Help with Buhlmann ZHL-16c GF

[doctormike]

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?

They are confusing two similar terms that have different meanings.

"Deep Stops" in the context of "not believing in them" usually refers to "Pyle Stops". This was the theory promoted by a marine biologist in the 1990s (Richard Pyle) who found that if he added a stop halfway to the surface on deep dives, he felt better. This was in addition to any calculated profile. Many dive computers added this option, even though there is no decompression science behind it - it was just an empirical observation. To the contrary, one of the criticisms of this approach is that you may actually be ongassing your slower compartments that aren't already saturated, potentially making your decompression stress greater! There are smart people arguing on both sides of this, and it's above my pay grade to take a position from a scientific point of view, but that is what the term "deep stops" often refers to.

On the other hand, every generated deco profile (bubble model or modified Buhlmann) has a number of stops at different depths. So you may prefer to generate a profile that has you outgassing more at shallower depths for a variety of reasons (choice of deco gas, site conditions, etc..), but your deepest stop is still your deepest stop, and you have to do it if you are going to follow any sort of calculated profile.

So preferring profiles with less deco at depth and more in the shallows isn't the same thing as not believing in deep stops.

 

[tursiops]

Hi turisops,

I got the NOAA NDLs from tables off the internet: http://www.ndc.noaa.gov/forms/New_NOAA_Air_Deco_Tables_2008.pdf maybe yours are older?

Ahh, yes, mine are from the NOAA Diving Manual. The tables now follow the latest Navy tables!

 

[uncfnp]

If you haven't seen these yet you might find them interesting. The first is a Shearwater video discussing their deco model and tissue loading bar graft

https://m.youtube.com/watch?v=O81vX79X_mA

And this is some recommended reading from the Shearwater site on deep stops.

https://d2uy6nbe9j4vef.cloudfront.net/wp-content/uploads/2012/08/Deep-Stops.pdf

 

[scubadada]

Is your Multideco an iOS app?

I'm running it on a PC but runs on MAC, Android, iPhone, some others. Apparently available in iPhone App store MultiDeco dive decompression software - VPM-B and ZHL deco

---------- Post added September 30th, 2015 at 09:38 AM ----------

They are confusing two similar terms that have different meanings.

"Deep Stops" in the context of "not believing in them" usually refers to "Pyle Stops". This was the theory promoted by a marine biologist in the 1990s (Richard Pyle) who found that if he added a stop halfway to the surface on deep dives, he felt better. This was in addition to any calculated profile. Many dive computers added this option, even though there is no decompression science behind it - it was just an empirical observation. To the contrary, one of the criticisms of this approach is that you may actually be ongassing your slower compartments that aren't already saturated, potentially making your decompression stress greater! There are smart people arguing on both sides of this, and it's above my pay grade to take a position from a scientific point of view, but that is what the term "deep stops" often refers to.

On the other hand, every generated deco profile (bubble model or modified Buhlmann) has a number of stops at different depths. So you may prefer to generate a profile that has you outgassing more at shallower depths for a variety of reasons (choice of deco gas, site conditions, etc..), but your deepest stop is still your deepest stop, and you have to do it if you are going to follow any sort of calculated profile.

So preferring profiles with less deco at depth and more in the shallows isn't the same thing as not believing in deep stops.

I wasn't clear, I was referring to the former, deeper stops dictated by the decompression algorithm, not stops deeper than designated by the algorithm (like 2 min at half your maximum depth for a recreational dive)

 

[Lorenzoid]

How so?

Shearwater has simplified it by giving you three levels of conservativeness. They've then set the Lo and Hi GF numbers to translate into low/mid/high levels of conservativeness. It appears to me that Shearwater has done exactly what they should have done. I see no red herring or anything misleading.

If a diver goes into deco, accidentally or on purpose, the GF settings will handle that to the desired level of conservativeness. . . .

Well, yes to all that. But your comment doesn't address my question of how the GF-Low number relates to conservativeness in a no-stop or "recreational" dive.

I've spent a lot of time thinking about this, and it still isn't clear to me why one would choose different GF-Low values for different levels of conservatism in a no-stop dive. If the GF-Low value is totally irrelevant then I think "red herring" is a fair characterization of the choice of that value. It's clear to me how the GF-High value affects NDL conservatism.

So again, for example, Shearwater chose 45/95 for Low conservatism, 40/85 for Medium conservatism, and 35/75 for High conservatism in Recreational mode. The only theory I can come up with is that perhaps the inverse relationship for choosing GF-Low in relation to conservatism is that, the more conservative the setting, the faster it gets you off the bottom IF you happen to violate a NDL. In other words, maybe the thinking is that a diver who feels more conservative believes that getting closer to the surface sooner and doing the stop(s) closer to the surface is a more conservative option than doing the stop(s) deeper, since as a recreational diver the surface is your lifeline. That seems reasonable. But one could also argue that whether doing stops deeper or doing them shallower corresponds to a person being more conservative or less conservative gets into the thorny issues surrounding the merits of deep stops ("deep" as in just deep-er, not "Pyle stops").

 

[uncfnp]

Someone with a Petrel could easily fill in the table I posted earlier.

Feet High. Med. Low

50 49 62 74
60 30 39 49
70 21 27 34
80 15 20 24
90 11 14 19
100 9 11 14
110 7 9 11
120 6 7 9
130 4 6 7

Sorry. My formatting is completely screwed when posted. Will try and get a clearer posting.

 

[doctormike]

I wasn't clear, I was referring to the former, deeper stops dictated by the decompression algorithm, not stops deeper than designated by the algorithm

Right, I just think that sometimes when people say that they don't believe in deep stops, that's what they are talking about (Pyle stops).

 

[Divin'Papaw]

Well, yes to all that. But your comment doesn't address my question of how the GF-Low number relates to conservativeness in a no-stop or "recreational" dive.

I've spent a lot of time thinking about this, and it still isn't clear to me why one would choose different GF-Low values for different levels of conservatism in a no-stop dive. If the GF-Low value is totally irrelevant then I think "red herring" is a fair characterization of the choice of that value. It's clear to me how the GF-High value affects NDL conservatism.

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.

 

[scubadada]

Feet High. Med. Low

50 49 62 74
60 30 39 49
70 21 27 34
80 15 20 24
90 11 14 19
100 9 11 14
110 7 9 11
120 6 7 9
130 4 6 7

Sorry. My formatting is completely screwed when posted. Will try and get a clearer posting.

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

 

[Lorenzoid]

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.

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.