Feedback on recent two-tank and dive limits

[lowwall]

If a recreational diver is doing a "no decompression limit" (NDL) dive, he has the perception/expectation that at any time during the dive he is able to go straight to the surface (for whatever reason). On a NDL dive, his computer at the start of the ascent phase should not be telling him that he has to do multiple stops to the surface.

This is the thing you keep getting wrong. The computer won't be telling a diver to do multiple stops to the suface on an NDL dive.

There's something you aren't getting about either 1 or 2 below.

1) When using Gradient Factors the NDL time is solely determined by the GF High. For example, whether your computer is set to 45/85 or 85/85, the NDL time will be the same on the same dive profile,

2) As long as you do not violate NDL, you can go directly to the surface no matter what the GF Low setting is. For example, whether your computer is set to 45/85 or 85/85, the ascent limitations will be identical on the same dive profile if you leave the bottom before exceeding NDL. GF Low determines the depth of the first stop, but only if you actually need to do a stop,

3) OK, so you violate your NDL, what happens next? Well, what happens is you get to do deco. This is where different GF Low settings make a difference and you may well end up doing an extra couple of stops if your GF Low is set low enough. But either way you will still be stuck doing some deco. If you don't want to do deco, then don't violate NDL.

 

[JMarc]

My position on computers using Bhulmann with GF (air, nitrox and oxygen capable) is as entry level technical diving computers. Yes, they can be used by recreational level divers, but with caution.

And my position (which I think is shared by the other participants) is that your rationale for your recommendation of specific caution for ZHL16 + GF is at odd with my understanding of the state of deco knowledge. The reasons I've to recommend specific caution before using GF just don't apply for NDL diving. The reasons I've to recommend caution for NDL diving when using a computer apply to all computers.

Additionally, I couldn't reproduce the examples of ZHL16 + GF exit profiles you gave. That means either that we have a serious mis-understanding about what you were computing, or that the number you gave weren't intended to be exact. In the later case, you need to know that they were so far from the one I got with planners that they aren't a base for any valid reasoning.

 

[rongoodman]

If a recreational diver is doing a "no decompression limit" (NDL) dive, he has the perception/expectation that at any time during the dive he is able to go straight to the surface (for whatever reason). On a NDL dive, his computer at the start of the ascent phase should not be telling him that he has to do multiple stops to the surface.

Citations, or it didn't happen. If a computer is showing required deco stops, deep or otherwise, it is no longer an NDL dive, by definition. It is possible for two different computers, or the same computer with different conservatism settings, to disagree as to just what constitutes an NDL dive.

 

[David Carron]

If a recreational diver is doing a "no decompression limit" (NDL) dive, he has the perception/expectation that at any time during the dive he is able to go straight to the surface (for whatever reason). On a NDL dive, his computer at the start of the ascent phase should not be telling him that he has to do multiple stops to the surface.

This absolutely cannot happen. It doesn't matter what the gradient factors are set to, an NDL dive tautologically has no stops before the surface.

What can happen is, where the NDL has been exceeded and with a low enough GFlo, stops might be distributed deeper than you might expect.

 

[34109411]

One should always dive to their own DC and remember a divemaster is just a guide. You can ascend to increase NDL anytime you like. I bought my Perdix as it does not have any audio or vibration alarms. I ended up turning off the LOW NDL alert. Apart from re reading the settings also spend some time studying gradient factors, GF99 and Surf GF which are added safety factors for your diving.

If you have less than the 1 hour surface interval that's fine but you will get shorter NDL times. The longer the SI the better. I really find I enjoyed as a recreational diver learning and using the features of my Perdix.
There was a reason I bought it and it was for its large digital display and the fact I can put things where I want and use colours I like.

I am responsible for my own dives not my dive buddy or my guide.

Even if my dive i relatively shallow i always take plenty of time between dive also sometimes i will do 3 dives in a row. During the SI i will eat or sleep like bear under a tree. You are right on with you last affirmation be self reliant.

 

[Blackcrusader]

That said, I just checked and my current settings are as follows:

Conservativism: medium (GF 40/85)
Safety stop: set to “adapt”

So if your alarm went of then it would be due to having a low NDL alert setting on yes. On my Perdix no audio or vibration just a change of colour and alert. I have turned of my low NDL alert as I prefer to see the NDL time.

 

[34109411]

so lizzz don't overthink this and you will be fine. One last point i didn't re-read the entire tread but i suspect also you had a short interval between the two dive at almost the same depth.

I am not gonna re-invent the whell here deeper dive first, shallower dive after. Put plenty of time in between dive or use PLAN on the sherwater to know where you are going at.

Put an alarm for low NDL i put mind at 10 minutes so that's plenty of time to figure out what to do. What else like other suggest leave that GF at MED and you will be good to go. Personally i will always do a 3 minutes safety stop so i mind is not at adapt.

Be safe and go diving !

 

[Belzelbub]

If a recreational diver is doing a "no decompression limit" (NDL) dive, he has the perception/expectation that at any time during the dive he is able to go straight to the surface (for whatever reason). On a NDL dive, his computer at the start of the ascent phase should not be telling him that he has to do multiple stops to the surface.

That is exactly what happens every time I use my Shearwater and Garmin. Both running Buhlmann with GFs, and get this. Both running a custom GF. When I begin my ascent I do not find that there are multiple stops upcoming. I only have the optional safety stop. That's it. Nothing else.

A safety stop is appropriate (and optional) for NDL diving. Unexpected multiple stops to the surface, which to the inexperienced basic scuba diver may be interpreted as mandatory is not appropriate and may cause anxiety which is not good.

A safety stop is optional. That's true with dive computers running Buhlmann with GFs. That's true for pretty much any dive computer that I'm familiar with. My Oceanics running DSAT had an optional safety stop as well.

As long as you stay within the NDL on a computer running Buhlmann with GFs, you don't get surprised with multiple stops on the ascent. Only the optional safety stop will trigger. And it's optional. No penalty for skipping. If you violate NDL, then yes, you will potentially have stops to clear on ascent. But that's true for all dive computers, regardless of algorithm.

Buhlmann with GF is appropriate for decompression dives however when used for recreational NDL dives use with caution.

You keep saying, but you don't provide a reason. At least not a valid reason. The reasons you provided come from a misunderstanding of how GFs work.

Based on what I've been able to discern from your posts, you seem to be clinging to the notion that if GF is set at 40/85 NDL will be determined by that 40 number. Multiple posters have tried to tell you that the 40 doesn't matter in NDL. NDL will ONLY be determined by the 85. Therefore, NDL using 40/85 will actually be longer than if using 70/70.

Guys, this is the Basic Scuba section of Forums. Basic level scuba divers are a long way from doing decompression dives. The philosophies for NDL dives and decompression dives are a universe apart. When you commit yourself to a proper decompression dive you cannot make a quick dash to the surface when things go wrong.

Agreed. This is the Basic Scuba section. I've pointed that out a few times. Most of us are keeping this in the NDL realm. You seem to keep bringing up that an NDL dive turns into a decompression dive without warning when the diver begins the ascent.

If I dive with a Buhlmann computer running GFs set at 15/85, 40/85, or 85/85, they will all be perfectly acceptable for NDL dives. They will actually all have the same NDL time. As long as I don't exceed the NDL, I can make a direct ascent to the surface should that be necessary.

My position on computers using Bhulmann with GF (air, nitrox and oxygen capable) is as entry level technical diving computers. Yes, they can be used by recreational level divers, but with caution.

All computers should be used with caution. I'll ask this question again. You've failed to answer it previously. Why do you feel that more caution is needed with a computer running Buhlmann with GFs instead of one running DSAT or RGBM with selectable conservatism settings?

 

[34109411]

That is exactly what happens every time I use my Shearwater and Garmin. Both running Buhlmann with GFs, and get this. Both running a custom GF. When I begin my ascent I do not find that there are multiple stops upcoming. I only have the optional safety stop. That's it. Nothing else.

A safety stop is optional. That's true with dive computers running Buhlmann with GFs. That's true for pretty much any dive computer that I'm familiar with. My Oceanics running DSAT had an optional safety stop as well.

As long as you stay within the NDL on a computer running Buhlmann with GFs, you don't get surprised with multiple stops on the ascent. Only the optional safety stop will trigger. And it's optional. No penalty for skipping. If you violate NDL, then yes, you will potentially have stops to clear on ascent. But that's true for all dive computers, regardless of algorithm.


You keep saying, but you don't provide a reason. At least not a valid reason. The reasons you provided come from a misunderstanding of how GFs work.

Based on what I've been able to discern from your posts, you seem to be clinging to the notion that if GF is set at 40/85 NDL will be determined by that 45 number. Multiple posters have tried to tell you that the 40 doesn't matter in NDL. NDL will ONLY be determined by the 85. Therefore, NDL using 40/85 will actually be longer than if using 70/70.


Agreed. This is the Basic Scuba section. I've pointed that out a few times. Most of us are keeping this in the NDL realm. You seem to keep bringing up that an NDL dive turns into a decompression dive without warning when the diver begins the ascent.

If I dive with a Buhlmann computer running GFs set at 15/85, 40/85, or 85/85, they will all be perfectly acceptable for NDL dives. They will actually all have the same NDL time. As long as I don't exceed the NDL, I can make a direct ascent to the surface should that be necessary.

All computers should be used with caution. I'll ask this question again. You've failed to answer it previously. Why do you feel that more caution is needed with a computer running Buhlmann with GFs instead of one running DSAT or RGBM with selectable conservatism settings?

oh my mares M1 was running RGMB this is only things i understand in the dog fight between you two

 

[Belzelbub]

OP here. Honestly, I’ve done 50+ dives with this computer, including 100’+ ones, and have never had it buzzing alarms at me like on the dive in my first post and that’s a big part of my I was concerned.

It's a great computer. Don't let someone that doesn't understand how it works scare you away. It's perfectly fine for recreational diving and even a bit beyond should that be in the plan.

I've got over 100 dives on my Shearwater. I have had the 5 minute safety stop trigger, and low NDL alerts happen from time to time, but not once have I been surprised by an unexpected deco stop.

I’ll take the initiative to re-read the settings, since I’ve had it a couple years, but will likely just defer to whatever it tells me next time - even if the divemaster says things are fine, if only for my peace of mind.

Good idea. It should always be you deciding how the dive should go. Divemaster may be on a different computer (set more liberal) or may be on Nitrox while you may be diving air. Have your computer set for what you are doing. If you find NDL getting low, you can ascend a bit, but still keeping with the group to extend it a bit. If it gets too low, definitely signal the DM and let them know.