PfcAJ
Contributor
His relevance.
Slow tissue on gas from stops
- Thread starter rossh
- Start date
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Diver-Drex
Contributor
If I plan a dive on my desk top planner running ZHL B at GF 50/80 for a 130fsw dive for 25minutes using EAN28 and EAN50 I get two stops on my ascent - 40fsw for 2minutes and 20fsw for 11min.
If I add a segment to the dive at 65fsw for 2min my stops are 40fsw for 2min and 20fsw for 12min. The algorithm compensated for additional gas loading during the 65fsw segment by adding 1min at the 20fsw stop.
If I do the same dive to 130fsw for 25min at GF 30/80 I get three stops - 60fsw for 4:40, 40fsw for 1min and 20fsw for 10min.
During these various threads GF 50/80 seems to be a sensible choice. If someone was looking to control bubble growth adding a stop at 65fsw and letting the GF 50/80 planner compensate seems to be reasonable. But GF 30/80 adds time at depth and reduces time on the shallow stops. Can anyone provide insight into why this is?
I don’t think it’s unreasonable to add a brief stop at depth as long as your plan or PDC accounts for it. But it looks like ‘forcing’ ZHL-B to give a deep stop by using GF 30/80 backfires. Any thoughts?
If I add a segment to the dive at 65fsw for 2min my stops are 40fsw for 2min and 20fsw for 12min. The algorithm compensated for additional gas loading during the 65fsw segment by adding 1min at the 20fsw stop.
If I do the same dive to 130fsw for 25min at GF 30/80 I get three stops - 60fsw for 4:40, 40fsw for 1min and 20fsw for 10min.
During these various threads GF 50/80 seems to be a sensible choice. If someone was looking to control bubble growth adding a stop at 65fsw and letting the GF 50/80 planner compensate seems to be reasonable. But GF 30/80 adds time at depth and reduces time on the shallow stops. Can anyone provide insight into why this is?
I don’t think it’s unreasonable to add a brief stop at depth as long as your plan or PDC accounts for it. But it looks like ‘forcing’ ZHL-B to give a deep stop by using GF 30/80 backfires. Any thoughts?
What a load of made up and insulting rubbish.
You keep loosing each technical discussion on this topic, and now your only retort is to fabricate insults about other peoples intelligence, as thinly veiled ad hominem attacks. How very "academic" of you.
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Hard to know what exactly from your description, and would need some hard sample plans to really comment. You might be tricking your calculation with the 65 ft being done on bottom mix, while the plain deco ascent will have 50% being used at this point. On small time deco dives, the 1.6 at 70ft really speeds up the overall deco.
All the deco models, when allowed to function without too much user interference, will properly compensate and account for on- gassing during the stops. That is the basic operation of any model. There is no need to be manually adding extra shallow time - nothing is missed out by using GF 30/80, or any other deeper stop model plan like VPM-B.
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Ross,
You have never quite got your head around the concept of decompression efficiency (the least risk for the same decompression time), or that this deep stops debate has always been all about that.
You believe in VPM-B. Fair enough. Let's say you plan a decompression dive using VPM-B (whatever conservatism level you choose), and it tells you that the total decompression time is X. Then someone else comes along and plans the same dive using a GF approach with shallower first stops, but with the GF adjusted so that the total decompression time is still exactly X; the same as your VPM-B approach.
It is an indisputable fact that your VPM-B approach will result in less supersaturation in faster tissues early in the ascent, and more supersaturation in slower tissues later in the ascent (because of the issue being discussed in this thread). In contrast the GF approach will result in more supersaturation in fast tissues early in the ascent, and less supersaturation in slower tissues later in the ascent. Both you and the GF diver believe their approach is optimal. Its like two deluded people both claiming to be Jesus; at the very least they cannot both be right (and most probably they are both wrong).
Which is why your statement....
....is nonsense.
They only "properly compensate" to satisfy the assumptions of the person who wrote the model. And in case you are tempted to launch off into a speech about your belief that GF is not a model, that would be missing the point. You clearly believe that VPM-B is a model, and I have articulated one of a number of ways in which someone might generate a decompression profile of exactly the same length but which distributes the pattern of supersaturation across the range of tissues very differently. I repeat, they can't both be right.
So which is right? This is where we come back to the current state of our available evidence, which suggests that decompression approaches prescribed by bubble models (and RD) incorporate deep stops that are too deep for the most efficient decompression (least risk for the same decompression time). I would remind you that all that evidence is pointing in the same direction, and there is none that supports the degree of emphasis placed on deep stops by bubble models. Relating that back to the subject of the thread, the protection of faster tissues early has not fulfilled the promise that many thought it would, and protection of slower tissues later in an ascent appears more important than many assumed.
Simon M
You have never quite got your head around the concept of decompression efficiency (the least risk for the same decompression time), or that this deep stops debate has always been all about that.
You believe in VPM-B. Fair enough. Let's say you plan a decompression dive using VPM-B (whatever conservatism level you choose), and it tells you that the total decompression time is X. Then someone else comes along and plans the same dive using a GF approach with shallower first stops, but with the GF adjusted so that the total decompression time is still exactly X; the same as your VPM-B approach.
It is an indisputable fact that your VPM-B approach will result in less supersaturation in faster tissues early in the ascent, and more supersaturation in slower tissues later in the ascent (because of the issue being discussed in this thread). In contrast the GF approach will result in more supersaturation in fast tissues early in the ascent, and less supersaturation in slower tissues later in the ascent. Both you and the GF diver believe their approach is optimal. Its like two deluded people both claiming to be Jesus; at the very least they cannot both be right (and most probably they are both wrong).
Which is why your statement....
rossh:
....is nonsense.
They only "properly compensate" to satisfy the assumptions of the person who wrote the model. And in case you are tempted to launch off into a speech about your belief that GF is not a model, that would be missing the point. You clearly believe that VPM-B is a model, and I have articulated one of a number of ways in which someone might generate a decompression profile of exactly the same length but which distributes the pattern of supersaturation across the range of tissues very differently. I repeat, they can't both be right.
So which is right? This is where we come back to the current state of our available evidence, which suggests that decompression approaches prescribed by bubble models (and RD) incorporate deep stops that are too deep for the most efficient decompression (least risk for the same decompression time). I would remind you that all that evidence is pointing in the same direction, and there is none that supports the degree of emphasis placed on deep stops by bubble models. Relating that back to the subject of the thread, the protection of faster tissues early has not fulfilled the promise that many thought it would, and protection of slower tissues later in an ascent appears more important than many assumed.
Simon M
UWSojourner
Contributor
Your posts in this thread are just the same old stuff you've peddled many times before. As we've shown over and over, VPM-B-style deep stops come at a cost. The cost is higher decompression stress caused by the continued on-gassing and delay in off-gassing of the slower compartments.
See this post for a discussion. What it shows for the dive discussed is that VPM-B incurs about 33% more decompression stress due to VPM's deep stop "feature".
What benefit is VPM-B providing that justifies the increased supersaturation exposure for the same runtime? All the evidence suggests the answer is "None".
Diver-Drex
Contributor
If we accept that ZHL-B is effective what is the potential harm in adding a deep stop and allowing the algorithm to make the adjustments? Aside from more overall time in the water what is the downside? How is it different than a multi level dive where the PDC tracks offgassing and ongassing during the shallower segment and calculates the ascent profile based on that data?
Hello Diver-Drex,
Other than less efficient decompression, effectively not much. However, the point is similar to the one I made to Ross above. If you are prepared to do that extra decompression time that would accrue from stopping deeper then, depending to some extent what you mean by deep stop(s), the current evidence would suggest that your risk of DCS may be smaller if you did that extra time on your shallow stops, and omitted the deep stop(s).
Simon M
Other than less efficient decompression, effectively not much. However, the point is similar to the one I made to Ross above. If you are prepared to do that extra decompression time that would accrue from stopping deeper then, depending to some extent what you mean by deep stop(s), the current evidence would suggest that your risk of DCS may be smaller if you did that extra time on your shallow stops, and omitted the deep stop(s).
Simon M
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