Help with Buhlmann ZHL-16c GF

[Jax]

Is your Multideco an iOS app?

Yes, also. MultiDeco dive decompression software - VPM-B and ZHL deco

---------- Post added September 30th, 2015 at 11:21 AM ----------

I think that there may be more to it than that. I'm not sure, but maybe someone who really knows this stuff can chime in here.

With a "recreational" dive (I hate that term), there is only one thing that your computer is calculating - your NDL. That is based on the gas that you are breathing (which doesn't change during these dives), your current depth, and your previous depth/time profile. More precisely, what it is doing is calculating a current amount of N2 loading in the leading compartment and figuring out what the NDL is for that value at your current depth.

We are just assuming that GFL has no affect on N2 loading because in deco diving it determines the depth of the first stop. We are also assuming that Shearwater determines GFL in Rec mode just in case of inadvertent deco. I'm not sure that both of those assumptions are correct.

What GFL and GFH do is create a line on the classic GF graph:

For an NDL dive, the ascent follows this line continually, and the slope has to be flat enough so that ascending continually at 30-60 FPM (or whatever you use) will result in hitting the surface before you hit the M line. So these two numbers TOGETHER determine the slope of that line, and the slope of that line should determine the NDL, right? That is, the NDL is calculated so that you limit N2 accumulation below the level that would result in the slope of your ascent line hitting the M line before the surface. At some degree of tissue loading, you will no longer be able to generate a straight line for that ascent.

I'm not sure I can figure out exactly HOW the GFL setting affects NDL, but it seems like it might...

Same with my simulations on MultiDeco

---------- Post added September 30th, 2015 at 12:37 PM ----------



It's still not at all clear to me why a more conservative GF hi must be associated with a lower GF lo, giving deeper stops. The GF lo is relatively far from the M-value compared with the GF hi. If a GF lo of 45 is safe with a GF high of 95, why is is not just as safe (or safer) with a GF high of 85 or 75? As long as you behave yourself, it seems like the surfacing GF is the last, most important, determinant of risk of DCS. Perhaps I have some basic misunderstanding, would love to hear comments from those knowledgeable in the topic

I will take a stab at this, so please refer to the graph.

The Hi GF number is more about the Nitrogen loading.

The Lo GF is more about the Bubbles.

The absolute most conservative is the 50/50 line. Lots of stops, not pushing the off-loading of gasses, few(est) bubbles.

Some people are more concerned about bubbles, so they push the Lo GF from Zero to say, 40.

Some people are more concerned about the Tissue Saturation of N₂, so they push the Hi GF from 100 to ~ 70.

The more toward the center 50% that either value is, is the more conservative.


Some divers I know swear by 40/90, because it gets them out of deco fastest with the least chance of bubbles.

For me, I dive 20/80 for "regular" on and off again diving as "a little bit of conservatism" due to age, fitness, etc.

If I am diving a lot, but not deeper than 130, I go to 30/70.

If I go deeper, I would do a 40/70 or maybe 40/80 depending how the previous dive made me feel. I am very in tune with how I feel after a dive.

 

[scubadada]

Some divers I know swear by 40/90, because it gets them out of deco fastest with the least chance of bubbles.

Is there data supporting 40/90 as out of deco fastest/least chance for bubbles?

Just for comparison 1st dive, 100 feet/45 minutes/32%, obviously deco profile differs:

45/95 12:05 total deco time, Shearwater low conservatism
40/90 15:05
40/85 18:05 Shearwater medium
35/75 24:00 Shearwater high

Just trying to understand, thanks. Bubble/nitrogen load makes some sense

 

[rhwestfall]

which is echoed in the VPM w/ GF's as you can only change the GF(High), thus you are adding "nitrogen loading calcs" to the bubble model that VPN is.....

a recent dive kicked my but at VPM +3, w/ deco on 50%. A couple choices were; +4 (long deco), 100%, or implement VPM w/ GF(high).....


this has been an awesome discussion.....

 

[doctormike]

I think that we may be oversimplifying this - the modified Buhlmann models are having to adjust for an ever changing leading compartment, and all of those N2 loading parameters change with changes in the profile. It's not as simple as saying that 50/50 is more conservative, that's reducing a very complex (and largely unknown) physical system to two numbers.

But to use that example, why wouldn't 50/40 be more conservative than 50/50? All that means is that after finishing your 50/50 profile, you then spend a little more time at your last stop, where you are no longer ongassing, and you drop your supersaturation of the leading compartment another 10% of the way to the M line. Since by that stop, all of the compartments are offgassing, the lower the GFH, the less inert gas pressure when you surface, and presumably the less decompression stress.

On the other hand, 40/50 might or might not be safer than 50/50, it would depend on whether or not the ongassing of the slow compartments at the first stops and the less efficient decompression was made up for by the reduced bubble initiation at depth. Making the GFL very low does start your stops earlier, but you may also be ongassing your slower (unsaturated) compartments, until one of them becomes the leading compartment. Ideally (explained well in Doppler's blog post), you are shallow enough to have efficient offgassing and are above the gas transition point (where there is more offgassing than ongassing). Too complex to guess!

Bubbles and N2 saturation are two manifestations of the same underlying phenomenon, so I don't see how they are separable into conflicting requirements.

---------- Post added September 30th, 2015 at 03:42 PM ----------

Using 45/85 instead of 45/95 is no "safer" (although safer might be the wrong word), what it means is that you will have shallower first deco stop (compared to someone running 10%) and you will have a longer 20ft safety stop than the person running 45/95 as you off gas more.

Don't understand this. Wouldn't a person running 45/85 surface with less inert gas tissue pressure than someone running 45/95?

 

[uncfnp]

I am going to give this another layman's try. I do not think that GFL will influence NDL because it is an ascent factor. It effects the ascent profile based on the total dive time and tissue loading. As I understand it. The Low GF does not come into play until you start the ascent then it will determine the first DECO stop needed to prevent the TC's from exceeding the specified percentage of the M value In an attempt to prevent bubble formation. Simply put, it is the bubble factor not a time factor. If you have not exceeded NDL, no stops are needed prior to the SS hence GFL does not factor into the ascent.

Any graphed line that represents both GF values will be stair stepped to allow for the time spent at each stop.

With both values, the lower number has the highest conservative level with 0 being there is no supersaturation and 100 being where the leading compartment is at the Buhlmann M value line and highest risk for DCS.

 

[CptTightPants21]

Don't understand this. Wouldn't a person running 45/85 surface with less inert gas tissue pressure than someone running 45/95?

Yes, a person would have less inert gas in their system. What I meant by "no safer" is that it is not like a person running 45/95 will get bent and the person running 45/85 will not get bent. DCS is still a mystery and subject to a myraid of different factors dive and personal diver related.

---------- Post added September 30th, 2015 at 04:52 PM ----------

I am going to give this another layman's try. I do not think that GFL will influence NDL because it is an ascent factor. It effects the ascent profile based on the total dive time and tissue loading. As I understand it. The Low GF does not come into play until you start the ascent then it will determine the first DECO stop needed to prevent the TC's from exceeding the specified percentage of the M value In an attempt to prevent bubble formation. Simply put, It is the bubble factor not a time factor.

Any graphed line that represents both GF values will be stair stepped to allow for the time spent at each stop.


With both values, the lower number has the highest conservative level with 0 being there is no supersaturation and 100 being where the leading compartment is at the Buhlmann M value line and highest risk for DCS.

I disagree.
Yes it relates to bubble formation, but it also relates to time.

You mention the stair stepped graph, what is causing that stair step formation?----You are hitting the new M value slope created from both GFs, you need to wait, off gas (the vertical of the step) and then continue your ascent (the horizon of the step) until you hit the slope again....that is what is causing the stair step view of the deco.

The standard Bulhmann M-Value slope (essentially 100/100 in GF speak) is increasingly upward sloping. What that means is that at deeper depths there is a large difference between changes in your inert inert tissue pressure and the value in which you will breach an M-value and take a DCS hit (according to the model). As you get shallower, the M-value is much closer to the ambient pressure line and therefore changes in pressure will have a greater effect on making a diver hit against that M-value line

http://blog-imgs-45.fc2.com/c/h/a/charopapa/mvaluefigure1.jpg

The picture isn't the best, but you can clearly see how the M value line and Ambient pressure line are much closer together at 0 then they are deeper depths.

What causes the standard Bulhmann model to have so much deco at the shallower depths and almost none at deeper depths is the narrowing of the M value. A diver can at depth ascend and be between the M value line and the ambient pressure line for a large part of his ascent and only hit the M value line (and need to do a deco stop) once he is at 20 or 30 ft.

The lower GF is a percentage of the M value for each compartment. Depending on the dive, some are on gassing, some are already saturated, some may be already off gassing.

Your NDL time using Bulhmann with GFs is the point in the dive where your tissue loading is such that you can ascent at 30ft per minute and never breach the lower GF in any of the 16 compartments and have all 16 compartments end with equal or less than nitrogen loading of the High GF. If you would breach the lower GF in ANY compartment, you will do a deco stop even if your overall nitrogen loading is still pretty benign.



This is what I have read, learned from others, and extrapolated. I don't want to show myself as some kind of master. I would welcome someone with a far greater understanding to correct anything I have misstated.

 

[doctormike]

Yes, a person would have less inert gas in their system. What I meant by "no safer" is that it is not like a person running 45/95 will get bent and the person running 45/85 will not get bent. DCS is still a mystery and subject to a myraid of different factors dive and personal diver related.

Yeah, I guess it's semantics, I see what you are saying. Overall, the rate of DCS is very low, so there may not be a statistically significant, clinically detectable difference in DCS rates for a study of 45/85 vs 45/95, given the limits of sample size and presuming dives with low overall TC loading (which would be an independent variable, separate from the GF chosen)

But less inert gas HAS to correlate with less DCS stress at some level, and there might be a greater risk surfacing at 95% M value when compared to 85%, especially if there were other risk factors at play (PFO, scar tissue, poor peripheral circulation, etc..).

Good point, though!

 

[CptTightPants21]

Yeah, I guess it's semantics, I see what you are saying. Overall, the rate of DCS is very low, so there may not be a statistically significant, clinically detectable difference in DCS rates for a study of 45/85 vs 45/95, given the limits of sample size and presuming dives with low overall TC loading (which would be an independent variable, separate from the GF chosen)

But less inert gas HAS to correlate with less DCS stress at some level, and there might be a greater risk surfacing at 95% M value when compared to 85%, especially if there were other risk factors at play (PFO, scar tissue, poor peripheral circulation, etc..).

Good point, though!

All correct.

I would like to add there are other factors that would dictate deciding between the two including total gas supply, O2 clock, past dive profiles, and future dives planned that day.

 

[doctormike]

I am going to give this another layman's try. I do not think that GFL will influence NDL because it is an ascent factor. It effects the ascent profile based on the total dive time and tissue loading. As I understand it. The Low GF does not come into play until you start the ascent then it will determine the first DECO stop needed to prevent the TC's from exceeding the specified percentage of the M value In an attempt to prevent bubble formation. Simply put, it is the bubble factor not a time factor. If you have not exceeded NDL, no stops are needed prior to the SS hence GFL does not factor into the ascent.

Any graphed line that represents both GF values will be stair stepped to allow for the time spent at each stop.

OK, that makes more sense... Still waiting for something from Shearwater or another real expert, but I think that you are right.

I guess another way of saying this would be that a dive computer does the following:

1) Tracks N2 loading

2) Calculates point in run time at which - staying at constant depth - a no stop ascent to the surface would result in overpressure of the leading compartment to be equal to GFH.

3) Generates an NDL by subtracting the current runtime from #2.


Since the only thing that makes it a no stop dive is that at the NDL you can ascend directly and surface at GFH, it really doesn't matter what you set GFL to, that only becomes a consideration when NDL is exceeded and a mandatory stop is calculated.

With both values, the lower number has the highest conservative level with 0 being there is no supersaturation and 100 being where the leading compartment is at the Buhlmann M value line and highest risk for DCS.

Well, here some real world limits come into play, at some point the numbers get so low that they pass the point of diminishing returns. A VERY low GHL will result in inefficient decompression, so unless you have unlimited gas and unlimited time, that won't work either. For example, if you use 1/70, then you would rise a small distance off the bottom and then stop, waiting for the 1% towards the M value in the leading compartment to slowly offgas back to the ambient pressure line. Then up again for a very small step, etc... But with each of these slow, inefficient steps, the slower compartments are ongassing, until one of them becomes leading, then everything is recalculated. Theoretically, you could surface this way (in a hard hat, I guess), but this makes the point that even conservatism has it's limits.

 

[uncfnp]

...//...
Your NDL time using Bulhmann with GFs is the point in the dive where your tissue loading is such that you can ascent at 30ft per minute and never breach the lower GF in any of the 16 compartments and have all 16 compartments end with equal or less than nitrogen loading of the High GF. If you would breach the lower GF in ANY compartment, you will do a deco stop even if your overall nitrogen loading is still pretty benign.



This is what I have read, learned from others, and extrapolated. I don't want to show myself as some kind of master. I would welcome someone with a far greater understanding to correct anything I have misstated.

I only have common sense to work with but I get what you are saying. As I said it is an ascent factor limiting the percentage of M value and as you said that as BT increases so does tissue loading so theory would suggest that at some point the tissue loading allowed by the HGF alone would exceed a set LGF, unless of course the LGF is set at 100. At what point this would happen, I have no idea but I would suspect that for the average recreational diver, the effect would be minimal.

---------- Post added September 30th, 2015 at 05:38 PM ----------

...//...

Well, here some real world limits come into play, at some point the numbers get so low that they pass the point of diminishing returns. A VERY low GHL will result in inefficient decompression, so unless you have unlimited gas and unlimited time, that won't work either. For example, if you use 1/70, then you would rise a small distance off the bottom and then stop, waiting for the 1% towards the M value in the leading compartment to slowly offgas back to the ambient pressure line. Then up again for a very small step, etc... But with each of these slow, inefficient steps, the slower compartments are ongassing, until one of them becomes leading, then everything is recalculated. Theoretically, you could surface this way (in a hard hat, I guess), but this makes the point that even conservatism has it's limits.

Agreed. Speaking theoretically only.

---------- Post added September 30th, 2015 at 05:40 PM ----------

Yeah, I guess it's semantics, I see what you are saying. Overall, the rate of DCS is very low, so there may not be a statistically significant, clinically detectable difference in DCS rates for a study of 45/85 vs 45/95, given the limits of sample size and presuming dives with low overall TC loading (which would be an independent variable, separate from the GF chosen)

But less inert gas HAS to correlate with less DCS stress at some level, and there might be a greater risk surfacing at 95% M value when compared to 85%, especially if there were other risk factors at play (PFO, scar tissue, poor peripheral circulation, etc..).

Agreed. Semantics perhaps but it is safer.