Qs About Science of Diving Content

[Dan G]

I just finished Section 3 of the Science of Diving course, which is "Decompression Theory." I read some content I have questions about.

From the "Dive computer" section:

The mathematical model works only with the correct dive profile, which is a multi-level profile with the deepest depth first, followed by subsequently shallower depths.

Dive computers were designed for multi-level diving and are an excellent tool if they are used properly and conservatively. Use the computer with deep-to-shallow profiles, not deep-shallow-deep profiles. The computer continuously computes nitrogen absorption based on the actual depth and time actually spent there.
Therefore, if divers go deep then shallow, their nitrogen absorption is computed at a slower rate at the shallow depth. However, on descending again to greater depth, the theoretical nitrogen absorption is increased accordingly. As indicated earlier, the residual nitrogen affects that process, yet the computer does not accurately account for this affect. Therefore, the computer is not calculating absorption the same way your body is, which is potentially hazardous.

Is this a fairly common pov from certifying agencies? I do not remember seeing anything about this in the directions for my dive computer. I was unaware that reverse profiles were an issue for computers and thought that this theory would be thought of similarly to needing the first dive of the day to be the deepest.

This section is in regards to diving after flying:

SSI’s recommendations are:

1. A minimum surface interval of 24 hours is required to be reasonably assured a diver will remain symptom free upon ascent to altitude in a commercial jet airliner (pressurized to an altitude of 2500m).
2. A surface interval of more than 24 hours is recommended after daily, multiple dives over several days or dives that require decompression stops. The greater the surface interval before flight the less likely decompression sickness will occur.

Are most agencies still using 24 hrs despite DAN's more recent 18 hr recommendation?

 

[Angelo Farina]

I can try answering just on the first part.
In principle, a properly-programmed computer should take care of any diving profile, accounting for nitrogen migration in and out of any tissue, depending on instantaneous depth (pressure) and on the amount of nitrogen already present in each tissue.
But in reality on most computers a number of simplifications are done, based on schematic models such as Bulhman.
The effectiveness of these models has been verified empirically only for a number of diving profiles.
Very strange profiles could result in phoenomena which are not properly taken into account in these over-simplified computational models.
Hence the recommendation to follow simple, widely tested profiles, where we know that the computer is "reasonally safe".

 

[rick00001967]

Better for an agency to recommend a 24 hr interval to be more conservative. Less liability that way.
If you were to follow DAN recommendations they are less conservative.

 

[RobPNW]

I'm not even following the logic of those statements. If the computer is calculating based on real time profile changes how can it not be accurate? Something is missing from those statements.

 

[L13]

The mathematical model works only with the correct dive profile, which is a multi-level profile with the deepest depth first, followed by subsequently shallower depths.

SSI is incorrect. The model "works" for all profiles, but has not been validated for all profiles.

Dive computers were designed for multi-level diving and are an excellent tool if they are used properly and conservatively. Use the computer with deep-to-shallow profiles, not deep-shallow-deep profiles. The computer continuously computes nitrogen absorption based on the actual depth and time actually spent there.
Therefore, if divers go deep then shallow, their nitrogen absorption is computed at a slower rate at the shallow depth.

Correct so, far but the rest is somewhat wrong.

Dive computers correctly implement the model for all profiles. The model "works" for all profiles. However, the model has only been validated for deep to shallow profiles. If the theory behind the model is close enough to correct, it should be "correct" for all profiles. For the profiles tested, the model performed "well", so it can probably be trusted. For profile types not tested (shallow to deep, sawtooth, etc.) you are taking the model on faith. Most divers trust the model to be close enough for all profiles, given the extensive testing on the model for square and deep to shallow profiles.

Specifically this is incorrect:

As indicated earlier, the residual nitrogen affects that process, yet the computer does not accurately account for this affect.

The computer does account for this as accurately as it does for any profile.


And this is true for all profiles:

Therefore, the computer is not calculating absorption the same way your body is, which is potentially hazardous.

But, we assume it is close enough for the validated profiles and many assume it is close enough for all profiles.

 

[quietlife4me]

Ha! I was reviewing the Science of Diving content last night and had the exact same thought! As a computer nerd for the past 25 years that’s not “how computers work” was my first thought but figured some litigation lawyer was getting creative.

 

[BlueTrin]

Reverse profile dangerous?

Just finished the section on reverse profile diving in "Deco for Divers" and was surprised to learn that there is no scientific basis for discouraging reverse profile dives, and that they are already being done in commercial, scientific and military diving. Yet this is against everything I've...

scubaboard.com

Reverse dive profiles

Recently read a very good post giving a review of diving with a cruise line in multiple locations. One of the recommendations was that one operator should change the Order of their dives to get back to port more quickly. As I read the dive profiles the effect would be to create a reverse...

scubaboard.com

Reverse Profiles - do we have it backwards?

Deepest dive first has been one of the tenets of safe diving for years. Recently the DAN Hotline got a call about this following an article on the web [Click here for the original article].The core issue is that the conclusions of the Workshop on Reverse Profiles at the Smithsonian Institute did...

www.dansa.org

Computers That Properly Penalize Reverse Profiles: Undercurrent 05/2006

 

[Angelo Farina]

I'm not even following the logic of those statements. If the computer is calculating based on real time profile changes how can it not be accurate? Something is missing from those statements.

The real time profile is just the input to the algorithm.
You can provide the most detailed and accurate input data, but if the algorithm is flawed, or over-simplified as in this case, then the results can be wrong.
Also the opposite is true: even if you employ a powerful multiphysics FEM simulation program (such as Comsol), modelling all the physics of blood flow, gas diffusion in liquid and solid tissues, with a detailed meshed geometry of your body coming from an hi-res CT scan, yet if you feed that program with inaccurate input data can give bad results.
Your diving computer is running a very basic algorithm , the equivalent of a number (8÷16) of parallel RC circuits.
The values of the time constants RC for each of them is tuned to approximate the behaviour of an human tissue.
Such a model is a gross oversimplification of the human body.
Under simple and favourable input data sets, it was verified that such a model works reasonably well.
But there is no certainty on how it will behave when driven by a set of input data never verified before.

 

[BlueTrin]

@RobPNW to explain what Angelo said in simpler words: if the data used to calibrate your model does not include many reverse profile, are you sure that your model is working for reverse profiles?


If you use something like Buhlman, it is assuming your body can be expressed in a certain number of tissues and each of them can off gas and on gas using a certain formula function of depth and the amount of gas already on the tissues.

Then these parameters were calibrated using data so that a certain level of safety was reached through experimentation.

Now Eric Baker’s gradient factors were added as an arbitrary shift to the original Buhlman for added safety.

If few of the data for calibration included reverse profile, then you can make a case that it’s not heavily tested.

It’s not like this mathematical construct is exactly representing your body but is rather a simple assumption that seems to hold against the data/experimentation?

 

[DeepSeaExplorer]

I just finished Section 3 of the Science of Diving course, which is "Decompression Theory." I read some content I have questions about.

From the "Dive computer" section:

The mathematical model works only with the correct dive profile, which is a multi-level profile with the deepest depth first, followed by subsequently shallower depths.

Dive computers were designed for multi-level diving and are an excellent tool if they are used properly and conservatively. Use the computer with deep-to-shallow profiles, not deep-shallow-deep profiles. The computer continuously computes nitrogen absorption based on the actual depth and time actually spent there.
Therefore, if divers go deep then shallow, their nitrogen absorption is computed at a slower rate at the shallow depth. However, on descending again to greater depth, the theoretical nitrogen absorption is increased accordingly. As indicated earlier, the residual nitrogen affects that process, yet the computer does not accurately account for this affect. Therefore, the computer is not calculating absorption the same way your body is, which is potentially hazardous.

Is this a fairly common pov from certifying agencies? I do not remember seeing anything about this in the directions for my dive computer. I was unaware that reverse profiles were an issue for computers and thought that this theory would be thought of similarly to needing the first dive of the day to be the deepest.

This section is in regards to diving after flying:

SSI’s recommendations are:

1. A minimum surface interval of 24 hours is required to be reasonably assured a diver will remain symptom free upon ascent to altitude in a commercial jet airliner (pressurized to an altitude of 2500m).
2. A surface interval of more than 24 hours is recommended after daily, multiple dives over several days or dives that require decompression stops. The greater the surface interval before flight the less likely decompression sickness will occur.

Are most agencies still using 24 hrs despite DAN's more recent 18 hr recommendation?

This is outdated thinking based on using dive tables and was the justification against reverse profiles.

Just FYI, the PADI manual states there is no evidence that reverse profiles are any less safe, but then urges caution because there isn’t a lot of data. Then goes on to state that the dive computer is constantly recalculating the NDL and should easily do the math on reverse profile.

Same with the 24 hour time to fly, the best recommendation is 18 hours now.