How does the table math work?

[Blackwood]

So you did not get the point, that teaching dive computers is easier, more complete, and less error prone than teaching dive tables?

I'm not sure I agree that teaching dive computers is necessarily more complete (though it certainly has the potential to be) - as I said, it's a function curriculum, etc.. Easier is debatable. With a table, it's easier to demonstrate the depth/time relationship with a large number of examples.


In any case, it's long been my position that the average basic dive class should put more emphasis on PDCs, since the vast majority of divers will go on to use PDCs almost exclusively.

 

[rstofer]

So it is a dive planning tool that does not go underwater?

Basically an inexpensive dive computer that you cannot get wet?

Yup!

They can do what the tables do but they can only be used on the surface. They have a neat 'Back' button that allows you to alter the depth for the next planned dive thus giving you the as many ANDLs versus depth for the next dive as you wish. But they can't get wet...

Richard

 

[tesnusxenos]

Yah, we always talk a lot. Especially when someone makes an interesting post, like yours.

BTW, I can also tear my jeep apart and fix it, and also my DPV, and my motorcycle, and not to mention also my scuba gear (although I do take the regs themselves to an authorized service center annually / but I do understand the principle of the piston regulator / I'm just too pressed for time to take a DEMA class from Scubapro on Mark 20 & S600 maintenance).

I take things apart and figure out how they work too. But, I understand that not all people have that desire...some are content just to use stuff and that is cool too. 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!

 

[Reg Braithwaite]

Wow.... I am surprised that this topic caused so much conversation. I understood how to use the tables, I just wanted a clarification on the math behind them.

Welcome to the internet ;-)

But back to your original question. Quite honestly, I think the "User Interface" for the standard tables is terrible in a way, because although it is simple to use them as instructed to get a result, they do not build a coherent "mental model" of the process.

As noted before, the residual nitrogen TIME is adjusted for depth. So if the table shows a No Deco Limit (NDL) of 30 minutes at a certain depth, it is saying that if you have no residual nitrogen in your body, if you remain for up to 30 minutes at that depth and surface using the proper protocol, you have an acceptably small likelihood of suffering from decompression sickness.

Given a dive to a certain depth and a surface interval of a certain amount, you may discover that you have 22 minutes of Residual Nitrogen Time at that depth. This is a complicated way of saying that the residual nitrogen remaining in your body is equivalent to spending 22 minutes at that depth.

Since that depth had an NDL of 30 minutes, you now dive to that depth for no more than eight minutes while still maintaining an acceptably small likelihood of suffering from decompression sickness.

The reason why the RNT is greater for a shallower dives is that you would need to spend more time at a shallow depth to accumulate that much nitrogen. So really, all those different RNTs are a way of expressing the quantity of nitrogen in your tissues. The different numbers for different depths serve to simplify the calculation for someone who just wants to follow the table by rote, but complicate things if you want to try to infer the underlying process from the mechanics of the tables.

On the other hand, our existing models for the underlying process are actually quite complicated. If we discuss tissue compartments and gas gradients and so forth, we could work out a dive plan by hand from first principles, but the math would be heinous.

The models we have do not have a direct and obvious correlation to the tables, either. For example, if you know that you have a NDL of 30 minutes at a certain depth, you might assume that you accumulate nitrogen at a rate of 1/30th of your maximum residual nitrogen per minute. But this may not be so, the accumulation may follow a curve.

Likewise you may mistakenly assume that the tables follow depth in a straightforward way, but upon closer examination you will discover that they are closer to following pressure in atmospheres, such that 60 feet of depth does not have twice the pressure of 30 feet of depth but rather only 50% more pressure.

So swerving back to our digression, teaching a table does not directly teach an understanding of the underlying process, nor is it a direct simplification of the process. But it serves a certain limited useful purpose, as does the computer.

My personal observation is that I learned almost nothing about the process in my OW class, I quickly realized that the instructor was just trying to rush us through the course and into his dive shop. I did my OW checkout while on vacation, and the instructor there sold me on taking the "Nitrox Diver" program, and I felt like it was the "missing manual" for the tables.

I am not qualified to compare and contrast agencies or curriculae, but I can tell you that I felt that the Nitrox course did a lot to answer questions the OW course raised. There were and still are a lot of questions in my mind, but it was a start.

 

[Blackwood]

Since that depth had an NDL of 30 minutes, you now dive to that depth for no more than eight minutes while still maintaining an acceptably small likelihood of suffering from decompression sickness

to complete that sentence: "after making a direct ascent to the surface at xx feet per minute."

 

[Reg Braithwaite]

to complete that sentence: "after making a direct ascent to the surface at xx feet per minute."

Funny, in my first draft of that post I think I had something about returning to the surface using the protocol associated with the table in both places. For example, the most common tables mandate a certain ascent rate and strongly suggest a certain safety stop.

But you're right, if you ascend too quickly you are not safe and conversely, if you ascend too slowly you can actually accumulate more nitrogen down deep.

 

[rstofer]

Funny, in my first draft of that post I think I had something about returning to the surface using the protocol associated with the table in both places. For example, the most common tables mandate a certain ascent rate and strongly suggest a certain safety stop.

But you're right, if you ascend too quickly you are not safe and conversely, if you ascend too slowly you can actually accumulate more nitrogen down deep.

To further complicate matters, PADI stops the dive time when you BEGIN a direct ascent to the surface.

NAUI stops the dive time when you REACH the surface. They do allow you to ignore any time spent at a shallow safety stop.

Consider a dive to 120 fsw with and NDL of 12 minutes and 4 minutes to ascend. That leaves just 8 minutes to descend and swim around according to NAUI. In fact, if you descend at 60 fpm, there is really only 6 minutes of actual bottom time. Six minutes ascending and descending and 6 minutes swimming. Hardly worth going.

Richard

 

[Blackwood]

Funny, in my first draft of that post I think I had something about returning to the surface using the protocol associated with the table in both places. For example, the most common tables mandate a certain ascent rate and strongly suggest a certain safety stop.

But you're right, if you ascend too quickly you are not safe and conversely, if you ascend too slowly you can actually accumulate more nitrogen down deep.

It's still there... third 'paragraph'

I 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.

 

[SteveAD]

The most recent incantation of PADI OW essentially replaces the tables with the eRDP; they don't even include the table in the Crew Pack, just the calculator. The calculator is a lot easier to use and will give the Adjusted NDL (ANDL) for the next dive given the parameters of the previous dive(s) and the SIT. It's pretty slick! There is even a multilevel version for those that are interested.

That said, eliminating the tables or limiting the discussion isn't a good thing from my point of view. I like to see the context of being a 'K' diver and the alternatives for SIT and ANDL for the next dive. But that's just me! I started using NAUI tables back in '88 and I'm still comfortable with them today.

The PADI Nitrox course uses tables extensively so if the OW student didn't use them earlier, they will use them in this class.

But, when you add the complication of O2 loading and MOD limits to N2 loading, perhaps a computer is something to consider. I bought the Dive Rite Nitek Duo for that very purpose. But I will always have the tables for backup.

Richard

PADI has not replaced the tables with the ERDP. There are several versions of the crew pack available, some with tables, some with ERDP, just as they used to be available with the wheel. It is up to the instructor and or dive center which version to provide to the student.

I still supply and teach tables (and will continue to do so) because I feel that is the only way that students will develop an understanding of what a computer will tell them. Teaching students to dive computers without giving them a solid grounding in the theories they are based on is the same as teaching kids in school to use calculators without teaching them to do basic math.

 

[rstofer]

PADI has not replaced the tables with the ERDP. There are several versions of the crew pack available, some with tables, some with ERDP, just as they used to be available with the wheel. It is up to the instructor and or dive center which version to provide to the student.

I still supply and teach tables (and will continue to do so) because I feel that is the only way that students will develop an understanding of what a computer will tell them. Teaching students to dive computers without giving them a solid grounding in the theories they are based on is the same as teaching kids in school to use calculators without teaching them to do basic math.

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