Compartments and Deco
- Thread starter 261311
- Start date
Please register or login
Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.
Benefits of registering include
- Ability to post and comment on topics and discussions.
- A Free photo gallery to share your dive photos with the world.
- You can make this box go away
chrpai
Contributor
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?
2) Are "fast" compartments fast because they on gas faster or because they can hold relatively less? (I.e. is the compartment "faster" or "smaller"?)
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?
2) Are "fast" compartments fast because they on gas faster or because they can hold relatively less? (I.e. is the compartment "faster" or "smaller"?)
More compartments doesn't necessarily make a model more conservative. And be sure not to confuse compartments (theoretical mathematical constructs) with actual body parts.
In addition, the common models assume compartments ongass and offgass at equal rates, on definable exponential curves. Hence the half-times. While those models have proven safe to dive, we don't know whether or not real body tissues behave that way.
All we have is mathematical models. Even Wieneke's RGBM is based on modeling bubble dynamics, not on observing what happens in a real diver's body. I don't mean to sound dour, and I love that we have the models to keep us safe. But the models don't pretend to know what happens in our bodies when diving.
DevonDiver
N/A
I'm still waiting for my morning coffee to percolate... but I'll have a crack at this without caffeine..forgive me for any in-eloquence 
In recreational (no-deco) diving, the 'controlling' compartment will be the tissue compartment that reaches NDL first. On a single dive, that will be the fastest compartment. On repetitive and/or multi-day dives, it may be a slower compartment - because the slower compartments will be the ones retaining nitrogen from previous dives (slower half-times to off-gas).
The NDL limit doesn't necessarily correspond to complete saturation. That depends on which tissue compartment is controlling. The compartment half-time versus ascent speed plays a critical role in this. NDL in recreational diving is the state where the diver retains the ability to ascend directly to the surface at a given controlled speed. Thus, the NDL indicates a level of saturation that can be safely off-gassed within the parameters of that direct, controlled ascent. For fast tissues, that may permit complete tissue saturation. For slower tissues, it won't.
In essence, staying inside an NDL means you are limiting tissue saturation to a level where it can be off-gassed sufficiently (within 'M-Values') on a direct ascent to the surface. The controlling tissue compartment half-time must be sufficiently fast to permit controlled ascent (usually ~10mpm) to the surface (1atm) without absorbed nitrogen reaching a critical pressure where it will leave tissue solution and form bubbles.
For simplicity, recreational diving assumes a constant maximum ascent rate. Tissue saturation and safe off-gassing has to fit within that parameter. This restricts dive depth/duration to the faster tissues.
When you exceed an NDL, a slower compartment (or compartments) are sufficiently saturated to become controlling. The longer-deeper the dive, the more slow compartments are saturated. Ascent speed equates to the time needed to sufficiently off-gas those compartments. Deco stops are merely a means to limit that descent speed, whilst simultaneously maximizing the time spent with the greatest safe pressure differential across the lungs-blood-tissue (using nitrox deco gas also maximizes that differential).
True saturation - as I believe the term is used in commercial diving - indicates all theoretical tissue compartments are fully saturated (at or beyond six half-times) to ambient pressure, even the slowest ones.
In deco calculation (the math) tissue 'speed' is dictated entirely by the half-time. That half-time speed is a constant for a given compartment. As others have said, it is only a mathematical construct. The primary factor believed to dictate tissue on/off-gas speed is vascular supply - how much blood supply reaches a given tissue. Bones have little, fat a little more, muscles lots.. Given that body mass isn't a factor used to calculate deco, we can assume that current theory doesn't place much emphasis on the volume of gas held by a given tissue compartment.
In Deco for Divers, Mark describes half-time speed (via vascular supply) in an analogy - that of a warehouse loading trucks for delivery. The limitation on delivery is caused by the number/frequency of trucks that arrive to be filled up - rather than the size of the warehouse.
In recreational (no-deco) diving, the 'controlling' compartment will be the tissue compartment that reaches NDL first. On a single dive, that will be the fastest compartment. On repetitive and/or multi-day dives, it may be a slower compartment - because the slower compartments will be the ones retaining nitrogen from previous dives (slower half-times to off-gas).
The NDL limit doesn't necessarily correspond to complete saturation. That depends on which tissue compartment is controlling. The compartment half-time versus ascent speed plays a critical role in this. NDL in recreational diving is the state where the diver retains the ability to ascend directly to the surface at a given controlled speed. Thus, the NDL indicates a level of saturation that can be safely off-gassed within the parameters of that direct, controlled ascent. For fast tissues, that may permit complete tissue saturation. For slower tissues, it won't.
In essence, staying inside an NDL means you are limiting tissue saturation to a level where it can be off-gassed sufficiently (within 'M-Values') on a direct ascent to the surface. The controlling tissue compartment half-time must be sufficiently fast to permit controlled ascent (usually ~10mpm) to the surface (1atm) without absorbed nitrogen reaching a critical pressure where it will leave tissue solution and form bubbles.
For simplicity, recreational diving assumes a constant maximum ascent rate. Tissue saturation and safe off-gassing has to fit within that parameter. This restricts dive depth/duration to the faster tissues.
When you exceed an NDL, a slower compartment (or compartments) are sufficiently saturated to become controlling. The longer-deeper the dive, the more slow compartments are saturated. Ascent speed equates to the time needed to sufficiently off-gas those compartments. Deco stops are merely a means to limit that descent speed, whilst simultaneously maximizing the time spent with the greatest safe pressure differential across the lungs-blood-tissue (using nitrox deco gas also maximizes that differential).
True saturation - as I believe the term is used in commercial diving - indicates all theoretical tissue compartments are fully saturated (at or beyond six half-times) to ambient pressure, even the slowest ones.
In deco calculation (the math) tissue 'speed' is dictated entirely by the half-time. That half-time speed is a constant for a given compartment. As others have said, it is only a mathematical construct. The primary factor believed to dictate tissue on/off-gas speed is vascular supply - how much blood supply reaches a given tissue. Bones have little, fat a little more, muscles lots.. Given that body mass isn't a factor used to calculate deco, we can assume that current theory doesn't place much emphasis on the volume of gas held by a given tissue compartment.
In Deco for Divers, Mark describes half-time speed (via vascular supply) in an analogy - that of a warehouse loading trucks for delivery. The limitation on delivery is caused by the number/frequency of trucks that arrive to be filled up - rather than the size of the warehouse.
It's all theoretical. Compartments are simply a way to calculate how we think nitrogen and helium act in our bodies so we can create decompression models to get us out of the water safely. While there have been some bubble studies done there is no set number of compartments in the body. The number of compartments used by each model is simply what works for the mathematical equations of that model. If you're planning on writing your own decompression model then knowing more about them would be useful. There are a few books out there on the subject. I have several of Bruce Wienke's books but haven't read through any of them from cover to cover. I don't know if many have! If you just want to know more about decompression diving then learn the various models. Purchase Multi-Deco and play around with various profiles. But keep in mind that no decompression model will guarantee that you do not experience DCS.
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)
"Fast" compartments reach equilibrium with the inspired gas faster -- but any compartment, at equilibrium, will have the same nitrogen tension as any other compartment at equilibrium. Does that make sense? It's kind of like saying you can dissolve a half cup of sugar in a cup of water -- but if the water is hot, the sugar will dissolve faster than it will in cold water. It's the same amount of sugar in the end, though.
M-values are basically the maximum nitrogen tension that can be present in that compartment without bubbling; Msub0 is the tension at the surface. NDLs for each compartment are calculated as the nitrogen tension that can be offgassed during the prescribed ascent time (at the prescribed rate, from that depth) to be at or below Msub0 when you reach the surface. Of course, it gets more complicated as you begin to include concepts like tolerated overpressure gradients (compartments that won't immediately bubble if they exceed their M-values) and when you begin to consider bubble theory, because gas in bubbles has dynamics related to both tension and ambient pressure.
It gets very mathematically complex very quickly, but I do believe the bottom line is this: As our own Rick Murchison has said, decompression is measuring with a micrometer, marking with chalk, and cutting with an axe. The theory is elegant and can be as complicated as you want to make it, but the correlations with physiology are at best imperfect.
If the topic interests you, in addition to reading Mr. Powell's excellent book, you may want to pick up the "Mysterious Malady" DVD, which you can purchase on the GUE website (gue.com). It is a very well made series of interviews with the scientists who are as much at the forefront of decompression theory and experimentation as we have these days (where not very much is being done any more). What you learn from listening to them is that there is still a very large amount we don't know or understand about decompression sickness and how and why it occurs.
M-values are basically the maximum nitrogen tension that can be present in that compartment without bubbling; Msub0 is the tension at the surface. NDLs for each compartment are calculated as the nitrogen tension that can be offgassed during the prescribed ascent time (at the prescribed rate, from that depth) to be at or below Msub0 when you reach the surface. Of course, it gets more complicated as you begin to include concepts like tolerated overpressure gradients (compartments that won't immediately bubble if they exceed their M-values) and when you begin to consider bubble theory, because gas in bubbles has dynamics related to both tension and ambient pressure.
It gets very mathematically complex very quickly, but I do believe the bottom line is this: As our own Rick Murchison has said, decompression is measuring with a micrometer, marking with chalk, and cutting with an axe. The theory is elegant and can be as complicated as you want to make it, but the correlations with physiology are at best imperfect.
If the topic interests you, in addition to reading Mr. Powell's excellent book, you may want to pick up the "Mysterious Malady" DVD, which you can purchase on the GUE website (gue.com). It is a very well made series of interviews with the scientists who are as much at the forefront of decompression theory and experimentation as we have these days (where not very much is being done any more). What you learn from listening to them is that there is still a very large amount we don't know or understand about decompression sickness and how and why it occurs.
Half times are directly analogous to "half-lives" in radioactive decay. Decompression is all about having enough decay in the tissue gas load to actually off-gas and excrete that gas (via the lungs) but not creating such a steep off gassing gradient that the gas bubbles in tissues before it can be eliminated via the lungs.
Mr Carcharodon
Contributor
Yeah it's all theoretical and works 99.95% of the time which seems like a pretty good theory. Yeah you could do a covolution integral and get away from discrete compartments but why bother. It would be better to discuss why things breakdown.
Most of the time because people don't follow the parameters of a model at all, the so called "deserved hits".
Similar threads
