The "fast" tissues also tolerate much greater supersaturation, and when you combine that with their rapid off-gassing, they aren't going to force you into a decompression stop . . . which is the essence of exceeding your NDL's, right?
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PaulChristenson
Contributor
meekal:- that's why you do your homework (like me!) and get a SUUNTO (like me!) - even the lowest priced MOSQUITO uses the RGBM model. when you download the data to your PC and make all dem purty graphs you can play the dive like a VCR where on the NITROGEN loading graph; yes, it DOES show you offgassing as you come up from depth - but only in certain 'compartments' - [as noted elsewhere] - some of the "deeper" compartments continue to ON-gas.
Actually they use a modified RGBM model...and good luck finding out what they modified...
TSandM:
To me, yes. But something's bubbling to some degree in almost all of us on a "no deco" dive. I suspect its those tissues which would be considered "fast" in the models (half times in minutes).
Here's a good paper by Erik Baker on Buehlman mechanics
http://www.tek-dive.com/portal/upload/M-Values.pdf
Available all over the place on the web if that link desn't work, google the title = Understanding M Values
This has morphed into quite a discusion of models
To get back on track, Jason and Lynne have demostrated that:
1) Principles of decompression can be used to develop "rules" similar to tables.
2) CNS% and OTUs can be tracked with simple math
3) The variance between computer algorithms on a typical 60' recreational dive is as large as 18 minutes
4) A diver can follow some simple rules and be within the range of results generated by computers.
A computer is a tool, but its not a required tool for either multilevel or repetitive diving.
To get back on track, Jason and Lynne have demostrated that:
1) Principles of decompression can be used to develop "rules" similar to tables.
2) CNS% and OTUs can be tracked with simple math
3) The variance between computer algorithms on a typical 60' recreational dive is as large as 18 minutes
4) A diver can follow some simple rules and be within the range of results generated by computers.
A computer is a tool, but its not a required tool for either multilevel or repetitive diving.
WarmWaterDiver
Contributor
And, to keep on track,
5) You should understand whatever tool(s) you use (scuba or not).
6) All decompresion models are theoretical - there is no one "golden algorithm". All are based on an attempt to explain behavior in nature by mathematics.
6) Equilibrium is not the same as saturation or supersatruration. For the points of my discussion, the bar graph displays clearly illustrate the mathematics involved. The fast tissues quickly equilibrate to the rate they are absorbing gas and the rate at which they are passing gas to the next compartment - but typically only achieve 50% saturation before reaching this equilibrium (at 60 fsw and EAN32 - my dive at Blue Bead Hole on Statia, a 60 fsw muck dive in the sand flats, clearly illustrates this in my electronic logbook). Understanding of items in series vs. items in parallel is key. (Go to the TUSA or Apeks web site, download the software, and fiddle with the Dive Simulator function for a pictorial representation).
7) It is not uncommon for me to have my dive computer showing up to 26 hours 'time to desat' near the end of a vacation dive trip (I did take the trip to dive, after all). This is not uncommon when diving at more remote locations where Nitrox is unavailable. Others may not have this same occurance, but it does not mean it doesn't occur (a priori experience is not required - just do the math). How any one individual chooses to apply that information is an individual choice, but understanding of the tool is key to making an educated decision.
8. The statement 'every dive is a deco dive' was never questioned in this thread that I saw. However, there were references to 'recreational dive profiles' which would be different than 'decompression obligation incurred' dives - which are indicated when saturation is reached in a compartment.
A little knowledge can be a dangerous thing in some cases.
5) You should understand whatever tool(s) you use (scuba or not).
6) All decompresion models are theoretical - there is no one "golden algorithm". All are based on an attempt to explain behavior in nature by mathematics.
6) Equilibrium is not the same as saturation or supersatruration. For the points of my discussion, the bar graph displays clearly illustrate the mathematics involved. The fast tissues quickly equilibrate to the rate they are absorbing gas and the rate at which they are passing gas to the next compartment - but typically only achieve 50% saturation before reaching this equilibrium (at 60 fsw and EAN32 - my dive at Blue Bead Hole on Statia, a 60 fsw muck dive in the sand flats, clearly illustrates this in my electronic logbook). Understanding of items in series vs. items in parallel is key. (Go to the TUSA or Apeks web site, download the software, and fiddle with the Dive Simulator function for a pictorial representation).
7) It is not uncommon for me to have my dive computer showing up to 26 hours 'time to desat' near the end of a vacation dive trip (I did take the trip to dive, after all). This is not uncommon when diving at more remote locations where Nitrox is unavailable. Others may not have this same occurance, but it does not mean it doesn't occur (a priori experience is not required - just do the math). How any one individual chooses to apply that information is an individual choice, but understanding of the tool is key to making an educated decision.
8. The statement 'every dive is a deco dive' was never questioned in this thread that I saw. However, there were references to 'recreational dive profiles' which would be different than 'decompression obligation incurred' dives - which are indicated when saturation is reached in a compartment.
A little knowledge can be a dangerous thing in some cases.
I think I understand this . . . you mean saturation is reached as you are ascending, right? Not that you stayed at depth long enough to achieve saturation? Because that takes six half-lives, and for any but the fastest compartments, we're talking two hours or longer.
As I'm slowly trying to put a comprehensive picture of all this together in my mind, it sort of looks like this: All dives are decompression dives, in that all dives involved the absorption and offgassing of nitrogen. The ascents in all dives have to be shaped in a way that causes efficient offgassing and does not run too high a risk of excessive bubbling and symptomatic DCS. An "NDL" dive is a dive where the depth and time variables have been kept within an envelope that permits the diver to ascend -- at a deliberate and measured pace -- directly to the surface. (Note that even an NDL dive can be a DCS dive if the ascent rate is too fast.) The recommended pace now appears to be typically 30 fpm, but if the dive has been deep enough, a "safety stop" is advised. A safety stop appears to be to be a sort of decompression stop that you can probably safely blow off
When the shape of the decompression curve gets to the point where a 30 fpm ascent is no longer tolerable within safe margins (and those vary by model, too), then "mandatory" decompression stops get inserted into the process. These, in effect, slow the ascent rate (more and more as you get more loaded, and as you get shallower on your ascent).
If you think about it that way (if I'm making any sense), it's clear that there is no black and white cutoff between decompression and non-decompression dives. There's a gradation, and there's clearly a gray area where some models call a given dive a deco dive, and others don't. Anybody doing dives in that gray area is probably well advised to think about decompression strategies and perhaps alter their ascent behavior to make the dives safer. In fact, all of us are probably well served to learn more about decompression algorithms and where they come from, and what they have in common, and where they differ. We can then make more intelligent choices about shaping our ascents so that we maximize safety and are still efficient and rational, and stay within a safe gas supply margin.
WarmWaterDiver
Contributor
TS&M,
No, you're missing the point that all on-gassing travels through the fastest compartment in the model to all other compartments in series. Off-gassing is the reverse AFAIK. This is for the model I've been discussing. I have seen documentation that DCIEM has 4 compartments in parallel, and that's the only such model I'm aware of where all the theoretical compartments are in parallel. But I have no personal experience using that model - the dive computers I use as tools use a Buhlmann based algorithm, which is what I've been discussing.
Maybe cerich from Oceanic can chime in about compartments in parallel in the Haldanean model used in their dive computers - our first dive computers were Oceanic data Plus units.
If the compartments were all in parallel, the post that the first compartment reaches saturation within a few minutes of reaching the bottom would be correct - but it is not - once any compartment reaches 100% saturation, NDL time remaining equals zero - true for any compartment - which is why the 'controlling compartment' shifts during the dive. Going past 100% saturation increases the time and depths of decompression obligation stops - also true for any compartment.
Download the software and run the dive simulator using EAN32 and a 60 FSW square profile dive. Run the time at 60 FSW to 50 minutes at the bottom, and watch the 12 bar graphs in the lower right hand corner, as well as the display for NDL time remaining. The fastest compartment will get to about 50% saturation and hold there. The third, fourth, and fifth compartments will all be right at 77% or so, all togetrher ( which says to me this could be cut to one compartment here - why three with values all right together? - and explains why the Dive Rite model with 9 compartments parallels this model with 12 compartments so closely). Now on the simulator, make an ascent to 30 FSW and hold there for 1 minute. Note that no compartment starts to drop in % saturation until you start the ascent. Then ascend for a three minute safety stop at 15 FSW, then surface. watch the bar graphs and NDL times through this process. Once you see it graphically, it should make more sense.
BTW, this is my actual dive profile logged on Statia at Blue Bead Hole last November.
You can download the software from either www.tusa.com or www.apeks.co.uk - whichever is easiest for you. Go to either the IQ-700 or IQ-800 product pages at Tusa, or the Downloads section of the Apeks web site - the software is called PC Logbook. Make note of the code number to use for when you first run the software on the web page you download the software from. Once you install it, go to 'tools' then 'simulation', and take it from there.
'PGT' is Seiko abbreviation for 'percent gas tolernace'. When this value reaches the numeral 8, NDL time = 0. This is a holdover from programming in base 8 on the early Seiko (Dive Rite) dive computers with their chip sets I think (educated guess - otherwise, why not make it 10 and each pixel or numeral increase mean a 10% increase in approach to NDL limits?). This is the value of the 'controlling compartment' at any point in the dive. Watch this value too.
Then, take a dive profile to say 100 FSW and hang around for 50 minutes - you will enter deco obligation, and it will be clear when this happens with the big red bar on the screen as well as when you see a compartment hit 100% saturation and you see NDL become equal to zero.
You have a medical background - think about medicinal uptake if that helps. I remember my Physical Chemistry prof used metabolizing ethanol as the illustration for third order reaction kinetics (something he said almost all his students could relate to). Or just plain old math - I had seen you post you were a math major!
For electrically inclined folks, the perspective of items in series in a circuit vs. items in parallel in a circuit should be a piece of cake.
The 'black and white' between deco obligation incurred vs. no deco obligation incurred sure should be 'balck and white' for any single model - the math is the math - but there are numerous models which only apply so well to any one population distribution (think again about medicine and side effects in certain percentage of test subjects). This is why there is no definitive single model, and no guarantee you won't get DCS just because you followed the model. Any empirically derived model will have outliers in the data set the model is generated from - if not, either the data set is not robust (too small a data set or to narrrow a range of data collection conditions), or the modeler 'cooked the books' with the data - not necessarily intentionally. But you can never tell who might be an outlier on any given day or point in time - including yourself. So, you use your organic Mark 1 brian in addition to any other tools - such as making a 'deep stop' and a 'safety stop' and keeping your ascent profile just so, and how you feel that day, and what you did the day before, etc. in your realm of personal judgement and risk analysis / risk mitigation / risk acceptance criteria - which also varies by individual.
But if you are going to use a tool, best to understand the tool you're using.
No, you're missing the point that all on-gassing travels through the fastest compartment in the model to all other compartments in series. Off-gassing is the reverse AFAIK. This is for the model I've been discussing. I have seen documentation that DCIEM has 4 compartments in parallel, and that's the only such model I'm aware of where all the theoretical compartments are in parallel. But I have no personal experience using that model - the dive computers I use as tools use a Buhlmann based algorithm, which is what I've been discussing.
Maybe cerich from Oceanic can chime in about compartments in parallel in the Haldanean model used in their dive computers - our first dive computers were Oceanic data Plus units.
If the compartments were all in parallel, the post that the first compartment reaches saturation within a few minutes of reaching the bottom would be correct - but it is not - once any compartment reaches 100% saturation, NDL time remaining equals zero - true for any compartment - which is why the 'controlling compartment' shifts during the dive. Going past 100% saturation increases the time and depths of decompression obligation stops - also true for any compartment.
Download the software and run the dive simulator using EAN32 and a 60 FSW square profile dive. Run the time at 60 FSW to 50 minutes at the bottom, and watch the 12 bar graphs in the lower right hand corner, as well as the display for NDL time remaining. The fastest compartment will get to about 50% saturation and hold there. The third, fourth, and fifth compartments will all be right at 77% or so, all togetrher ( which says to me this could be cut to one compartment here - why three with values all right together? - and explains why the Dive Rite model with 9 compartments parallels this model with 12 compartments so closely). Now on the simulator, make an ascent to 30 FSW and hold there for 1 minute. Note that no compartment starts to drop in % saturation until you start the ascent. Then ascend for a three minute safety stop at 15 FSW, then surface. watch the bar graphs and NDL times through this process. Once you see it graphically, it should make more sense.
BTW, this is my actual dive profile logged on Statia at Blue Bead Hole last November.
You can download the software from either www.tusa.com or www.apeks.co.uk - whichever is easiest for you. Go to either the IQ-700 or IQ-800 product pages at Tusa, or the Downloads section of the Apeks web site - the software is called PC Logbook. Make note of the code number to use for when you first run the software on the web page you download the software from. Once you install it, go to 'tools' then 'simulation', and take it from there.
'PGT' is Seiko abbreviation for 'percent gas tolernace'. When this value reaches the numeral 8, NDL time = 0. This is a holdover from programming in base 8 on the early Seiko (Dive Rite) dive computers with their chip sets I think (educated guess - otherwise, why not make it 10 and each pixel or numeral increase mean a 10% increase in approach to NDL limits?). This is the value of the 'controlling compartment' at any point in the dive. Watch this value too.
Then, take a dive profile to say 100 FSW and hang around for 50 minutes - you will enter deco obligation, and it will be clear when this happens with the big red bar on the screen as well as when you see a compartment hit 100% saturation and you see NDL become equal to zero.
You have a medical background - think about medicinal uptake if that helps. I remember my Physical Chemistry prof used metabolizing ethanol as the illustration for third order reaction kinetics (something he said almost all his students could relate to). Or just plain old math - I had seen you post you were a math major!
For electrically inclined folks, the perspective of items in series in a circuit vs. items in parallel in a circuit should be a piece of cake.
The 'black and white' between deco obligation incurred vs. no deco obligation incurred sure should be 'balck and white' for any single model - the math is the math - but there are numerous models which only apply so well to any one population distribution (think again about medicine and side effects in certain percentage of test subjects). This is why there is no definitive single model, and no guarantee you won't get DCS just because you followed the model. Any empirically derived model will have outliers in the data set the model is generated from - if not, either the data set is not robust (too small a data set or to narrrow a range of data collection conditions), or the modeler 'cooked the books' with the data - not necessarily intentionally. But you can never tell who might be an outlier on any given day or point in time - including yourself. So, you use your organic Mark 1 brian in addition to any other tools - such as making a 'deep stop' and a 'safety stop' and keeping your ascent profile just so, and how you feel that day, and what you did the day before, etc. in your realm of personal judgement and risk analysis / risk mitigation / risk acceptance criteria - which also varies by individual.
But if you are going to use a tool, best to understand the tool you're using.
Charlie99
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Warmwater, I don't know which model you are talking about, but the only deco model that I'm aware of that uses serial ongass/offgass is the DCIEM. Although everything comes in a serial path via very the very short time constant of lungs to blood exchange, the models generally just ignore that and treat that exchange as instantaneous. The the exchange from blood to the other "compartments" is treated in Buhlmann, DSAT, neo-Haldanian, etc. as a parallel event.
In your earlier post, where you said "However, there were references to 'recreational dive profiles' which would be different than 'decompression obligation incurred' dives - which are indicated when saturation is reached in a compartment. " <emphasis added>, it would be more correct to say that a decompression obligation is incurred when the maximim allowable value, aka M-value, is reach in a compartment.
Here's the numbers behind the statement:
If you look at the Erik Baker M-value article and compare the tabulated M-values with the inspired ppN2, you can easily see why the fast compartment just goes to 50% of the M-value and no further.
60FSW on EAN32 is (60+33-2)= 91FSW absolute pressure. Times 0.68 FN2 = 62FSW ppN2. Assumiing that zero on the graph is the 24.5FSW starting ppN2 sat point, and that 100% is the 97FSW limits of ZHL16C 1B compartment, halfway on the graph would be (24.4+97)/2, which is pretty close to the 62FSW inspired ppN2.
For the ZHL16x compartment 3, the M-value is 73.8 FSW. The inspired ppN2 of 62fsw is about 76% of the way from the 24.5fsw starting point, and the max value of 73.8fsw. In other words, even after fully saturating the 12.5 minute compartment, you will never reach the limit.
In your earlier post, where you said "However, there were references to 'recreational dive profiles' which would be different than 'decompression obligation incurred' dives - which are indicated when saturation is reached in a compartment. " <emphasis added>, it would be more correct to say that a decompression obligation is incurred when the maximim allowable value, aka M-value, is reach in a compartment.
Here's another way of analyzing your observations. Again, instead of saying that the first compartment gets to 50% saturation, this way of looking at it says that the first compartment, even when approaching saturation at the inspired pressure of N2, it will only get to about 50% of maximum allowable compartment pressure / M-value.WarmWaterDiver:
Here's the numbers behind the statement:
If you look at the Erik Baker M-value article and compare the tabulated M-values with the inspired ppN2, you can easily see why the fast compartment just goes to 50% of the M-value and no further.
60FSW on EAN32 is (60+33-2)= 91FSW absolute pressure. Times 0.68 FN2 = 62FSW ppN2. Assumiing that zero on the graph is the 24.5FSW starting ppN2 sat point, and that 100% is the 97FSW limits of ZHL16C 1B compartment, halfway on the graph would be (24.4+97)/2, which is pretty close to the 62FSW inspired ppN2.
For the ZHL16x compartment 3, the M-value is 73.8 FSW. The inspired ppN2 of 62fsw is about 76% of the way from the 24.5fsw starting point, and the max value of 73.8fsw. In other words, even after fully saturating the 12.5 minute compartment, you will never reach the limit.
WarmWaterDiver
Contributor
my PC's getting hosed up - I'll have to resume at another date & time (curse you Windows ME!).
Thanks, Charlie, that's the way I understand it, too.
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