applies in all cases. Otherwise, stops times are doubled" mean that SI's less than 90 minutes are allowed if you double the stop lengths?

For example is a series of repetitive dives to 40' 170 minutes with 30 minutes SI OK if you do 2 minutes stops at 20' and 10' after each dive?

 

[plot]

Comparing my SSI tables to my PADI tables, I think the SSI tables are more friendly on the eyes and less intimidating... but both are straight forward to follow.


...now the fact that I took my SSI ow and was never given dive tables until afterwords, and even then they were PADI dive tables... and 6 months later I actually got the SSI tables I learned on... well, that's all a whole nother story.

 

[Blackwood]

For example, does "A 90 minute SI applies in all cases. Otherwise, stops times are doubled" mean that SI's less than 90 minutes are allowed if you double the stop lengths?

Aye, however I don't prefer it.

For example is a series of repetitive dives to 40' 170 minutes with 30 minutes SI OK if you do 2 minutes stops at 20' and 10' after each dive?

I'd double the stops on each repetitive dive. If the first dive calls for one minute at 20 and 10, the next will call for two minutes each, and the third will call for four minutes each.

Per this methodology, there isn't any hard lower limit to what constitutes a surface interval. I imagine you may next ask if I'd do those dives with a 1 minute surface interval and the answer is no. At some point judgment comes into play.

 

[Charlie99]

Per this methodology, there isn't any hard lower limit to what constitutes a surface interval. I imagine you may next ask if I'd do those dives with a 1 minute surface interval and the answer is no. At some point judgment comes into play.

Interesting. Some of your other posts show a comittment to DIR diving. I would expect you to fully understand any dive planning tool you are using and advocating. From the numbers you posted above, it appears that are referring to the 5ThD-X Minimum Deco Air table. The copies of that table that I have seen, such as http://www.5thd-x.com/xducation/pdf/ratiodeco.pdf (p14 of 18) all very clearly say "Minimum Surface Interval: 60 mins"

Do you have a link for such a table that does NOT have the 60 minute interval requirement? If so, it should be corrected, as it can very easily be shown that this is a requirement for safe diving when the N2 loading for previous dives is accounted for only through an extended surface interval and lengthed mandatory deco stops.

Your original post in this thread was, in it's entirety

They most certainly could be simpler. There really is no need to show a series times and "pressure groups" for each depth. Depth and NDL is sufficient when coupled with proper ascent technique.

Even though it took a few posts, hopefully it is now clear that proper ascent technique is NOT a magic bullet that eliminates the need to account for nitrogen loading of previous dives. One may use extended surface intervals and additional decompression stops instead of pressure groups, but the loading must be accounted for.

If you DO still stand by your original post, please correct me.

 

[PerroneFord]

Charlie,

Do you believe it is possible, through ascent profile, and gas selection, to exit the water "clean", or nearly so? Meaning that you could simply strap on more tanks and dive without concern for the previous dive? Or that you could get out of the water, pack your gear and jump on a plane?

 

[Charlie99]

Charlie,

Do you believe it is possible, through ascent profile, and gas selection, to exit the water "clean", or nearly so? Meaning that you could simply strap on more tanks and dive without concern for the previous dive? Or that you could get out of the water, pack your gear and jump on a plane?

Yes, in at least one extreme case fly-immediately-after-dive is fine. You'd have to ascend slow enough that bubble formation is virtually nil, and then spend enough time on O2 that your average depth is no more than about 10'. In figuring average depth, assume that your depth is -33' (as in negative depth) for all of the time you are on O2.

The 10' average is just a ballpark number pulled out of the air ---- the real number would be whatever is appropriate for the model you choose to use, such that the all compartments have a deco ceiling of no more pressure than the 8,000' cabin altitude.

-------------

The 2nd general area where one can ignore previous dives in decompression calculations is for saturation dives. Clearly, prior N2 loadings are not relevant if you spend enough time at depth, that for all practical purposes the compartments are fully saturated.

--------------

The "saturation dive" concept can then be extended to case where, although the very slowest compartments are not saturated, that all of the compartments that are controlling or setting decompression are near saturation. This will happen in many mandatory decompression dives, but is NOT usually the case in recreational diving.
Edit - additional comment: Although you can't completely ignore the first dive, if you run a bunch of dives through V-planner, what you will see is that doing two more or less identical dives back-to-back will there will be a strong effect on NDL or deco. OTOH, if you pair up a shallow dive and a deep dive, then they will have much less affect on each other and for all practical purposes you can ignore the first dive. In a short deep dive followed by a shallow one, the deep dive is limited by fast compartments, the shallow one by the slower ones, and the deep dive doesn't change NDL for the 2nd significantly. The same sort of thing applies for a reverse profile series.


The above is also related to why GI3 posts that when doing repetitive dives, the shallow dive should be first and the deeper one second. This does NOT apply in the normal NDL/min deco (or even "mild deco") diving regime.

At first glance, it may appear that the Min Deco tables quoted by Blackwood assume that you leave the water "clean". If you study it a bit closer, you will actually see that the N2 loading is taken into account by conservatism in the table and minimum surface intervals.

Of course, the whole emphasis on slow ascents and "clean profiles" is that, once you have significant bubble formation in tissues (as opposed to the venous system), then offgassing is severely slowed and all bets are off as to decompression calculations.

Charlie Allen

 

[Blackwood]

Even though it took a few posts, hopefully it is now clear that proper ascent technique is NOT a magic bullet that eliminates the need to account for nitrogen loading of previous dives.

There is no such thing as a magic bullet. You can never come out of the water completely clean without sitting on oxygen.

I was stating that I believe there to be simpler ways to convey information than an array (noting that the first post said that PADI-like tables are complicated). Nothing more, nothing less. Yes, I stand by that.

No, I don't adhere strictly to 5dx methodology. Personally I won't sit less than 90 minutes while diving near NDLs. Having sat through Georgitsis's seminar, I am aware of the assumptions behind his methodology. The 90/60 rules are arbitrary, and I tend toward the 90. However, I do believe that by increasing stops, one can account for >60 minutes.

I don't have a commitment to any particular dive philosophy. I don't call myself DIR. I agree with a large part of the DIR structure, and that may come out in my posts.

 

[miketsp]

..snip..
The 10' average is just a ballpark number pulled out of the air ---- the real number would be whatever is appropriate for the model you choose to use, such that the all compartments have a deco ceiling of no more pressure than the 8,000' cabin altitude.
..snip..

Just to comment one detail.
Your deco ceiling would have to be for a steady state 8.000' with a 5min reduction to 40.000' (or whatever your planned altitude is) to ensure that you wouldn't have problems in the case of a loss of cabin pressure.

 

[Charlie99]

Just to comment one detail.
Your deco ceiling would have to be for a steady state 8.000' with a 5min reduction to 40.000' (or whatever your planned altitude is) to ensure that you wouldn't have problems in the case of a loss of cabin pressure.

Generally fly-after-dive calculations only consider the steady state 8,000' altitude and ignore the possible short term exposure to higher altitudes possible in an explosive decompression situation.

In real life, explosive decompression is often combined with major airframe failure and loss of flight control. When your plane is falling out of the sky, potential DCS is not of much importance. If you do survive, then the short period of exposure, combined with breathing O2 limits the effect.

============================

I pulled the “10’ average depth” limit out of the air in the previous post. Here’s a calculation that shows for BuhlmannZH-L16 that the maximum average depth for immediate fly-after-dive is about 10.76’.



Assumptions: 8,000’ altitude = 10.91psia=0.742ata=24.5fsw
ZHL16a/b/c compartment 16 (635 min halftime) M0 is 41.8fsw and deltaM is 1.0359.
M value at altitude is (33-24.5)fsw * 1.0359 = 8.8fsw less than M0.
41.8-8.8= 33fsw of N2 in the slow compartment at altitude.

The inspired ppN2 = 33fsw at 33fsw/(0.79 Fraction N2) + 2.0fsw H2O + CO2 = 43.76fsw, or 10.76’ of depth below sea level.

The limits for the faster compartments are higher, so limiting the average depth of the dive to 10.76' has some safety factor built in. For example, repeating the same calculation for the 109 min halftime compartment results in a 18.5' avg depth limit.

Technically, instead of calculating averages, the compartment loadings should be tracked as exponentials. But with long halftimes like 635 minutes, just linearly averaging is a reasonable approximation.

If you want to calculate in a fudge factor for a short term excursion to 40,000' Mike, be my guest. I don't have a clue as to how to account for that if I wanted to.

=================

One warning about the above calculations --- the time-to-fly times calculated by an unpadded Buhlmann algorithm are significantly shorter (by about a factor of 2) than the standard fly after dive times. Take a case where you exit the water with the slowest compartment right at the limit --- 41.8fsw. At sea level inspired ppN2 is (33-2)*0.79 = 24.5fsw. Per the above calculations, the limit at altitude in compartment 16 is 33fsw. It turns out that the midpoint between the 41.8fsw and the SI inspired ppN2 of 24.5fsw is 33.15 fsw. In other words, after one halftime of 635 minutes (10-1/2 hours) the slowest compartment will decay from the maximum allowed surfacing value and the maximum allowable value for ascending to 8,000'. In plain words, a 10-1/2 hour time to fly.