What should I trust?

[DaleC]

Forget the table entirely and use the computer for both planning and filling up the log book

I don't know if you meant it this way but I wouldn't just forget about the tables (if not please disregard).

One should study the tables and familiarize themselves with the various ratios of depth to NDL limits. This helps when either your computer fails or gives you a funky reading. If you are at 100 feet for forty five minutes on air and the computer says you are fine you may want to remember the NDL's suggest otherwise.

I also use my tables for pre dive planning and determining the best mix for the dive based on the NDL I want (along with other tools).

Another way I've wondered if a computer can be "less safe" is that it allows you to fully saturate yourself (within NDL limits) on a non square profile dive. If one uses the air tables (for example) a diver will usually descend to 100ft for 20 minutes max and then ascent directly. On a computer one can dive to 100 for 20 then ascend to 80 for several more minutes then up to sixty for some more minutes etc... surfing the edge of the NDL for as long as the gas lasts. I don't know if there really is more N loading this way or not but it's something I've been wondering about.

 

[Dr Neil]

Tables and computers don't mix.

While the table assumes you did the entire dive at the maximum depth, the computer will calculate your profile as you dive. You can use the tables in planning your dive, but you will get a very conservative dive profile.

It's good to keep in practice by using your tables after the fact, just don't combine the two while diving.

 

[Interceptor121]

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?

 

[Thalassamania]

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?

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.

 

[AbyssalPlains]

Just a general newb question.

Did some dives, with my nice new dive computer that up until now I had only tested in the pool... perfect, both recorded as No Deco. Dive tables though - another matter. Rounding up from my max depth they put me into Deco. The general consensus from everyone I asked is "trust the computer, its more accurate".

Is this the advice you would give? Does anyone even record the pressure groups etc in their logs or do they just record bottom times and depths and SI and note that their computer stated it was No Deco??? Am I just being typically picky about things bothering about this?

What you observed is the precise reason why you're using a computer instead of tables. Tables are "dumb" - if you do a dive to 120 feet, stay there for 3 minutes, then slowly work your way up to shallower depths where you spend the remaining 30 minutes of your dive, the table won't know that and treat the dive as if you had dropped to 120 feet and stayed there for the entire 33 minutes of the dive. As a result, you will have a huge deco obligation according to the table. The computer, on the other hand, is "smart." It samples your depth every few seconds, so it knows the ACTUAL times you spend at any depth at any time during the dive. It takes all this information and calculates your resulting no-deco limit in real time. That's why a computer lets you do dive profiles that are totally out of the question using tables.

 

[Thalassamania]

... That's why a computer lets you do dive profiles that are totally out of the question using tables.

Just for the sake of complete accuracy it should be pointed out that there are table based procedures such as, "repping up" that permit the use of standard tables for multilevel dives.

 

[gcbryan]

It also should be pointed out that if you are on the 4th dive of the day, all using a computer, and the computer goes out mid-dive reverting to a table while underwater might not be all that useful.

That's not to say that if you still have a dive watch with time and depth you shouldn't be able to continue to dive (with a general knowledge of NDL's) and some conservative modifications to your dive profile.

 

[Thalassamania]

Might I point you in the direction of Mike Emmerman's paper on dive computer logs in the AAUS Dive Computer Workshop Proceedings that you can find at RUBICON?

 

[gcbryan]

Might I point you in the direction of Mike Emmerman's paper on dive computer logs in the AAUS Dive Computer Workshop Proceedings that you can find at RUBICON?

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

What is the pertinent info in that paper?

 

[Thalassamania]

Mike wrote a piece on how to move from a computer to a table based repetitive dive group.