Why Aren't Dive Tables/Computers Customizable?

[HJeffreyM]

One of the things I was surprized to learn during dive training was that everyone, no matter what their size, sex, age or physical characteristics, uses the same non-customizable dive tables.

Given the same dive profile, the same computer would give my wife and I (Me: 49, 6'3" 240/She: 39, 5'0", 112) the same nitrogen tissue bar load with the same time to desat.

Common sense would lead me to believe that she and I might off gas at significantly different rates. Am I wrong about this? Why can't these tables/tissue loading models be customized to individual physical characteristics?

 

[mikemill]

Because all the models are based off of statistical approximations. The fact is that there is no formula that given a bunch of parameters will give you a 100% correct answer. We simply don't have the knowledge at this time.

 

[DevonDiver]

If you remember back to your OW course, you will see that individuals can add greater conservatism to their dive tables, by adding extra theoretical depth to their dive planning,

 

[Turbosaurus]

Maybe I am incorrect, but from a purely mathematical standpoint, wouldn't it all be proportional even if there was a difference?

You are bigger, so you have bigger lungs, so you take in more air, so you probably absorb more nitrogen into your tissues,(since you have more tissues), and probably release more nitrogen with each breath you exhale at the surface, (since your breaths are bigger) but you also have more nitrogen to get rid of. I would think that on a % basis its going to be about the same for both of you. If you are going on an assumption that your body would rid itself of more nitrogen in the same time period in a surface interval, wouldn't the opposite would also be true? That you would absorb more nitrogen under pressure in the same time frame?

My friend Christine and I dive together and we're both about the same size. She's always dragging me up when she hits 700psi and I still have 1,100+ left. That means we are the same size, but she just breathed in a lot more nitrogen (because she breathed in more total air), but there is no adjustment for that either.

on the opposite side of that argument, I am a smoker and she is not, therefor the nice clean pink insides of her lungs should be more efficient as gaseous exchange, but again, no adjustment.

Honestly I do not think our body mass or lung capacity or any personal attributes have anything to do with it. It was my understanding it is a matter of purely physical laws. The issue with nitrogen saturation is solely due to the effects of extra atmospheric pressure on how nitrogen as a gas behaves in solution, so time and depth are the only relevant measures. Does anyone know if that's right?

 

[LeadTurn_SD]

One of the things I was surprized to learn during dive training was that everyone, no matter what their size, sex, age or physical characteristics, uses the same non-customizable dive tables.

Given the same dive profile, the same computer would give my wife and I (Me: 49, 6'3" 240/She: 39, 5'0", 112) the same nitrogen tissue bar load with the same time to desat.

Common sense would lead me to believe that she and I might off gas at significantly different rates. Am I wrong about this? Why can't these tables/tissue loading models be customized to individual physical characteristics?

Because the calculations are based on theoretical models, not actual living tissue.... at least that is how I understand it. The models have been tested enough with a broad enough selection of the population to know that they work the majority of the time, on the majority of dives, with the majority of divers (as long as you don't press too hard on the edges of the model "envelope").

I think what you are proposing may be possible in the future, but my guess is that it would require implants or some way of transmitting real time data from within your body to the dive computer.

Interesting to think about this stuff though.

Best wishes.

 

[Interceptor121]

Because the calculations are based on theoretical models, not actual living tissue....
Best wishes.

The data on dive computer and tables are based on empirical data over a theoretical model. The computer only computes the adjusted non decompression time taking into account the time spent at certain depth but has no more advanced model than the table. It just takes off any rounding.
Which means that there is no knowledge of underlying mathematics to map the nitrogen absortion and release but just a set of test data gathered in real or decompression chamber dives.
So as there is not an equation that takes all various parameters into account (weight, age, fitness, conditions) and besides those factors may or not be interrelated so finding out the dependecies could be a very expensive exercise.
Which means that the dive computer will have a set of standard tables and then a few more conservative profile which you should use at your discretion and that should do the job for you

I met an guy 65 who got bent without breaking any limits of his computer and since he uses the more conservative profile on his computer and that is about it
So in short you are not going to get what you ask for without a significant leap in the underlying research

 

[vladimir]

Maybe I am incorrect, but from a purely mathematical standpoint, wouldn't it all be proportional even if there was a difference?

You are bigger, so you have bigger lungs, so you take in more air, so you probably absorb more nitrogen into your tissues,(since you have more tissues), and probably release more nitrogen with each breath you exhale at the surface, (since your breaths are bigger) but you also have more nitrogen to get rid of. I would think that on a % basis its going to be about the same for both of you. If you are going on an assumption that your body would rid itself of more nitrogen in the same time period in a surface interval, wouldn't the opposite would also be true? That you would absorb more nitrogen under pressure in the same time frame?

My friend Christine and I dive together and we're both about the same size. She's always dragging me up when she hits 700psi and I still have 1,100+ left. That means we are the same size, but she just breathed in a lot more nitrogen (because she breathed in more total air), but there is no adjustment for that either.

on the opposite side of that argument, I am a smoker and she is not, therefor the nice clean pink insides of her lungs should be more efficient as gaseous exchange, but again, no adjustment.

Honestly I do not think our body mass or lung capacity or any personal attributes have anything to do with it. It was my understanding it is a matter of purely physical laws. The issue with nitrogen saturation is solely due to the effects of extra atmospheric pressure on how nitrogen as a gas behaves in solution, so time and depth are the only relevant measures. Does anyone know if that's right?

An article in the May 2006 edition of Undercurrent had this to say in an article about research into reverse profiles:

The researchers chose the dive profiles to comply with the work-shop recommendations, but they shortened the time for guinea pigs so that their exposure would approach the no-decompression limits. As there is a direct relationship between DCS susceptibility of a species and its body mass, guinea pigs have a much lower DCS susceptibility than humans. So, “human” depth limitations should carry much lower risks of DCS for guinea pigs.

I would not assume that we can extrapolate from species of different body masses to individuals of different body masses.

 

[DA Aquamaster]

I agree - the diference in mass between guinea pigs and humans is substantial - several times greater than the mass diffreneces within humans that dive.

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Regarding the OP's question:

To a large degree the correction in tables to accommodate larger mass individuals has already occurred. When I started diving in the early 1980's everyone used the US Navy tables or a cosmetically altered variation of them. The US Navy tables had been based primarily on study and control groups of males in good physica with an average age of 23. And even then the US Navy tables had a hit rate of 1% on square profiles to the NDL or on deco profiles and that hit rate rose to 4% on square repetitive profiles. Consequently, in Navy use, repetitive dives were seldom done and decompression dives were normally not done without an on board recompression capability. Navy divers also adopted next greater depth and time protocols that added additonal safety factors and square profiles were generally avoided and these safety factors became an essential part of using the tables in recreational settings where the divers were often older, in les than stellar physical condition and increasingly female.

In the late 1980's many agencies adopted reduced NDL's on the US Navy Tables based on doppler ultrasound research that were designed to reduce the occurence of "silent" bubbles. Safety stops also became very common, especially on dives deeper than 60' and these changes in effect adjusted the tables for a more normal civilian/recreational diving population and reduced the importance (but not entirely in some cases the need) of using next greater depth and time as it was in many respects built into the tables and how they are used.

In the early 1990's other decompression tables and models began to appear, most of which are more conservative and better suited to recreational diving.

So in essence, modern recreational dive tables do take into account differences in diver size and condition, but it is in terms of using a larger body mass civilian diver as the common denominator. The option is still out there for divers to use the greater NDLs of the US Navy Tables, but they need to do so fully understanding the risks and need for to incorporate safety factors.

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With regard to computers:

Many dive computers now offer the ability to adjust the level of conservatism based on a percentage of the M values, by adjusting gradient factors or just on a simple linear scale. These are most often computers intended for technical diving but some are still relatively low cost puck sized computers that are also well suited to recreational nitrox diving. (Tusa IQ-700/Dive Rite Duo, Scubapro Tec 2 G).

Within the range of recreational computers conservatism levels vary greatly and by selecting a model at one end or the other of the range you can in effect add or remove conservatism based on your individual nody mass index and fitness levels.

However, even a "normal" average computer that does not offer user adjustable conservatism can be dove in a manner that adds conservatism. Most have a nitrogen loading bar that reflects the loading of what ever compartment is controlling the dive. The diver can use that bar to end the dive short of the NDL (perhaps at the top of the green or in the middle of the yellow rather than at the top of the yellow) to add conservatism. Similarly, on shorter, deeper dives where the faster compartments are controlling, the diver can stay at the safety stop a few minutes longer to allow the bar to recede out of the yellow and one or two notches into the green to add conservatism.

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Other custom table options:

A similar ability to adjust the conservsatism of tables exists with software using variable permeability and bubble gradient models. I use both Palm VPM, a program using a variable permeability model (VPM-A) and DPlan (a Palm based version of DPlanner) which is a bubble gradient model.

I essentially enter the dive plan and the software gives me the depth/time/table information (and on deco dives the deco schedule). The conservatism is adjustable on Palm VPM using settigns from 1-4 and is adjustable on DPlan my modifying the gradient factors.

There are pros ad cons. The user interface is much easier with DPlan, particularly in terms of multi-level dives repetitive dives, but you need to understnad how gradient factors work to adjust the shape of the curve as a single set of gradient factors will not be the same in terms of conservatism on widely different dive profiles.

Palm VPM on the other hand offers an easy 1-4 level of conservatism but you need to exprerss the profile in a particular text format and then import it to Palm VPM.

There are also computer based applications that can be done on a laptop and they are becoming more attractive for daily on-the-boat use given the availability of smaller laptops and toughbooks that are more ammenabel to the potential abuse that may occur in diving. But I still like my Palm as it stays water tight on the dive boat in what amounts to a specialized zip lock bag and does not add any significant weight or bulk to carry around.

 

[D_B]

I agree with DA, you can use your tables, or computer differently depending on depth, temperature of the water, current, even the vis and how your feeling that day .. and you should

 

[HJeffreyM]

I think what you are proposing may be possible in the future, but my guess is that it would require implants or some way of transmitting real time data from within your body to the dive computer.

Maybe a glove with something like a pulse oxymeter/doppler sensor in it? It could be linked directly to a DC worn on the wrist.

Just an idea....