Help with Buhlmann ZHL-16c GF

[doctormike]

In my mind, it's for the event where a diver goes into deco, whether that be intentional or not. Diving a Petrel in Rec mod doesn't preclude going into deco. They've set up the conservatism settings to handle that if it occurs.

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...

 

[scubadada]

I've been wondering, why does a lower GF hi necessarily go with a lower GF lo in the conservativeness ratings? Are the surfacing and the deeper stops GFs directly related, , linear, of the same magnitude of risk, etc.? I've talked to individuals who set custom GF levels and choose to go with a more conservative (lower) GF hi and a more liberal (higher) GF lo. The most common explanation is that they choose not to do the deeper stops or do not "believe" in them. Does this general principle apply to conservative settings in VPM?

Again, yes, I understand and agree with that much. But what might be Shearwater's thinking behind the inverse relationship between GF-Low and conservatism, and what might be the thinking behind the specific GF-Low values they chose? Why a value of 35 for High Conservatism and a value of 45 for Low Conservatism?

Actually, in my previous post I think I stated it backwards. (See how befuddled I am by all this?) A lower GF-Low value would generate a deeper stop than a higher GF-Low value. So it seems to me that Shearwater's thinking is that deeper stops (in the event an NDL is exceeded) are more conservative than shallower stops.

I agree that whether exceeding an NDL is "intentional" or not doesn't really matter as far as the fact that in either case the computer needs to guide the diver to the surface safely by indicating to the diver to make one or more stops. But might there be some presumption that if a diver sets the computer to recreational mode, the diver is probably using a single tank, may not have deco training, etc.--in other words, the recreational diver might be ill-prepared to be doing much deco? If so, maybe the "conservative" thing to do would be to get the diver closer to the surface sooner rather than later, even if it means surfacing with more tissue loading. For a diver prepared to do deco, I believe there are many who believe doing stops deeper rather than shallower can help keep the bubbles from growing--that's sort of what the bubble models do, right? So I can see how a technical diver might choose a low GF-Low to be conservative. But might the thinking there change for a recreational diver? I have no idea. I would really like to understand the thinking behind the selection of GF-Low values for recreational, no-stop divers.

It's actually my question too, I probably did not state it understandably

 

[Lorenzoid]

Doctormike, that's exactly the kind of thinking I was hoping someone would offer. So if your understanding is correct that a diver ascending without making any stops would want to follow the GF line as closely as possible, then the GF-Low value is just as relevant to the computing the corresponding NDLs as the GF-High value. (But I suppose a no-stop diver's "safe" ascent could follow ANY path between the GF line and the Ambient line.) I hope someone like Doppler will elucidate.

 

[uncfnp]

Copying a table from Excel preserves formatting (pretty much)

Air NDLs (min)
Depth (ft)	PZ+	DSAT	PADI RDP	NOAA	Petrel (low)
60	48	57	55	60	49
70	35	40	40	48	34
80	26	30	30	39	24
90	19	24	25	30	19
100	16	19	20	25	14
110	12	16	16	20	11
120	10	13	13	15	9
130	8	11	10	10	7

From your values, it appears low conservatism matches PZ+ better than DSAT. I guess I will have to try this for myself, side by side. I've seen slightly different values for the Petrel, not sure why. Thanks for the response. Shearwater does not include these values in the owner's manual as Oceanic does

Thanks. Posting from iPad with limited options. I double checked and those are the numbers my Petrel 2 gives for low.

I guess it makes since that PZ+ and Buhlmann match since PZ is based on Buhlmann. And on DC's that offer CF settings on their PZ+ algorithms, likely they would correlate with the Petrel's higher conservative settings.

Lorenzoid. I think only Shearwater can give you the definitive answer you are looking for. Sent pm. Maybe they will join in the discussion.

 

[Scubanoobi]

Although the table shows 45/95 closely matches PZ+, in practice that is really not the case, at least with the profiles I tested with, 45/95 would give me about 10 minutes more NDL time than PZ+. As I mentioned earlier, I've found 40/85 to match very closely with my Geo (on my recreational profiles)


Sent from my iPhone using Tapatalk

 

[doctormike]

Doctormike, that's exactly the kind of thinking I was hoping someone would offer. So if your understanding is correct that a diver ascending without making any stops would want to follow the GF line as closely as possible, then the GF-Low value is just as relevant to the computing the corresponding NDLs as the GF-High value. I hope someone like Doppler will elucidate.

Well, here's where it gets confusing to me, and I would definitely welcome Doppler's sage analysis at this point!

If you make a graph of simply depth vs. time, then a no-stop ascent is just a straight line with a slope proportional to the ascent rate. But the GF line is depth vs. N2 pressure in the leading compartment, so it's not the same as an ascent line. But nevertheless, since there are no stops, it's a straight line. I'm just guessing that those two modifying numbers which generate the dive parameters might both contribute to NDL in some mathematical way that is beyond my current comprehension...

 

[scubadada]

Scubanoobi,

I'm having a blast playing around with my newly acquired MultiDeco. I've only had a chance to run 1st dives and have not looked into multiple dives yet. Buhlmann ZHL16-C 40/85 runs close to Oceanic PZ+, 45/95 runs pretty close to DSAT/PADI. I don't own a Petrel (yet) so can't compare the NDLs directly. Someone with a Petrel could easily fill in the table I posted earlier.

Thanks, Craig

---------- Post added September 30th, 2015 at 08:45 AM ----------



I've been wondering, why does a lower GF hi necessarily go with a lower GF lo in the conservativeness ratings? Are the surfacing and the deeper stops GFs directly related, , linear, of the same magnitude of risk, etc.? I've talked to individuals who set custom GF levels and choose to go with a more conservative (lower) GF hi and a more liberal (higher) GF lo. The most common explanation is that they choose not to do the deeper stops or do not "believe" in them. Does this general principle apply to conservative settings in VPM?

Although the table shows 45/95 closely matches PZ+, in practice that is really not the case, at least with the profiles I tested with, 45/95 would give me about 10 minutes more NDL time than PZ+. As I mentioned earlier, I've found 40/85 to match very closely with my Geo (on my recreational profiles)


Sent from my iPhone using Tapatalk

Same with my simulations on MultiDeco

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

Well, here's where it gets confusing to me, and I would definitely welcome Doppler's sage analysis at this point!

If you make a graph of simply depth vs. time, then a no-stop ascent is just a straight line with a slope proportional to the ascent rate. But the GF line is depth vs. N2 pressure in the leading compartment, so it's not the same as an ascent line. But nevertheless, since there are no stops, it's a straight line. I'm just guessing that those two modifying numbers which generate the dive parameters might both contribute to NDL in some mathematical way that is beyond my current comprehension...

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

 

[CptTightPants21]

Well, here's where it gets confusing to me, and I would definitely welcome Doppler's sage analysis at this point!

If you make a graph of simply depth vs. time, then a no-stop ascent is just a straight line with a slope proportional to the ascent rate. But the GF line is depth vs. N2 pressure in the leading compartment, so it's not the same as an ascent line. But nevertheless, since there are no stops, it's a straight line. I'm just guessing that those two modifying numbers which generate the dive parameters might both contribute to NDL in some mathematical way that is beyond my current comprehension...

I believe you are on the right track and I am no expert on the subject, but the following is my understanding (mostly from reading deco for divers and some internet articles:

The two gradient factors make a new NDL line like your chart shows. That new slope line is in each of the 16 compartments and each compartment has a different half time and a rate on on-gassing/off-gassing. You will have a deco stop if any ONE of the compartments breaches/meets its "new" GF M value line. You would then have to off-gas that compartment enough to continue your ascent. During the time you are waiting to off-gas the one compartment you could be saturated, still on-gassing, or off-gassing different combinations of the various compartments.

You will only have a no stop ascent line if your ascent stays below the GF created line. Using your rec dive no stop example, your NDL for the dive would be the point in time that a 30ft/sec ascent rate straight to the surface would have you end at your high GF.

 

[scubadada]

I believe you are on the right track and I am no expert on the subject, but the following is my understanding (mostly from reading deco for divers and some internet articles:

The two gradient factors make a new NDL line like your chart shows. That new slope line is in each of the 16 compartments and each compartment has a different half time and a rate on on-gassing/off-gassing. You will have a deco stop if any ONE of the compartments breaches/meets its "new" GF M value line. You would then have to off-gas that compartment enough to continue your ascent. During the time you are waiting to off-gas the one compartment you could be saturated, still on-gassing, or off-gassing different combinations of the various compartments.

You will only have a no stop ascent line if your ascent stays below the GF created line. Using your rec dive no stop example, your NDL for the dive would be the point in time that a 30ft/sec ascent rate straight to the surface would have you end at your high GF.

No stop, by definition, is where a normal ascent will allow you to surface directly at, or below, your GF hi. GF lo does not kick in until you enter deco, and determines your deeper stops, to not exceed that fraction of your M-value during your ascent at a normal rate.

 

[CptTightPants21]

I don't think any gradient factor hi or lo needs to be associated with each other. Just like many things some numbers eventually get used in conjunction a lot and become almost "standard". Trimix mixes might be a good example.

The two GF numbers were created for different purposes. GF hi was meant to give a margin of safety for your surfacing nitrogen levels. So a lower GF hi would be more conservative and have doing longer stops to off gas more at depth before going to the surface.

The GF lo number was being used to generate deep stops and mimic the bubble models.

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.