Ascent rates. Why do you choose to do it the way you do it.
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raymond phule
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Except PADI.
In the old days we were told not to ascend faster than your smallest bubbles. Currently we have all these fancy gizmos that beep, flash and wail if you exceed a 30fpm assent rate.
Im with Catherine on this. Unless there is an emergency, whats the hurry? I love slow assents, I like hovering at 20 then 15 and again at 10 followed by the slowest possible assent I can manage. Do I feel better? I dont remember how I felt 30 years ago after following my bubbles to the surface but It must have been pretty good. Now, after a very slow assent and the requisite safety stops I just feel old. But then again, Im am!
diver 85
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What do they teach now, I know what it 'used to be'....
raymond phule
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Not faster than 60 feet/min but that you can ascend slower.
My recent class was PADI, They taught the 60/30 combination, although our instructor did tell us that the rest of the world had moved to a 30 fpm maximun ascent rate. Thank you TS and M, your post was the answer to my question.
I find the off gassing, on gassing question interesting also. It seems that if our bodies could be compared to the vessel of liquid in the henry's law illustrations, you would be on gassing until you had ascended to a level which the vessel was in equilibrium, then off gassing as you decreased the pressure. The compartment idea is foreign to me, are you saying that our bodies represent a collection of different vessels which reach equilibrium at different speeds?
I find the off gassing, on gassing question interesting also. It seems that if our bodies could be compared to the vessel of liquid in the henry's law illustrations, you would be on gassing until you had ascended to a level which the vessel was in equilibrium, then off gassing as you decreased the pressure. The compartment idea is foreign to me, are you saying that our bodies represent a collection of different vessels which reach equilibrium at different speeds?
Yes, that's the way the theory is constructed. You see, gas equilibrates across the alveolar membrane (i.e. between inspired gas and blood) very quickly, but then that gas has to be transported to tissues and diffuse out of capillaries into interstitial space and then into cells. Poorly perfused tissues like ligaments, tendons and joint spaces will have a relatively slow delivery of gas to them, but also a slow washout. Very well perfused tissues, like brain and nerves, will have more rapid delivery, and particularly rapid uptake of lipid-soluble gases. And as Rick Murchison has observed, warm tissue has more perfusion than cold, so even the delivery and washout of gas to the same part of the body can be different as the dive progresses.
The multi-compartment models attempt to address this, but it gets even more complicated, because they introduce the concept of "allowable supersaturation", which is greater in fast "tissues" than in slow ones. That's beyond the scope of this thread, I think.
But the bottom line is that the multi-compartment structure of the models is an attempt to simulate the behavior of gas in a real organism. The compartments do NOT correlate with specific structures or tissues in the human body.
Charlie99
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I'll split it into two posts ---- one for WHY and one for WHAT.
The background and the why:
My dives are recreational dives, more often than not on air. Usually 2 or 3 dives per day. Having an SAC down around 0.4cfm, it is pretty easy for me to push or slightly exceed the no-stop limits of even my relatively liberal Oceanic computer.
When I first started diving near the limits I had the typical post-dive fatigue. Nitrox did indeed help. What helped even more was changing my ascent and stop profile, and now I no longer have post-dive fatigue whether diving air or nitrox.
Although it isn't always clear because things get expressed in different ways, but there is really a lot more consensus on the appropriate ascent and stops profile that it would seem at first.
The GUE minimum deco ascent rules at first glance may not seem to have much in common with the Naui half-depth stop, or Pyle stops, or the Marroni studies, or the output from bubble model programs, or the output from dissolved gas programs with the gradient factor hack, but in reality, all of these have some core elements in common.
They all share the dilemma of wanting to stay deep in order to avoid bubble formation, yet at the same time want to ascend in order to stop further ongassing and to promote offgassing. A good ascent profile is a balance between these two competing items.
An ascent profile is also a balance between complication vs "goodness". At some point, having a simple profile that one can remember and execute wins out over further optimization.
If you look at the series of mandatory stops from any decompression algorithm -- whether the latest bubble model or the earliest work of J.S. Haldane ---you will see a general shape of faster ascent when deep and slower as you get shallow. Yes, the pure dissolved gas models tend to take you shallow faster than do the bubble models, but that overall "shape of deco" is there. For dive profiles that do not have mandatory decompression obligation, this curve doesn't really show up, unless you are doing more sensitive tests, such as the doppler bubble monitoring tests of Dan Europe. But you can force most deco programs to develop the same sort of shape of deco curve for non-deco dives by cranking conservatism up to very high levels, by cranking in very low gradient factors, etc. Adding arbitrary deep stops by the Pyle method develops this same slower as you get shallower profile since they are equal time stops, but are at closer and closer depth spacings as you get shallower.
An important point to remember is that it has been shown in various studies that what counts is the profile as averaged over a short averaging period of a couple of minutes. (A series of 1 minute stops every 10' is pretty much equivalent to a constant 10fpm ascent. DAN Europe found that the most critical parameter was total time of ascent, rather than the instantaneous ascent rate, etc.). When you look at things in this light, the well tested 15' safety stop is crudely equivalent to slowing the ascent rate during the last 30' of the ascent, in much the same as the GUE 1@30', 1@20', 1@10' slows the ascent rate for the last 30 or 40'.
Putting all of this together with my typical dive profiles which are within normal recreational limits I have found that for me the right tradeoff between simplicity and "ideal profile" is to have 3 stops on my ascent.
The background and the why:
My dives are recreational dives, more often than not on air. Usually 2 or 3 dives per day. Having an SAC down around 0.4cfm, it is pretty easy for me to push or slightly exceed the no-stop limits of even my relatively liberal Oceanic computer.
When I first started diving near the limits I had the typical post-dive fatigue. Nitrox did indeed help. What helped even more was changing my ascent and stop profile, and now I no longer have post-dive fatigue whether diving air or nitrox.
Although it isn't always clear because things get expressed in different ways, but there is really a lot more consensus on the appropriate ascent and stops profile that it would seem at first.
The GUE minimum deco ascent rules at first glance may not seem to have much in common with the Naui half-depth stop, or Pyle stops, or the Marroni studies, or the output from bubble model programs, or the output from dissolved gas programs with the gradient factor hack, but in reality, all of these have some core elements in common.
They all share the dilemma of wanting to stay deep in order to avoid bubble formation, yet at the same time want to ascend in order to stop further ongassing and to promote offgassing. A good ascent profile is a balance between these two competing items.
An ascent profile is also a balance between complication vs "goodness". At some point, having a simple profile that one can remember and execute wins out over further optimization.
If you look at the series of mandatory stops from any decompression algorithm -- whether the latest bubble model or the earliest work of J.S. Haldane ---you will see a general shape of faster ascent when deep and slower as you get shallow. Yes, the pure dissolved gas models tend to take you shallow faster than do the bubble models, but that overall "shape of deco" is there. For dive profiles that do not have mandatory decompression obligation, this curve doesn't really show up, unless you are doing more sensitive tests, such as the doppler bubble monitoring tests of Dan Europe. But you can force most deco programs to develop the same sort of shape of deco curve for non-deco dives by cranking conservatism up to very high levels, by cranking in very low gradient factors, etc. Adding arbitrary deep stops by the Pyle method develops this same slower as you get shallower profile since they are equal time stops, but are at closer and closer depth spacings as you get shallower.
An important point to remember is that it has been shown in various studies that what counts is the profile as averaged over a short averaging period of a couple of minutes. (A series of 1 minute stops every 10' is pretty much equivalent to a constant 10fpm ascent. DAN Europe found that the most critical parameter was total time of ascent, rather than the instantaneous ascent rate, etc.). When you look at things in this light, the well tested 15' safety stop is crudely equivalent to slowing the ascent rate during the last 30' of the ascent, in much the same as the GUE 1@30', 1@20', 1@10' slows the ascent rate for the last 30 or 40'.
Putting all of this together with my typical dive profiles which are within normal recreational limits I have found that for me the right tradeoff between simplicity and "ideal profile" is to have 3 stops on my ascent.
Charlie99
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1. I ignore maximum depth, and instead base my profile upon my average depth over the last 10 or 15 minutes. For example, if I drop down to 120' for 5 minutes to look at some black coral and a longnose hawkfish, and then start working my way back up a wall and have spent a lot of time up at 70', then I do my ascent as if starting from 70', not 120'. (I recommend that those divers doing the NAUI half-depth stop also apply this logic to their choice of "half depth")
2. I figure out how long I want to take on my ascent. Never less than 6 minutes. Rarely more than 12. This is a judgement call, based on experience with similar dive profiles/multiple dive sequences and observation of how long it takes to get the N2 bargraph on my Oceanic computer back into the green section (More or less equivalent to 80% of Buhlmann M-value, or a gradient factor of 80 or 0.8 depending upon program).
If I'm up against NDL limit on a long 60' dive, it will take longer to offgas the heavily loaded medium speed compartments than it would on a dive to 100'+ to near NDL which is controlled by faster compartment; so my ascent time from 60' would actually be longer than from 100' near NDL dive.
This is also where you crank in a fudge factor for how well behaved your profile has been (surging up and down, yo-yo-ing up and down around coral heads vs. nice clean gradually ascending multi-level profile), and how you are feelng that day.
Anyway, you come up with a desired total ascent time.
Current runtime + desired minutes of ascent give you what runtime you will be making your final ascent. REMEMBER THAT FINAL SURFACING RUNTIME, and of course your TOTAL ASCENT TIME.
3. I pick out my deep stop depth. For my dives it will be 35' to 50'. 50' if coming up from 120'. 35' if coming up from spending time at 60'. Work out 30fpm ascent time to that point, and add 1 or 2 minutes. That tells you the time you will LEAVE the first stop.
4. While hangin at the deepest stop, figure out how to split the remaining time between another intermediate stop and a final 20- to 10' glide. If 6 minutes total ascent, it will be something like 3 for the shortest, 2 for an intermiate stop, and just 1 for the deepest stop. Again, you weight things a bit according to where you are coming from. If doing a direct ascent from a square profile dive at 100', then you will spend more of the total ascent time at the first two deeper stops than you would on an ascent from a dive to 60'.
5. Intermediate stop --- kind of splits the difference between your first 35' to 50' stop and the final 20' to 10' slide. The length of the stop will also be longer than the 1st stop, but shorter than the 2nd stop. Once you have figured out the time, all you have to remember is at what runtime you depart this stop.
6. I start the last stop around 20' and then slowly glide upward during the entire period so that I end it around 10' when it is time to surface. (You do still remember that 1 number you decided upon at the beginning of the ascent, right?). As with the other things, the specifics of the stop get fudged a bit accoring to what sort of dive I'm coming up from. If I'm very heavily loaded in the slower compartments from a third dive of the day in the 40-50' range, then this stop may actually start at 15' and go up as shallow as 6' or 8'. OTOH, coming up directly from a deep dive, it will be weighted all towards the 20' end.
In any case, people I dive with often can just look at where I am in the water column and know when I'm ready to ascent.
===================================
At first reading, the above may seem to be complicated, but in reality it isn't that difficult. After a while you will indeed have a pretty good gut feeling as to how many minutes you should be taking on any given ascent. And then splitting it into 3 stops is IMO, a reasonable tradeoff between complexity and closeness to an ideal "shape of deco" curve.
Charlie Allen
2. I figure out how long I want to take on my ascent. Never less than 6 minutes. Rarely more than 12. This is a judgement call, based on experience with similar dive profiles/multiple dive sequences and observation of how long it takes to get the N2 bargraph on my Oceanic computer back into the green section (More or less equivalent to 80% of Buhlmann M-value, or a gradient factor of 80 or 0.8 depending upon program).
If I'm up against NDL limit on a long 60' dive, it will take longer to offgas the heavily loaded medium speed compartments than it would on a dive to 100'+ to near NDL which is controlled by faster compartment; so my ascent time from 60' would actually be longer than from 100' near NDL dive.
This is also where you crank in a fudge factor for how well behaved your profile has been (surging up and down, yo-yo-ing up and down around coral heads vs. nice clean gradually ascending multi-level profile), and how you are feelng that day.
Anyway, you come up with a desired total ascent time.
Current runtime + desired minutes of ascent give you what runtime you will be making your final ascent. REMEMBER THAT FINAL SURFACING RUNTIME, and of course your TOTAL ASCENT TIME.
3. I pick out my deep stop depth. For my dives it will be 35' to 50'. 50' if coming up from 120'. 35' if coming up from spending time at 60'. Work out 30fpm ascent time to that point, and add 1 or 2 minutes. That tells you the time you will LEAVE the first stop.
4. While hangin at the deepest stop, figure out how to split the remaining time between another intermediate stop and a final 20- to 10' glide. If 6 minutes total ascent, it will be something like 3 for the shortest, 2 for an intermiate stop, and just 1 for the deepest stop. Again, you weight things a bit according to where you are coming from. If doing a direct ascent from a square profile dive at 100', then you will spend more of the total ascent time at the first two deeper stops than you would on an ascent from a dive to 60'.
5. Intermediate stop --- kind of splits the difference between your first 35' to 50' stop and the final 20' to 10' slide. The length of the stop will also be longer than the 1st stop, but shorter than the 2nd stop. Once you have figured out the time, all you have to remember is at what runtime you depart this stop.
6. I start the last stop around 20' and then slowly glide upward during the entire period so that I end it around 10' when it is time to surface. (You do still remember that 1 number you decided upon at the beginning of the ascent, right?). As with the other things, the specifics of the stop get fudged a bit accoring to what sort of dive I'm coming up from. If I'm very heavily loaded in the slower compartments from a third dive of the day in the 40-50' range, then this stop may actually start at 15' and go up as shallow as 6' or 8'. OTOH, coming up directly from a deep dive, it will be weighted all towards the 20' end.
In any case, people I dive with often can just look at where I am in the water column and know when I'm ready to ascent.
===================================
At first reading, the above may seem to be complicated, but in reality it isn't that difficult. After a while you will indeed have a pretty good gut feeling as to how many minutes you should be taking on any given ascent. And then splitting it into 3 stops is IMO, a reasonable tradeoff between complexity and closeness to an ideal "shape of deco" curve.
Charlie Allen
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