Feedback on recent two-tank and dive limits

[David Carron]

Told him several pages back I only use Tec Mode. Wrote that on page 2 of this thread.
I've never used REC mode on my Perdix.

Did I reply to you somewhere?

 

[JMarc]

I've never dove with it set-up in anything other than Tec mode because information is a good thing.

Information overload is a thing.

I also always dive with the Perdix in tech layout, because switching would be worse. But for someone who only does rec dives, the rec layout is more convenient. And for someone with a Peregrine, the one layout available.

 

[Nick_Radov]

My point is, if the diver had been using a computer by OCEANIC with DSAT or straight Buhlmann (dual algorithms). When simply set on DSAT it would have given him a longer NDL bottom time for the first dive and automatically provided a predictable 3min safety stop at 15 ft. The NDL bottom time for the second dive would probably be shorter and again automatically provided a 3 min safety stop at 15 ft. It works for NDL recreational diving why convolute it with GFs.

Oceanic dual algorithm dive computers do not run "straight Buhlmann". They run "Pelagic Z+" which is an undocumented proprietary algorithm based on Bühlmann ZHL-16C. It appears they have made their modifications by reducing the gradient factors and (maybe) artificially increasing the altitude above sea level but hide the specific numbers from users. Personally I don't think it's too smart to trust a manufacturer who keeps this stuff secret and can't point to any reliable scientific data to support their proprietary algorithm, but whatever.

Pretty much all modern dive computers can be configured to show a predictable 3 minute safety stop at 15 ft for all dives within the NDL if that's what you want. This is implemented as an override on top of the underlying algorithm. In other words if you want a particular predictable safety stop then whether the computer is running ZHL-16C or DSAT or RGBM or whatever makes zero difference. If the device has a confusing user interface which fails to clearly differentiate between a recommended safety stop versus a required deco stop then that's an entirely different problem, and also unrelated to the specific deco algorithm. At some point users are going to have to RTFM.

I don't think there is any reliable scientific evidence showing a significant difference in DCS incidence (or other outcomes) between DSAT versus Bühlmann ZHL-16C with any particular gradient factors (including straight 100/100). Oceanic's implementation of DSAT also incorporates some undocumented proprietary adjustments which differ from the PADI RDP so it's a mystery what's going on inside their black box. Oceanic claims that their implementation of DSAT will give longer NDLs than Z+ so I would assume that DSAT is slightly more risky, at least for single dives, but who knows?

If you want to propose a change to recreational diving practices and equipment then at least get your facts straight instead of wasting everyone's time and confusing the new divers. Frankly I think you're unclear on the basics (even at the recreational NDL level) and don't understand this stuff nearly as well as you believe you do. Let me leave you with this quote from Jim Barksdale: “If we have data, let’s look at data. If all we have are opinions, let’s go with mine.”

 

[scubadada]

As per the post by @Nick_Radov, I have extensive experience diving DSAT and Buhlmann ZH-L16C with GF for no stop and light deco dives. I have never dived PZ+. I have previously posted on some of the differences between DSAT and Buhlmann with a GF high of 95, including some of the more dramatic differences I have seen

Thread 'DSAT vs GF high 95, examples'

Sep 28, 2021

I have written previously about my experience diving DSAT and Buhlmann ZH-L16C with a GF high of 95. I have an example of what I have previously reported. This dive is a first dive of the day on the Castor in Boynton Beach. It was relatively deep with a max depth of 107 feet and an avg depth of 73 feet. DSAT gave me a minimum NDL of 6 minutes. A GF high of 95 on my Teric gave me 1 min of deco at 10 feet. My surfacing GF was 79.

This is a dive later in the week. It is the 4th dive of the day, following 3 moderately deep and long dives. This was a...

• scubadada

 

[old frogman]

Oceanic dual algorithm dive computers do not run "straight Buhlmann". They run "Pelagic Z+" which is an undocumented proprietary algorithm based on Bühlmann ZHL-16C. It appears they have made their modifications by reducing the gradient factors and (maybe) artificially increasing the altitude above sea level but hide the specific numbers from users. Personally I don't think it's too smart to trust a manufacturer who keeps this stuff secret and can't point to any reliable scientific data to support their proprietary algorithm, but whatever.

Pretty much all modern dive computers can be configured to show a predictable 3 minute safety stop at 15 ft for all dives within the NDL if that's what you want. This is implemented as an override on top of the underlying algorithm. In other words if you want a particular predictable safety stop then whether the computer is running ZHL-16C or DSAT or RGBM or whatever makes zero difference. If the device has a confusing user interface which fails to clearly differentiate between a recommended safety stop versus a required deco stop then that's an entirely different problem, and also unrelated to the specific deco algorithm. At some point users are going to have to RTFM.

I don't think there is any reliable scientific evidence showing a significant difference in DCS incidence (or other outcomes) between DSAT versus Bühlmann ZHL-16C with any particular gradient factors (including straight 100/100). Oceanic's implementation of DSAT also incorporates some undocumented proprietary adjustments which differ from the PADI RDP so it's a mystery what's going on inside their black box. Oceanic claims that their implementation of DSAT will give longer NDLs than Z+ so I would assume that DSAT is slightly more risky, at least for single dives, but who knows?

If you want to propose a change to recreational diving practices and equipment then at least get your facts straight instead of wasting everyone's time and confusing the new divers. Frankly I think you're unclear on the basics (even at the recreational NDL level) and don't understand this stuff nearly as well as you believe you do. Let me leave you with this quote from Jim Barksdale: “If we have data, let’s look at data. If all we have are opinions, let’s go with mine.”

Mate, I accept the fact that you like scientific evidence, good on you.

My issue is not with the computer, the algorithm or the GFs. My issue is with the acclimatization of a new diver at basic level scuba diving with transition to dive computers. A dive computer with a simpler algorithm interface is going to be easier for the diver, in particular if the diver is just interested in NDL recreational diving. I feel the DSAT algorithm with safety stops is a better choice.

Sure, if the new diver from the start of has aspirations to go technical and has a mindset like yours, a computer with Buhlmann with GF may be a better choice. This diver may even want to do your GUE fundies course, why not. Super cool. However, this diver is probably in the -5% of new basic level divers.

This is the basic level section of the forum and most probably applies to the 95+% of divers who just want to do relaxed NDL recreational dive. Maybe look at marine life, maybe take a few pics, maybe bag a lobster and have a few proper beers and a nibble of sushi after the final dive of the day.

 

[inquis]

A dive computer with a simpler algorithm interface is going to be easier for the diver, in particular if the diver is just interested in NDL recreational diving. I feel the DSAT algorithm with safety stops is a better choice.

I don't feel choosing High, Medium, or Low conservatism is terribly confusing (Shearwater's approach). It's pretty easy to do nothing (speculatively, as is the case with the majority of users), and there is a sensible default (Medium = GF x/85).

When you say you feel DSAT is "a better choice", is that because you feel it's safer than the GF default setting (it's not for Shearwater computers, BTW) , because you feel the longer NDL times are worth the (slightly) increased risk, because the user is not exposed to any setting to increase conservatism (which is frequently also not true), or some other reason? FWIW, conservatism is typically added to DSAT-based computers by pretending the dive is at a higher altitude.

 

[old frogman]

Sounds a lot like your concern is not with the algorithm but with its user interface as presented by specific manufacturers.

That is a very astute observation. Very clever chap.

 

[Blackcrusader]

Information overload is a thing. I also always dive with the Perdix in tech layout, because switching would be worse. But for someone who only does rec dives, the rec layout is more convenient.

Well that's just your opinion. As mentioned all users need to do is RTFM.
I am a recreational diver and I only do recreational dives and I prefer the tec mode as I can choose the layout I want.

The user interface layout on my Perdix is very convenient. Air mode is very simple as well but I want the extra information at hand.

 

[Blackcrusader]

Did I reply to you somewhere?

Yes lol

 

[Nick_Radov]

My issue is not with the computer, the algorithm or the GFs. My issue is with the acclimatization of a new diver at basic level scuba diving with transition to dive computers. A dive computer with a simpler algorithm interface is going to be easier for the diver, in particular if the diver is just interested in NDL recreational diving. I feel the DSAT algorithm with safety stops is a better choice.

Aha, now I see the source of your confusion. The usability or ease of use of a particular dive computer has almost nothing to do with what deco algorithm it runs whether that's DSAT, Bühlmann, RGBM, or something else. Those are orthogonal issues. Safety stops can be automatically overlaid on any deco model so DSAT has zero advantage (or disadvantage) on that point. Once you understand how this stuff is actually implemented in code it will make a lot more sense.