what is the evidence for the long compartment?

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SailNaked

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So as i was contemplating Flying in under 24 hours like recommended, I wondered what evidence exist to support the long(slow) compartment. what is it supposed to simulate, and if it is truly physiological then wouldn't the gas come out about as slow as it went in, giving your body plenty of time to deal with it?:coffee:
 
I'm having some trouble understanding your questions, so I will try to make a response as I understand it. You can correct me if I got some things wrong.

First of all, the most commonly used rule for flying after diving is 12 hours for a single dive and 18 hours for multiple dives. The 24 hour rule for DAN was changed well over a decade ago.

Next, there is no clear relationship between any of the theoretical tissues, or compartments, and any specific tissue. Tissues that would be long would include something like bone.

Next, tissues load or unload nitrogen in relation to halftimes. A long compartment (say 2 hours) would take 2 hours to become halfway saturated, then another 2 hours to become 75% saturated, and so on. But it is more complicated than that because the saturation is not absolute. Let's say that you did a dive to 99 FSW for 20 minutes. At that time, the fastest compartment (5 minutes) will be roughly 94% saturated for that depth. Your 2 hour compartment will just be getting started on the road to saturation. As you ascend, the 5 minute compartment will rapidly reach a safe level, but the 2 hour compartment will be much slower in releasing its nitrogen. With only one dive to that depth, though, even though it is releasing its nitrogen very slowly, the fact that it did not ever gt that full in the first place means that it was never really a major threat.

Now, start doing more dives in a reasonable length of time, dives that are continually reaching NDLs, and the fact that the slow compartment was slow in releasing its gas becomes a factor. It will have more and more residual nitrogen on each dive.

I do not know what you meant by "giving your body plenty of time to deal with it." If the slow compartments are slow to eliminate gas, then the body needs more time to deal with it.
 
Next, tissues load or unload nitrogen in relation to halftimes. A long compartment (say 2 hours) would take 2 hours to become halfway saturated, then another 2 hours to become 75% saturated, and so on. But it is more complicated than that because the saturation is not absolute. Let's say that you did a dive to 99 FSW for 20 minutes. At that time, the fastest compartment (5 minutes) will be roughly 94% saturated for that depth. Your 2 hour compartment will just be getting started on the road to saturation. As you ascend, the 5 minute compartment will rapidly reach a safe level, but the 2 hour compartment will be much slower in releasing its nitrogen. With only one dive to that depth, though, even though it is releasing its nitrogen very slowly, the fact that it did not ever gt that full in the first place means that it was never really a major threat.

Now, start doing more dives in a reasonable length of time, dives that are continually reaching NDLs, and the fact that the slow compartment was slow in releasing its gas becomes a factor. It will have more and more residual nitrogen on each dive.

I do not know what you meant by "giving your body plenty of time to deal with it." If the slow compartments are slow to eliminate gas, then the body needs more time to deal with it.

Remember, tissues load and unload according to half-times until they exceed their capacity - then they bubble over. I would consider 2hr half time (120 minutes) to be a middle compartment. My Oceanic (DSAT) model uses 12 compartments with the slowest being 320, 400, and 480 minutes. I believe I have seen models with a 620 (or 680) minute compartment. The real problem is that these slower compartment can become saturated at shallower depths. Where a 5 minute compartment does not produce an NDL until it saturates at something on the order of 100 feet, these slower compartments will saturate at a lesser depth - like 30 to 40 feet for the slowest compartment. I have used the DiveNav software to look at the loading of these slow compartments when doing 4 or 5 dives per day in the 80 to 100 ft range for 3 or more days. The slower compartments become the leading compartments as time progresses and a 12 hour SI does not do much to empty them. I was just trying to understand what was happening on a live-a-board trip that did not involve flying. I am not sure that 18 SI to fly is really enough if the slow compartments were approaching saturation.
 
Ok maybe I should say more,

half times and compartments are part of a theory that maps math to physiology, it was created by a physicist to give the navy tables to use to manage diving, it mostly works and we use it, but it is a mistake to say that it is acutally what is happining.

what is happinging is that nitrogen gas is being forced into solution in the body, at some unknown rate, and we assume that rate varies based on the type of tissue in the body, muscle, blood, bone, fat, brain, etc we theorize that since they are different they absorb gas at different rates.

our computers are great at math so you can have as many compartments/tissues/half lives as you like. short compartments fill up faster and unload faster they are responsible for causing you to need to deco at depth, the long ones take much longer and fill up slowly over time and multiple dives, these "compartments" are the ones that cause a 12-18 hour or more surface interval before you are back to normal sea level saturation.

remember that this is just theory, so what I am asking is what is the evidence that these long compartments are representing anything real. just because my computer uses it does not mean that it is real. (except for people that are certain the govenment will save them in a disaster). if these tissues take a long time to absorb gas then can we assume they also would take a long time to off gas, and if they are doing it slowly then can we assume the body can deal with it over time.
 
if these tissues take a long time to absorb gas then can we assume they also would take a long time to off gas, and if they are doing it slowly then can we assume the body can deal with it over time.

As I understand the theory, the long compartment tissues do not necessarily load or unload inert gas more slowly than the short compartments, it just takes them longer to reach saturation or desaturation....is that correct ??
 
At the end of his career... and his life... Prof. Buhlmann was trying to find a mathematical construct that would "fix" the basic theory upon which his algorithm and therefore his dive tables were built (a neo-Haldanian model). He knew something wasn't right with them -- people getting bent -- and long slow compartments where his suggested fix.

A better solution for us all -- in my opinion -- is to adopt the use of a more up-to-date theory that is not build upon the false premise that we can avoid the formation of bubbles if we prevent super saturation in controlling tissue compartments... I admire Buhlmann's work but this is a horrible assumption upon which to base a decompression algorithm.
 
If a compartment takes longer to reach saturation either it is a much larger compartment and this is not accounted for in the math, or it is doing it more slowly, it is my assumption that if it loads slow then it unloads slow, this is not necessarily so but make some sense.
 
Interesting discussion, I don't have anything to add except I would have loved to view this thread when I drew M compartment and tissue half times in my IDC lesson!!
 
In theory, tissues take on or release gases at different rates for essentially two reasons:

1. The more liquid they are, the faster they are. If our bodies were truly liquid, we could ascend very quickly.

2. The better perfused they are, the better. Perfusion refers to the blood flow within the tissue.
 
If a compartment takes longer to reach saturation either it is a much larger compartment and this is not accounted for in the math, or it is doing it more slowly, it is my assumption that if it loads slow then it unloads slow, this is not necessarily so but make some sense.

On/off gassing at the same ambient pressure...flying will subject said compartments to reduced ambient pressures. Modestly, you can expect cabin altitudes of about 7000'MSL during the average commercial aircraft flight. The gradient caused by that cabin altitude is the problem the model forecasts, that gradient will generally exceed M values presumably for several compartments.

If you wish to "test pilot" deco theory, have at it, it's your body but why? Why the urgency?
 
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