Feedback on recent two-tank and dive limits

[Blackcrusader]

since when we arrived they were going to take some discover scuba folks on a 100’ wreck dive (which I learned later).

Well that is a violation of the depth limits for a DSD for sure.

 

[Mobulai]

It's relevant when people exceed NDL as some do unknowingly. They dive and see something interesting and never check their depth or NDL.

If someone exceeds NDL unknowingly they shouldn’t be far off.. and GFLo still isn’t their problem at that point; it’s having enough gas to get out of Deco (and their best bet is riding the ceiling == GFLO=GFHi)
Edit: ok saying that in my head made me realize it would be beneficial to be able to modify GFLo at that point to GFHi

 

[jborg]

The issue is you have a dive computer and that you do not understand the settings or functions.
[...]

It really is imperative you learn about your DC.

I mean, honestly, not really? We're all dive nerds here and like to get into the details of optimal GFs and riding the SurfGF and whatnot, obviously. I implemented Buhlman myself for a toy dive computer that only showed SurfGF as an experiment... Nonetheless I think most recreational divers will be perfectly fine by leaving their computer at the default settings and knowing enough to follow what it says, which is what happened here...

I had a woman telling her guide I must have been going into deco as she did not even know her own Shearwater settings. Not only that her Shearwater was set on fresh water and she was salt water diving.

The salinity setting has precisely zero effect on NDL or deco.

Edit: I say this not because it isn't great to know all about ones equipment, but I think the group veers a little too much towards berating divers for not knowing every detail of their computer's implementation of whatever deco algorithm, and ideally the precise functioning of the deco algorithm in question... when I honestly think they shouldn't have to understand that. Understand the principle, and understand enough to follow the implementation.

 

[34109411]

The issue is you have a dive computer and that you do not understand the settings or functions.
Gradient Factors are not taught in OW or AOW classes. There are threads here on gradient factors.
Also you should upload your dive logs and study them so you learn from them. So yes after I bought my Pedix in 2018 I downloaded and read the user manual many times, updated my firmware and then spent many an hour learning what my Shearwater functions are. I've done over 1000 dives on it now and I look at my dive logs from time to time. Great piece of kit but you need to understand it.

Plenty of threads on this forum on how Gradient Factors and Shearwater settings work posted. I had a woman telling her guide I must have been going into deco as she did not even know her own Shearwater settings. Not only that her Shearwater was set on fresh water and she was salt water diving. I asked her if she knew what settings she was using and she had no clue. Just strapped it on and never looked at the settings.

It really is imperative you learn about your DC.

Also a guide should never be your decision maker for a dive you have to be independent of the guide.I often dive with guides who use Suunto devices and they have far less NDL on repetative dives than I do.
So they need to ascend to avoid deco as I can stay deeper for longer with plenty of NDL.

Even if you show NDL of 1 minute you are not in deco. Even at zero you are not in deco. Also learn about the adaptive stop and also your last stop depth settings. I set my last stop at 3m.

i will add that most people buy computer that doesn't suit there needs and don't event needs the 3/4 information it display hence they don't even have the training to understand what the computer display.

For a recreational diver that dive from time to time a suunto zoop should answer the general joe out there.

Be safe

 

[scubadada]

Hi @lizzzzz

You have received some good advice and a nice reading list. Briefly:

• Your medium GFs of 40/85 are reasonably conservative
• Though I may have missed it, I don't think you said what gas you were using. You and your son should consider nitrox certification if you don't already have it. Nitrox will give you longer NDLs
• The adaptive safety stop adds 2 min to your stop and further lowers your surfacing GF. You should learn to use the SurfGF on your computer. It will give you better control of where you will stand at the end of the dive
• Your surfacing GFs (GF99 on your graphs) were both in the 50s, quite conservative
• Learn to use the computer NDL planner so that you will know what to expect for your dive time, either on your first or on a repetitive dive.
• The Peregrine home screen is nicely customizable to contain just the information you need or want

You will have a much better idea about what I wrote after you read your computer's owner manual and do some of the reading you were given.

Best of luck for future diving with your son

 

[34109411]

Hi @lizzzzz

You have received some good advice and a nice reading list. Briefly:

• Your medium GFs of 40/85 are reasonably conservative
• Though I may have missed it, I don't think you said what gas you were using. You and your son should consider nitrox certification if you don't already have it. Nitrox will give you longer NDLs
• The adaptive safety stop adds 2 min to your stop and further lowers your surfacing GF. You should learn to use the SurfGF on your computer. It will give you better control of where you will stand at the end of the dive
• Your surfacing GFs (GF99 on your graphs) were both in the 50s, quite conservative
• Learn to use the computer NDL planner so that you will know what to expect for your dive time, either on your first or on a repetitive dive.

You will have a much better idea about what I wrote after you read your computer's owner manual and do some of the reading you were given.

Best of luck for future diving with your son

Nice answer!

 

[old frogman]

Nice answer!

Good advice. However, let us explore those supposedly recreational GFs for the various ascent profiles (approximate) for a recreational dive to 30 metres (100 ft) for 20 mins.
30/70 - 15m (1min), 12m (1min), 9m (2min), 6m (3min) and finally 3m (5min).
40/85 - 12m (1min), 9m (2min), 6m (2min) and finally 3m (3-4min).
50/85 - 9m (1min), 6m (2min), and finally 3m (3-4min).

70/90 - 6m (1min), 3m (3-4min). The only realistic ascent GF.

Compare with a more realistic ascent profile following a 100ft 20min dive:
PADI - safety stop at for 3 min at 5m
Buhlmann - deco of 4 min at 3m

Although GFs are based on the adjustment of the M value of Buhlmann table in practice, they are mimicking the Variable Permeability Model (VPM). If you use VPM for recreational diving, you will get the similar ascent profiles which are ridiculously long.

Buhlmann combined with GF - Haldane (dissolved gas model)
VPM - Bubble Model (free gas model)

 

[Belzelbub]

Although GFs are based on the adjustment of the M value of Buhlmann table in practice, they are mimicking the Variable Permeability Model (VPM). If you use VPM for recreational diving, you will get the similar ascent profiles which are ridiculously long.

In reality, though, recreational diving ascent profiles for a DC running Buhlmann with GFs does not result in ridiculously long ascents. We are in Basic, so not talking about dives where the NDL is exceeded. At most, you have a 5 minute optional safety stop if you are using adaptive stops, and the dive triggered. If the adaptive stops are turned off, then it's the standard 3 minute optional stop.

If Buhlmann with GFs was so bad, then why are so many DC manufacturers offering it instead of their proprietary models?

Back to the OP. You've got a good computer. It will do a great job of tracking theoretical tissue loadings for a variety of dives. Key is to understand what it's telling you. I would also recommend looking into adding SurfGF to your main screen. It's a good indicator of relative safety of the dive.

 

[JMarc]

If Buhlmann with GFs was so bad, then why are so many DC manufacturers offering it instead of their proprietary models?

GF allows to tweak the algorithm. Indiscriminate choice of GFs can be unwise. Roughly GFhigh gives the total time of stops, GFlow gives the depth distribution of the stops (note that GFlow has no effect if you have no mandatory stops, so for NDL it is irrelevant) and having your stops too deep is known to increase the risk of DCS, so that's not a "lower is more conservative" situation. See for instance this article by Dr Doolette from which I extract this conclusion

I choose my GF low to be about 83% of the GF high, for instance GF 70/85. Although the algebra is not exact, this roughly counteracts the slope of the “b” values. This approach allows me to believe I have chosen my GF rationally, is not so large a GF low as I am unable to convince my buddies to use it, and satisfies my preference to follow a relatively shallow stops schedule.

Note that one aspect for fixing that rule of thumb is convincing his buddies who are used to lower GFlow and he seems to be more adverse to lowering GFlow than to increasing it. @old frogman 's 30/70, 40/85 and 50/85 have a GFlow quite lower than Dr Doolette rule of thumb and I'd not use them with air or nitrox (I'm not knowledgeable enough to write about the choice with Trimix, but that's even further outside of the scope of Basic Scuba than this discussion on the choice of GFlow with air/nitrox which is relevant only when you have mandatory deco stops).

 

[Belzelbub]

GF allows to tweak the algorithm. Indiscriminate choice of GFs can be unwise.

No argument there. If you don't know what those numbers mean, best to keep it on one of the presets. Most likely the default. Of which a lot seem to use very similar presets, but some manufacturers do have odd choices.

Roughly GFhigh gives the total time of stops, GFlow gives the depth distribution of the stops (note that GFlow has no effect if you have no mandatory stops, so for NDL it is irrelevant) and having your stops too deep is known to increase the risk of DCS, so that's not a "lower is more conservative" situation. See for instance this article by Dr Doolette from which I extract this conclusion

Again, agreed. I read that article shortly after getting my Shearwater. Used that information to settle on a custom GF. Mostly, it's irrelevant for me as most of my dives are within NDL. It's for that handful that might be on the line. On my previous DC, I turned off the Deep Stops option based on the newer thinking, and this was inline with that.

Note that one aspect for fixing that rule of thumb is convincing his buddies who are used to lower GFlow and he seems to be more adverse to lowering GFlow than to increasing it. @old frogman 's 30/70, 40/85 and 50/85 have a GFlow quite lower than Dr Doolette rule of thumb and I'd not use them with air or nitrox

Yeah, I'm not quite sure where some of those numbers came from. For reference, below are the defaults used by Shearwater and Garmin. I believe some others use similar, but not all. Ratio has some different defaults, IIRC, including some with GFLo=GFHi, which would be fine for recreational as the GFLo wouldn't matter.
High: 35/75
Med: 40/85
Low: 45/95

Even those are well off the 83% that Dr. Doolette chose. More in the 46-48% range.

Main point was that a DC running Buhlmann with GFs does not automatically mean it's overly complex. Most computers offer some level of conservatism adjustment. If you don't go the custom route, a lot seem to offer 3 settings. Which, would be just fine for most recreational divers. They may not understand what those numbers mean, but High, Med, Low conservatism is pretty easy to understand.