How does the table math work?

[SteveAD]

I agree with you and applaud your approach. I like tables and the eRDP thing just doesn't leave me with the warm fuzzy feeling of tables.

I need to study the Wheel. There's something there to learn and I need to get after it. One of these days...

Richard

Actually the wheel is being "obsoleted" by the erdpml. There were always precision problems with the wheel as demonstrated by the fact that during the deco theory exam in the IE it was always briefed that if you can show the examiner how you properly got your "wrong " answer on your wheel, you could get credit for a correct answer. The erdpml elimininates the mechanical discrepancies.

 

[Thalassamania]

A few thoughts:

PADI's tables were developed from scratch, the operational parameters such as ascent rate and the definitions conform to those of the US Navy at the time that PADI developed their tables.

NAUI's tables were "developed" by cutting back the US Navy tables one step and changing some defintion (like bottom time) to make the result even more conservative.

The discussion of tables vs. computers is, frankly, nonsensical. Tables are snapshots of a model while computers are a cartoon of a model, but the model remains and is still the basis. Current diver education fails because divers do not understand this and do not understand the underlying model(s). Most instructors do not understand this either but somehow are able to content themselves with arguing over what was better, the book or the move version, without any comprehension of what the plot line was.

 

[Reg Braithwaite]

may take issue with the word "proper" though. Rather, I'd say "minimum".

My admittedly poorly stated point was that NDL is a function of ascent rate, something that is glossed over in most classroom discussions. Most divers are taught to go slower than 30 or 60 "because it's safer", but I find it's rarely spelled out that a 90fpm ascent on a 60fpm table invalidates the NDLs.

I agree with your point that a 90fpm ascent violates the assumptions used to build the table and therefore may increase the risk of decompression sickness above acceptable levels.

I also say that it is not entirely correct to say that the recommended ascent rate is a minimum. If you ascend too slowly from depth, you may get into trouble because you are accumulating nitrogen faster than off-gassing it. I am not an expert, but I believe this effect is most pronounced at depth.

Both the table and the function of a computer are based on a particular set of assumptions, and it can be dangerous to stray too far from those assumptions, no matter how well intentioned we may be.

 

[Thalassamania]

I agree with your point that a 90fpm ascent violates the assumptions used to build the table and therefore may increase the risk of decompression sickness above acceptable levels.

The ascent rate is a variable that is part of your table calculations. If you take a table to it's NDL and then violate the ascent rate you increase the stochastic risk of DCS. If you are back from the NDL a faster ascent rate is not likely to hurt you. A study of divers back in the 1970s showed that they were routinely doing 100 to 120 FPM and were not getting bent, but they were rarely out at their NDL.

I also say that it is not entirely correct to say that the recommended ascent rate is a minimum. If you ascend too slowly from depth, you may get into trouble because you are accumulating nitrogen faster than off-gassing it. I am not an expert, but I believe this effect is most pronounced at depth.

This is true ... but it'd have to be mightly slow. The real issue here is the ascent being slow enough to permit the uptake of sufficient nitrogen over sufficient time to result in changing the controlling tissue.

Both the table and the function of a computer are based on a particular set of assumptions, and it can be dangerous to stray too far from those assumptions, no matter how well intentioned we may be.

Yup, hell hath no fury as an unwarranted assumption.

 

[Blackwood]

I agree with your point that a 90fpm ascent violates the assumptions used to build the table and therefore may increase the risk of decompression sickness above acceptable levels.

I also say that it is not entirely correct to say that the recommended ascent rate is a minimum. If you ascend too slowly from depth, you may get into trouble because you are accumulating nitrogen faster than off-gassing it. I am not an expert, but I believe this effect is most pronounced at depth.

Ascent rate and NDL are inseparable terms. While there most certainly is a minimum decompression (a function of depth and rate) associated with each and every depth/time combination for which an NDL applies, I'd venture to say that maximum decompression may be beyond the limits of practicality.

For a direct ascent, D_max / Ascent Rate = decompression time. If that ascent rate is the maximum allowed by the given model, then that decompression time is a minimum (i.e. any less decompression and you have blown the NDL).

I believe that a direct ascent is rarely optimum, and thus I have a hard time calling it 'proper.'

 

[divestoclimb]

This is true ... but it'd have to be mightly slow. The real issue here is the ascent being slow enough to permit the uptake of sufficient nitrogen over sufficient time to result in changing the controlling tissue.

He's right that it can be a problem, but it's usually only noticeable with deeper dives that get you into decompression, and usually involve helium. For example, you do a trimix dive to 180 feet for 20 minutes and you plan on switching gases at 70 feet to a nitrox mixture (note that these numbers are made up and don't represent an actual dive profile). If it takes you a minute longer than you planned to ascend from 180 feet to 70 feet, not counting stops (equivalent to a 23 feet per minute ascent as opposed to 30), that's an extra minute you've spent breathing helium that must now be offgassed.

 

[Thalassamania]

Gas switches always add a bit of voodoo to the calculations.

 

[nereas]

... If my Mom or sisters want to dive without understanding why the table works that is fine with me as long as they know how to use it and do use it!

They would however be better off strictly learning how to use the dive computer, because tables are poorly taught and even less well understood.

 

[Thalassamania]

They would however be better off strictly learning how to use the dive computer, because tables are poorly taught and even less well understood.

It's the message not the media.

 

[nereas]

Gas switches always add a bit of voodoo to the calculations.

The DiveRite NitekHE does all the calculations for you, if you are planning to switch gasses.

So does the VR3.

You simply only need to input the gasses you are using correctly, and switch underwater appropropriately, and tell your computer what you have done, and then it gives you your next stop depth and stop time.

And V-Planner will have done the same in advance for the dive plan and gas planning. The V-Planner output also provides on-the-fly backups that you can memorize as well, or you can put it onto tech forearm-slates as a backup to your dive computer.

There is simply no need to do any calculations yourself in the 21st Century.

Unless you really want to, yourself, the hard way, either with a pencil and calculator or else with an M/S Excel spreadsheet.