What should I trust?

[Bubbletrubble]

All I can find is a citation for authors Lang, MA and Hamilton Jr, RW

What is the pertinent info in that paper?

@gcbryan: For some reason, the search function on the Rubicon Foundation site has certain limitations. In the future, if you have an author's name, I'd recommend searching the author listing first...which should allow you to locate the desired paper.

Thalassamania was referencing a section written by Michael Emmerman beginning on page 197 of a proceedings manuscript prepared from a workshop organized by the American Academy of Underwater Sciences in Sept. 1988. Here's a direct link to the rather lengthy document.

Emmerman was using the Huggins "No Bubble" NDL table along with an Edge or Skinnydipper computer which utilize the same algorithm. As we all know, many dive computers utilize a proprietary algorithm. For instance, one might find it difficult to match up a Suunto dive computer with a Suunto modified RGBM table (as far as I know these don't exist). Nevertheless, using PADI RDP tables (DSAT algorithm based on USN tables + some Doppler bubble studies) will probably yield a workable approximation of residual nitrogen time for dives going forward. Also, if one uses this methodology, it might be useful to note that "backing out" pressure groups in this way will likely yield NDLs that are more conservative than what the dive computer would report (even given a true square dive profile) since the computer-based algorithm may have already built-in some additional NDL penalties for future dives. For example, with a Suunto computer, NDL penalties on subsequent dives might be imposed for what the computer deems to be an ascent that was too fast (faster than 33 ft/min).

Enjoy the reading and dive safely...

 

[Interceptor121]

If my computer fails (and it has happened) my back up instruments (currently a Citizens Aqualand) provides sufficient information from me to switch to tables. All I really need is my max depth and time, since by now I have committed the NDLs to memory. Thus you "invitation" is easily met.

In some cases looking at the tables you would already be over the NDL at the maximum depth

Thanks for trying, see you at the hospital then?

 

[Thalassamania]

In some cases looking at the tables you would already be over the NDL at the maximum depth

Thanks for trying, see you at the hospital then?

What ever are you talking about?

 

[LeadTurn_SD]

In some cases looking at the tables you would already be over the NDL at the maximum depth

Thanks for trying, see you at the hospital then?

Do you mean that swicthing to for example the PADI RDP to determine NDL's for a repetitive dive using the max depth and bottom time of your previous dive would put you at more risk than diving a computer?

When the tables assume a "square" profile even on a multilevel dive, and thus will be more conservative on the following dive than a computer?

Or do you actually mean that a diver who maintains no awareness as to the max depth and elapsed time of their dive and simply "rides the computer"... who then is forced to switch to the tables without adequate information to use them properly?

Just don't understand your comment....

Best wishes.

Edit: Oh, wait I think I understand your point.... just not the "hospital comment". The diver swicthing to tables "on the fly" would be instantly more constrained by the "square" profile of the table, and following the tables would be forced end the dive early, and safely.

 

[diver 85]

In general, the computer---it will give you 'credit' for varying depths---tables are strickly a square profile ie you go up a little & you will not get 'credit' for that shallower depth.....

 

[gcbryan]

In some cases looking at the tables you would already be over the NDL at the maximum depth

Thanks for trying, see you at the hospital then?

I think the point is (I think) that if you know the single dive NDL's (or carry the tables with you if you don't) you can approximate remaining time after a computer failure.

If you are diving at 70 fsw for 20 minutes and your computer dies and you want to go to 50 fsw for 20 minutes you should be fine since your average weighted depth is 60 fsw, and your profile was going from deep to shallow, and 40 minutes at 60 fsw would be fine according to the tables.

If it's your second dive of the day reduce the NDL's accordingly. It's an approximation and it would keep you in the water.

(I'd love to read the Rubicon report but I have a slow connection and downloading 197 pages is too painful!)

 

[RJP]

This whole idea of taking the table in the water as back up of the computer is complete rubbish.

That's rubbish.

 

[Thalassamania]

Thou rubbish me the wrongth way.

 

[Red Sea Shadow]

This whole idea of taking the table in the water as back up of the computer is complete rubbish.
The table assumes you stay at a certain depth and then go to 5 meters for a safety stop, the computer takes into account your real dive profile the two things do not mix.
The best back up plan for a computer is another computer because if your computer had to fail it would be EXTREMELY difficult to go back to the table because:
1. You needed a timing device
2. You needed to know the residual NDL when the computer failed
3. You could only have a vague idea of how long you can really stay down if at point of computer failure you knew the residual NDL and therefore you could not have continued the dive

When a computer fails the procedure is to ascent at 5 meters and perform a safety stop of at least 3 minutes or longer air permitting.

You find this in all computer manuals so let's be practical and not talk theory here

I invite anybody that believe that the table can back up a computer to take the tables down and at a certain point put the computer in their pockets after the last reading of the NDL.
Now think about it are you going to go and read the table to continue the dive or you simply ascent at 5 meters?

I totally agree. For non-decompression diving, computer's best backup is another computer. Alternatively, if you know the dive site's topology, you can plan you dive using the tables, write your plan on a slate, and use it as backup (or as primary plan and keep the computer as backup if you wish).

 

[Interceptor121]

NDL readings from computers and table are not usually interchangeable unless the computer is based on the same model behind the tables.
Recreational tables from DSAT (PADI) are based on Spencer 60 minutes wash out model, NAUI are based on US Navy
In a classic example of Suunto or Uwatec computers both based on Buhlmann microbubble model the pressure group and reading of the computers cannot be interchanged with the PADI or NAUI table.
So unless you are taking exactly the same table on which the computer is based down with you, you have to abort the dive if the computer fails.

Even if you had the same tables on which the computer is based you could have problems as the computer would take into account the real dive profile
If you have been doing a dive where you had been spending say 10 minutes at 30 meters and 20 minutes at 18 meters and the computer failed just by reading the maximum depth of 30 meters and the dive time of 30 minutes you would have concluded that you had exceeded the NDL of 10 minutes according to the PADI tables or 8 minutes for the NAUI tables and the procedure would be to do an emergency decompression of 15 minutes at 5 meters.
Instead you had actually other 9 minutes at 18 meters according to PADI or 11 minutes for NAUI.
Now let's have a look at a typical computer manual
STEP 1:
Assess the situation calmly and then move promptly to less than 18 m [60 ft].
STEP 2: At 18 m [60 ft], slow down your ascent rate to 10 m/min [33 ft/min] and move to a depth between 3 and 6 meters [10 to 20 ft].
STEP 3: Stay there as long as you assess your air supply will safely allow. After reaching the surface stay out of the water for at least 24 hours.

So unless you have your tables based on the same model behind your dive computer and you have done a single level profile, which in real cases does not happen very frequently, you can't fall back on your tables

For practical reason the recommended safety procedure in case of computer failure remains the one of your computer manual.