Compartments and Deco

[Hatul]

The relationship between compartments and tissues is just theorical (but someone might argue that the blood flow has something to do with it).
Here how we code the compartments in our simulators:

You can use divePAL to see which one is the leading compartment. And you can even design your own decompression algorithm - just don't go diving for real with it

Alberto (aka eDiver)

Hello Alberto, I would have thought the tallest bar in your graph represents the leading compartment. What is the defn of "leading compartment"?

 

[eponym]

I'm going to take a stab at clarifying the differences between "saturation" and "NDL".

"Saturation" in one usage denotes the state of equilibrium that's reached when a diver's body has absorbed all the inert gas that it can hold at a given depth. The deeper you are, the more gas you can absorb, of course. See for example US Navy (Diving Manual 1999), Mark Powell (Deco for Divers), and John Lippman (Deeper into Diving). Another usage refers to compartments in deco models. A compartment is likewise "saturated" when the model determines that it has absorbed all the inert gas that it can hold at a given depth.

AFAIK, you could have 6 compartments or 60 compartments but at any given moment there would be 1 controlling compartment. When that one saturates, you've reached an NDL.

Ah, see that isn't clarified- simple enough to grasp though. What would happen if you've saturated more than one? Medical emergency or just a longer SI? Does the recognition of more compartments add to conservatism (ie. something with high blood flow that might be classed under a larger compartment being saturated sooner and thus NDL is reached)?

Then I'll add two questions that I'm asking myself while trying to help you:
1) When a compartment reaches an NDL limit, is it correct to describe that compartment as saturated or is that term reserved for true saturation dive with the compartment truly is saturated at equilibrium?

Saturation of a compartment in a model does not mean you've reached your NDL. Some fast compartments will reach saturation on a dive. For example, during a thirty-five minute cruise at sixty feet, a five-minute compartment becomes fully saturated. But you're not past the model's NDL and you can still make a controlled ascent, because that compartment will offgas fast enough on a controlled ascent to keep the gradient (dissolved vs. lung pressures) less than the model's maximum ratio (M-value, see TS&M, above) during the ascent. Ascent rates are part of the model, too.

NDL is the point in your dive when the ascent must begin because the model calculates that some particular compartment is approaching the point at which it will exceed its M-value during the ascent. This could for example be a sixty-minute compartment that's only half-saturated at the current depth, but which is going to exceed the M-value as you ascend because it offgases very slowly.

 

[DiveNav]

Hello Alberto, I would have thought the tallest bar in your graph represents the leading compartment.

No. because that compartment is off-gassing.

...What is the defn of "leading compartment"?

It is defined by .... the mathematical equation for the specific model you are using. In the Buhlmann case, the equation includes over-pressure gradients and half-times :shocked2:

Alberto (aka eDiver)

 

[boulderjohn]

Also, half times by the looks of it without doing any serious math relate to surface intervals heavily, yes?

I'll jump in on this one.

When Workmann adjusted the navy tables, he added a 120 minute compartment and made that the controlling compartment for surface intervals. It takes approximately 6 half times to approach 100%. Consequently, the Navy tables he created "washed out" after 6 X 120 = 720 minutes, or 12 hours. That led to very long surface intervals, which made sense for the kind of diving that the Navy does. This did not seem to make sense to recreational divers, though, because they were doing very different dives. When PADI did the research for the RDP, they discovered that for the kind of NDL dives they were doing, the controlling compartment was actually the 40 minute compartment. To add a level of conservatism, when they made the RDP, they chose the 60 minute compartment to control the surface intervals. That is why the PADI RDP washes out in 6 X 60 = 360 minutes, or 6 hours. This provides for shorter surface intervals and gets recreational divers back in the water faster.

 

[MXGratefulDiver]

I've always liked Richard Pyle's explanation ...

If you ask a random, non-diving person on the street to explain what's really going on inside a diver's body that leads to decompression sickness, the answer is likely to be "I don't know".

If you ask the same question of a typical scuba diving instructor, the answer will likely be that nitrogen is absorbed by body under pressure (a result of Henry's Law); and that if a diver ascends too quickly, the excess dissolved nitrogen in the blood will "come out of solution" in the blood to form tiny bubbles; and that these bubbles will block blood flow to certain tissues, wreaking all sorts of havoc.

Pose the question to an experienced hyperbaric medical expert, and you will probably get an explanation of how "microbubbles" already exist in our blood before we even go underwater; and that ratios of gas partial pressures within these bubbles compared with dissolved partial pressures in the surrounding blood (in conjunction with a wide variety of other factors) determine whether or not these microbubbles will grow and by how much they will grow; and that if they grow large enough, they may damage the walls of blood vessels, which in turn invokes a complex cascade of biochemical processes called the "complement system" that leads to blood clotting around the bubbles and at sites of damaged blood vessels; and that this clotting will block blood flow to certain tissues, wreaking all sorts of havoc.

You will likely be further lectured that decompression sickness is an unpredictable phenomenon; and that a "perfect model" for calculating decompression schedules will never exist; and that the best way to calculate the best decompression schedules is by examining probabilistic patterns generated from reams of diving statistics.

If, however, you seek out the world's most learned scholars on the subject of decompression and decompression sickness, the top 5 or 6 most knowledgeable and experienced individuals on the subject, the ones who really know what they are talking about; the answer to the question of what causes decompression sickness will invariably be: "I don't know". As it turns out, the random non-diving person on the street apparently had the best answer all along.


http://www.xenscuba.com/articles/scuba-diving-physics-and-fizzyology?showall=&start=1

... Bob (Grateful Diver)

 

[rjack321]

No. because that compartment is off-gassing.

In your figure the red bar (compartment) is labelled "-ongassing" though. As a matter of theory, the controlling compartment on an ascent cannot be ongassing. One or more of the offgassing compartments would define the deco ceiling.

 

[DiveNav]

..... As a matter of theory, the controlling compartment on an ascent cannot be ongassing. ....

Maybe in that particular point in time the diver was stationary .... in that scenario, as a matter of theory, based on previous portion of the profile you could still have other compartments off-gassing but NOT being the leading ones.

Alberto (aka eDiver)

 

[redacted]

In your figure the red bar (compartment) is labelled "-ongassing" though. As a matter of theory, the controlling compartment on an ascent cannot be ongassing. One or more of the offgassing compartments would define the deco ceiling.

Intuitively, that theory does not seem to be correct. The number of compartments is variable. So what happens with a one compartment model? Obviously, it is controlling. And, when you exceed NDL, it puts you in deco. So, does it necessarily start off-gassing at the start of ascent - no, I don't think so. Does a 2nd compartment necessarily change that? Again, I don't think so.

Perhaps the model is not credible if the theory is not true?

I'm glad I am just a recreational diver.