Falling back to tables after a computer failure

[Diver0001]

Dr. Deco,

Question:

Assuming your computer fails during a dive and you are "off the tables" but sure you haven't gone over the NDL's, then can you start with Z (PADI RDP) and fall back to tables for planning subsequent dives?

Background:

There is a discussion about this point going on in another thread. I am of the opinion that you can do this because Z represents the maximum theoretical N2 load you can build up without going over the NDL's.

The other opinion offered is that because you're off the tables that you need to apply the emergency deco rules and wait 24 hours because that's what the RDP says you need to do if you're off the table.

Can you please shead some light on this.

Thanks,
R..

 

[BIGSAGE136]

I always table the dive b4 I hit the water. You never know when failure will occur. It's also a good idea to buy a cheap Walmart waterproof watch as a backup timer.

 

[Dr Deco]

Hello ro:

Off the Table

I would guess that we have one of two situations. If you believe that the dive was one simply of gas loading, but you are not sure of the “dose,” one could simply pick a table dive that you knew was in excess of your gas loading and proceed from there.

If however you believe that gas bubble formation could have occurred, it is best to forgo diving for the remainder of the day and allow your body to “reset,” that is, the microbubbles to redissolve.


Dr Deco :doctor:

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

 

[MechDiver]

Dr. Deco,

Question:

Assuming your computer fails during a dive and you are "off the tables" but sure you haven't gone over the NDL's...

R..

Not that I can add to the answers, but I can't figure how you can be off the tables and not over NDL? If you exceed the table for a given dive, then you are in deco, are you not?

I'm not seeing something I guess

Also, wouldn't the Rule of 120 apply anyway if it was indeed NDL?

MD

 

[jviehe]

I was also trying to figure out whether Z would be enough. Computers work basically the same as tables except more accurate. If I understand it right, as long as you did not go over NDLs in the computer, you could not go over NDLs on the tables, and therefore you most would be a Z diver, and can now jump on the tables.

This assumes your keeping an eye on your computer as you should up until it failed, and then surfaced.

Dr Deco, would a 6 hour wait time theoretically remove all nitrogen for all but the slowest tissues?

 

[Diver0001]

Not that I can add to the answers, but I can't figure how you can be off the tables and not over NDL? If you exceed the table for a given dive, then you are in deco, are you not?

I'm not seeing something I guess

Also, wouldn't the Rule of 120 apply anyway if it was indeed NDL?

MD

Hi MD,

What I meant by that is that your computer can have a combination of max-depth/bottom-time that isn't possible on the square profiles and still not be over the NDL's.

Also I wouldn't think that you can switch back to the 120 "rule" on the fly unless you were keeping a rolling average depth.

R..

 

[MechDiver]

Hi MD,

What I meant by that is that your computer can have a combination of max-depth/bottom-time that isn't possible on the square profiles and still not be over the NDL's.

Okay, the light went on. Doh.
Thanks

Also I wouldn't think that you can switch back to the 120 "rule" on the fly unless you were keeping a rolling average depth.

R..

Yea, but I think you'd be able to wing it close enough for most dives. At least in this scenerio.

Good thread.

MD

 

[lamont]

If you're doing a multi-level dive and you know approximately how long you stayed at each depth, can you do a multi-level dive on tables like the RDP by moving up until you hit the new depth, then adding the X number of minutes that you stayed there to get your new pressure group, repeat?

I don't have my tables with me or I'd post an example -- but lets say you spend 10 mins at 90 ft and then move up to 50 ft for 40 mins. Can you get a valid pressure group off of the RDP for that?

I think another way of saying this question is if you can do a multilevel dive by assuming back to back square profile dives with zero suface interval?

 

[Charlie99]

(1)Computers work basically the same as tables except more accurate.

(2)If I understand it right, as long as you did not go over NDLs in the computer, you could not go over NDLs on the tables, and therefore you most would be a Z diver, and can now jump on the tables.

(3)Dr Deco, would a 6 hour wait time theoretically remove all nitrogen for all but the slowest tissues?

(1)Computers track all compartments. The PADI table tracks ONLY the 60 minute compartment (by pressure group), with NDLs for deeper than 40' dives being limited by a faster compartment that isn't tracked. You can get away with this because the faster compartments offgass faster than the 60min HT compartment and so tracking only that compartment for SI is actually conservative.

The YZ rule on the back call for a minimum SI of 3 hours in a series of 3 dives where any hits Y or Z. This appears to be an ad hoc method of adjusting the average depth (over both dive and SI time) down to around 24' so that you will never exceed the M values for the slower than 60 minute compartments. While in square profile diving, it is very unlikely that you will hit Z, it can be done with a single tank and reasonable SAC in a multi-level dive.
EX:
20min at 100'
9 min at 70'
20 minutes at 50'
20 minutes at 40'
This profile would hit Z in a single dive, and can be done with SAC of around 0.4cfm with an 80cu ft tank.
The above profile "rides the NDL" most of the way up, and is what I would consider a very agressive profile for NDL diving. With a computer failure after a dive like that, I'd call it a day, or do at least a 3 hour SI.

(2). As noted above, there are some unusual profiles that are within NDL (more properly, "all compartments are within M value limits of the model"), but are off the table.

(3). It's an exponential decay. The PADI RDP tracks only 60 minute compartment. After 6 hours, you will have only 1/2^6 or about 1.6% of M value. To put this into perspective, the PG A is at about 12% of M value.

 

[Charlie99]

If you're doing a multi-level dive and you know approximately how long you stayed at each depth, can you do a multi-level dive on tables like the RDP by moving up until you hit the new depth, then adding the X number of minutes that you stayed there to get your new pressure group, repeat?
I think another way of saying this question is if you can do a multilevel dive by assuming back to back square profile dives with zero suface interval?

SHORT ANSWER: This is essentially how the wheel works.

THE LONG, NERDY ANSWER: The PGs and times on the wheel are indentical to the RDP table. The wheel adds additional restrictions however, to keep from going beyond M-value limits on the faster than 60 minute compartments that the RDP/Wheel don't track.

The most significant restrictionsa are the 1) "ML" limits, and 2) minimum depth change values.---

MLs are slightly reduced NDLs that you must follow in multilevel calcs. For examplel 80' NDL=30 minutes. ML=28. 60' NDL is 55 minutes, ML is 46. In addition, if you come within 3 pressure groups of any ML, then 3 min stop at 15' is mandatory (analagous to 3 min mandatory SS for square profile dives within 3 PG of NDL).

Depth change rules take the form like "if 1st dive level is 120 or 130', then the next level must be 80' or shallower, 80' or 90' must ascend to 60' or shallower, etc.

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).