Why Aren't Dive Tables/Computers Customizable?

[DandyDon]

I'm sure irt has already been mentioned that many computers are... IF you read the manual.

I think I may have given up on suggesting the idea. I used to urge new divers to study the manuals, play with the puters, run Simulated dives on them - but I guess I don't bother with that anymore. I did with my home bud, ran Sim-dives with him, but he forgot and didn't say anything. His attitude is to wear it so that I can tell him what it says.

 

[gcbryan]

Ok, here's my question on the symmetry issue:

I go to say...66fsw on air, stay for 20 minutes and do a normal ascent. Total dive time: 30 minutes. I surface with two or three "nitrogen bars" on my Elite T3. Assuming I was at 3ATM at 66fsw, should it then take 90 minutes for those bars to dissapear at 1ATM? 60 minutes? Am I not getting this symmetry thing?

Confused....

I can't say exactly what a nitrogen bar represents on your computer however the symmetry is referring to compartment halftimes (it's an exponential thing). Most dive computers are representing your body with up to 16 compartments (loosely representing different types of tissues in your body). These on gas at differing rates...5 minutes compartment, 12 minutes compartment up to 600 or so minute compartments in some models.

A 5 minute compartment "fills up" halfway in 5 minutes. The half that's left fills half way in another 5 minutes and so on. After 6 halftimes it's considered essentially full. The same process happens in reverse for off gassing. The process is symmetrical in that sense.

Your computer is making these calculations even while you are out of the water on your surface intervals. Dr. Deco was just saying that in a perfect world you would try to keep things roughly the same after the dive as during the dive to keep the rates symmetrical (the same) since this is what you're computer is assuming in it's calculations.

This is only if you wish to learn more about deco theory in detail. Otherwise, do what your computer indicates and you will probably be fine. There are no guarantees with DCS or with diving in general and this is part of the point to be made as well I think.

 

[Dr Deco]

Hello Readers:

Submersible Blood Gas Monitors - Tissue Nitrogen Monitoring - - -

One of the first studies that I did while at Ocean Systems in the early 1970s was using a mass spectrometer to monitor tissue nitrogen [in rats]. It was a cumbersome and complicated device and did not yield useful data.

It was through studies such as this [and Doppler monitoring] that I gradually developed the concept that DCS was the result of:

1. gas loads in connective tissue rather than muscle tissue [muscles can contribute to venous Doppler bubbles and neurological DCS]. And

2. something other than gas loads was also playing a large role in the cause of DCS [later developed to “tissue micronuclei” generated by physical activity].There does not appear to be any way to monitor dissolved nitrogen, tissue micronuclei, or decompression bubbles in “critical” tissues at the present time.

Symmetrical Exchange if Gas

By this is ment [as others explained] that there is the same rate of elimination in a compartment as nitrogen uptake. This symmetry changes e.g., if the diver was very active on the bottom and very sedentary topside, or if the diver was very warm on the bottom and cold topside.

Likewise, if many decompression bubbles form, the nitrogen will be sequestered [trapped as free gas] in them and not eliminated as fast as dissolved nitrogen.


Dr Deco :doctor:

 

[Andy077]

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?

One thing, about you and your friend Christine, it doesnt matter how much N2 either of you breathed, measured by how much of the tank you used, the only thing that is relevant is the time the lung tissues were exposed to a higher ppN2 in the air in the lungs than in the tissues. So little N2 is absorbed into the blood that the ppN2 exhaled is nearly the same as inhaled, even under pressure. To put it another way, N2 absorption into the tissues is a function of (ppN2{air} -ppN2{tissues}) X time. The number of air exchanges in the lungs (how much of your tanks you used respectively), isnt relevant.

 

[I Dive]

It's not an exact science, you can follow the tables and/or your computer and still get DCS-or you can violate the tables and/or computer and walk away. If you can find a way to make it an exact science and consequently customize it to individual divers, you will be a very very wealthy person.

 

[Dr Deco]

Hello Readers:

Customized Tables

In a sense, all tables are “Customized” such that DCS will not occur. What can be wrong with that? Of course, what you mean is presented such that each individual diver could maximize bottom time for every dive. This would present problems for divers in a group since each would have different depress schedules – not convenient.

Some commercial diving companies in the 1970s did have individual schedules made for each diver. This was based on the diver’s height, weight, and percent fat. It was wishful thinking as much as anything else, but it probably looked good as a selling point.

Diving science, at least at the level of the recreational diver, is wedded to the Haldane concept of halftime compartments for the derivation of depress schedules. The noD limits are derived from actual dive data on human test subjects. What divers really desire to know is “Why can I not have my own set of NDLs?”

All NDLs are determined from test subjects in a given set of conditions – usually seated in a chamber. In recent decades, this has been modified by exercising divers. One good system had NDLs from US Navy schedules reduced by a set amount to correspond with data from Doppler ultrasound studies. This formed the basis, for example, for the PADI Recreational Dive Planner. Nonetheless, all divers are lumped together.

"Inexact Science"

While this is often said, what is a better statement is that most depressurizations can produce bubbles in many locations of the body – and make it appear chaotic. Bubble generation will depend on musculoskeletal activity in a given tissue, and nitrogen loads will depend on blood perfusion through those tissues. This cannot be determined in advance. In addition, there are other questions such as “Why do some people form bubbles more readily than others? I guess it is some biochemical constituent currently unknown. Does this change from day to day? Not really known.

You would need NDLs that were different for each activity level both from the viewpoint of nitrogen loading and nuclei generation.

Not really possible.:depressed:

Dr Deco :doctor:

 

[WetCell]

In a sense, all tables are “Customized” such that DCS will not occur. What can be wrong with that? Of course, what you mean is presented such that each individual diver could maximize bottom time for every dive. This would present problems for divers in a group since each would have different depress schedules – not convenient.

...

Not really possible.:depressed:

Dr Deco :doctor:

I disagree. In fact, I have wondered why we don't have this and the answer clearly is that "dive science" is hardly sophisticated enough to proceed.

I see three very limiting factors.

1) Dive tables are basically derived from Navy divers. Twenty-somethings in very good shape, the majority males. My 16 YO dive buddy (my son) has a frighteningly low SAC compared to the way I hog air. He is also 10% body fat (or less!) compared to my 30 something The dive tables exist somewhere in between him and me, and far favoring him. Oh, and don't say that tables are also done by university researchers--20 something college kids are the same as 20 something Navy seamen.

2) The recreational diving organizations are faced with the monumental task of safeguarding safety on an absolutely huge population spread of divers. As such, they must be very, very conservative.

3) Commercial diving has a vested interest in customization, but nowhere near the resources to do the job right.
​

I have a scale that does a good job at measuring my biometrics with respect to body composition. True, I do not have a hydrostatic weighing tank in my bathroom, but biometrics has come a long way using modern electronics and microprocessors. I use a dive computer that knows, at 2 second intervals, where I am in terms of depth, my gas usage over time and the temperature of the water I am in. It even knows if I am breathing EANXX, where XX is other than 21.

Now why can't these two talk? In other words, how about a dive computer that starts before the dive by getting the latest biometrics, uses that data along with the dynamic data of the dive and then predicts, after the dive, the state of the body WRT gas absorption?

I could even see this system taking into account age and body structure. I am 6' with medium (on the large size frame). People are surprised to find out how much I weigh because I "carry it well". In other words, my fat is pretty well distributed. The data from a skin fold exam and tape measurements could also be input to this system to fine tune the process.

The point is, we now have the technology to take a great leap forward in terms of decompression science at the individual level.

 

[TSandM]

wetcell, you could customize with your biometrics . . . if you had any idea how much and in what direction each variable would require the model to be adjusted. The fact is that we don't know that. As Dr. Deco said, monitoring the actual saturation of nitrogen in body tissues in a living creature is all but impossible. The models we use are mathematical models of how gases OUGHT to behave, and how they do behave in simpler systems. We know, from observation, that some people diving completely within the parameters of a model get DCS symptoms. Over time, we've come up with some thing we think increase the risk of this happening, but there is no quantitation of the increased risk created, for example, by being 10% dehydrated, or 20% above ideal body weight. Yes, you can measure those things, but if you don't know what their impact will be, you can't adjust the model in an accurate way.

What we know is that DCS is a rare event for people diving within the recreational range and observing the parameters of their model, whichever model they are using. The data points of people who DO get clear-cut DCS are few, and often not well documented. And those are the data we would need to begin to predict how variables affect bubble formation and symptom occurrence.

There's no money for large-scale studies of that kind. The best we can do is try to report our dives to DAN, so they can have some data with which to work to try to answer some of these questions.

 

[WetCell]

wetcell, you could customize with your biometrics . . . if you had any idea how much and in what direction each variable would require the model to be adjusted. The fact is that we don't know that. As Dr. Deco said, monitoring the actual saturation of nitrogen in body tissues in a living creature is all but impossible. The models we use are mathematical models of how gases OUGHT to behave, and how they do behave in simpler systems. We know, from observation, that some people diving completely within the parameters of a model get DCS symptoms. Over time, we've come up with some thing we think increase the risk of this happening, but there is no quantitation of the increased risk created, for example, by being 10% dehydrated, or 20% above ideal body weight. Yes, you can measure those things, but if you don't know what their impact will be, you can't adjust the model in an accurate way.

...the actual saturation of nitrogen in body tissues in a living creature is all but impossible.

I could not agree more; however, we have enough information available to make the estimates better. Regrettably, my knowledge on this subject is very weak, but it appears to me that the tissue compartment models are based on chamber studies. Has anyone taken a pound of human body fat and subjected it to air under pressure to determine Nitrogen absorption? The same with muscle? etc. Seems like an easy experiment to do, or perhaps the data is already available empirically--we do know the biochemistry of tissues.

What we know is that DCS is a rare event for people diving within the recreational range and observing the parameters of their model, whichever model they are using. The data points of people who DO get clear-cut DCS are few, and often not well documented. And those are the data we would need to begin to predict how variables affect bubble formation and symptom occurrence.

My first reaction to this is DUH! The point of recreational dive tables/computer models is to be so conservative that they insure to a high degree of certainty that rec divers will not get DCS. That is the problem. This discussion is about narrowing the envelope between the conservative general estimates and the specifics of an individual diver.

There's no money for large-scale studies of that kind. The best we can do is try to report our dives to DAN, so they can have some data with which to work to try to answer some of these questions.

I believe that I made that point. Although I cannot disagree with your suggestion to report dives to DAN, that will take far too long. I think we can move faster by trying a composition analysis; i.e., looking at the absorption rates of different tissues and then apply biometrics to define the geometry and quantity of the tissues and hence develop an accurate model of the individual diver. Frankly, I am surprised no one has done it this way, but then, I am unfamiliar with the literature. It is also possible that my reasoning is flawed. Dr. Dive, care to comment?

 

[TSandM]

wetcell, you COULD take a pound of human fat and expose it to ambient nitrogen and measure absorption -- but the results would have nothing whatever to do with what happens while diving, because nitrogen in that setting is delivered by perfusion. To know how to adjust models, you'd have to have a perfusion-based delivery system and vary both nitrogen tension in the perfusate and amount of perfusion -- and then you'd have to know how perfusion in fat in various portions of the body is affected by the temperature, activity and pressure changes while diving.

It might be possible to design and carry out such experiments for each kind of tissue you'd be likely to run into, and then try to amend the models using those data, but it would be a huge project and the math involved daunts me. And there is very little reason to carry out such studies, except for knowledge, because the existing models keep people pretty safe and allow as much bottom time as the majority of people can have on their gas supplies.