Feedback on recent two-tank and dive limits

[old frogman]

My takeaway from this discussion is as follows:
1. If I have just completed an open water or advanced open water dive course and have no intention of advancing into technical diving, I would stay well away from any computer using GFs. I would stick to recreational level dive computers with Bhulmann and/or DSAT algorithms.
2. If I have completed an advanced open water dive course and are looking at the possibility of advancing to advanced nitrox and decompression procedure course, a dive computer such which incorporates GFs would be a serious contender.

However, when using the dive computer (with GF) for recreational level NDL dives, I would avoid GFs lower than 70/70. My view is that GFs such as 45/95, 40/85 and 35/75 are designed more for planned decompression dives. If you use them for NDL recreational dives you may find yourself being told by the computer to do unplanned decompression, and there lies the risk.

As a further risk mitigation strategy, I would check at the dive plan screen on the computer before committing to the dive. This reduces the risk for square dives but for multilevel dives the risk may still be unknown.

 

[inquis]

when using the dive computer (with GF) for recreational level NDL dives, I would avoid GFs lower than 70/70. My view is that GFs such as 45/95, 40/85 and 35/75 are designed more for planned decompression dives

I can't tell what you're trying to say. Your first sentence says you would use a GFhigh above 70 for recreational diving. Then you state GFhighs of 75, 85, and 95 -- all above 70 -- are more designed for planned deco dives, which is NOT recreational diving by most people's definition. (Yes, the BSAC folks blur that line, but let's go with the majority view world-wide.)

In fact, DSAT was designed for repetitive NDL dives, and is fairly comparable to a GFhigh of 95 (and perhaps 90ish on later dives).

 

[inquis]

If you use them for NDL recreational dives you may find yourself being told by the computer to do unplanned decompression, and there lies the risk

In my view, that's a lack of awareness. If it tells you to do unplanned mandatory decompression, you screwed up long before that point.

 

[old frogman]

I can't tell what you're trying to say. Your first sentence says you would use a GFhigh above 70 for recreational diving. Then you state GFhighs of 75, 85, and 95 -- all above 70 -- are more designed for planned deco dives, which is NOT recreational diving by most people's definition. (Yes, the BSAC folks blur that line, but let's go with the majority view world-wide.)

In fact, DSAT was designed for repetitive NDL dives, and is fairly comparable to a GFhigh of 95 (and perhaps 90ish on later dives).

It appears that you have misunderstood what I said. I understand, you may lack nuance of the King's English. Please let me rephrase it simpler for you.

Recreational NDL diving - anywhere between 100/100 to 70/70.
Planned decompression diving - below 70/70, for example, 45/90, 40/85, 35/75. These GFs will give you deeper stops and over all longer ascent profiles.

 

[JMarc]

1. If I have just completed an open water or advanced open water dive course and have no intention of advancing into technical diving, I would stay well away from any computer using GFs. I would stick to recreational level dive computers with Bhulmann and/or DSAT algorithms.

I'm not aware of any computer proposing Buehlmann ZHL-16 outside GF. Scubapro proposes a series of adaptive algorithms named ZHL-8 something, the initial design was done by Pr Buehlmann after ZHL-16 was completed but the details are not published and Scubapro seems to continue to make the method evolve.1

RGBM seems on the EOL curve since Wienke's death, at least recent models of Suunto and Mares are using ZHL-16 + GF (and in the case of Mares some optional additional automatic adjustment, I don't know if and how Suunto has their own tweaks).

However, when using the dive computer (with GF) for recreational level NDL dives, I would avoid GFs lower than 70/70. My view is that GFs such as 45/95, 40/85 and 35/75 are designed more for planned decompression dives.

When ordering GF pairs, the usage I'm familiar with is to use GFhigh has the most significant member of the pair, thus 70/70 is lower than 45/90. And that ordering make sense as GFhigh is the main driver of the total stop time.

And again about the value of GHlow, 45/95, 40/85 and 35/75 seems to date from the time when aggressively deep stops were though to be useful, something which is not the current consensus for air and nitrox.

Planned decompression diving - below 70/70, for example, 45/90, 40/85, 35/75. These GFs will give you deeper stops and over all longer ascent profiles.

I've no planner available here to double check, but I'm pretty sure that 45/90 will give you less total stop time than 70/70.

 

[old frogman]

I'm not aware of any computer proposing Buehlmann ZHL-16 outside GF. Scubapro proposes a series of adaptive algorithms named ZHL-8 something, the initial design was done by Pr Buehlmann after ZHL-16 was completed but the details are not published and Scubapro seems to continue to make the method evolve.1

RGBM seems on the EOL curve since Wienke's death, at least recent models of Suunto and Mares are using ZHL-16 + GF (and in the case of Mares some optional additional automatic adjustment, I don't know if and how Suunto has their own tweaks).



When ordering GF pairs, the usage I'm familiar with is to use GFhigh has the most significant member of the pair, thus 70/70 is lower than 45/90. And that ordering make sense as GFhigh is the main driver of the total stop time.

And again about the value of GHlow, 45/95, 40/85 and 35/75 seems to date from the time when aggressively deep stops were though to be useful, something which is not the current consensus for air and nitrox.



I've no planner available here to double check, but I'm pretty sure that 45/90 will give you less total stop time than 70/70.

For a 20 min dive to 30 metres (100 feet). Those GFs change the concept of a NDL dive from a short safety stop in the shallows like about 3 or 6 metres for about 3 mins which is optional, to three or more decompression stops that can range from 3 metres to 15 metres. If you like to hang for 9 min + 3 min (ascent time), total time to surface 12 min, be my guest.

 

[Belzelbub]

My takeaway from this discussion is as follows:
1. If I have just completed an open water or advanced open water dive course and have no intention of advancing into technical diving, I would stay well away from any computer using GFs. I would stick to recreational level dive computers with Bhulmann and/or DSAT algorithms.

You've said that a few times, but you haven't really explained why. This sounds like a lack of understanding of what GFs actually are. To equate it to DSAT, RGBM or others, it is a conservatism factor.

Back to Buhlmann. Can you point me to a dive computer running Buhlmann that is not using GFs? That's pretty much how Buhlmann is modified to add conservatism. So, if you have a computer running Buhlmann that is not using GFs, then it is just running to full M value. Or, in GF terms 100/100. I would really not recommend that for a new diver.

OK. Let's now talk about recreational level dive computers running Buhlmann. I use a Perdix, which is definitely tech capable. I also have a Garmin Descent MK2s. Also tech capable. My daughters have Peregrine TXs. Apart from Trimix and CCR modes, they have just as much adjustability as my Perdix and Garmin. Garmin also has a more rec oriented DC in the Fenix 8. It's running Buhlmann. Guess what, it also uses GFs to add conservatism (not sure if this one allows custom GFs, or just the defaults.)

2. If I have completed an advanced open water dive course and are looking at the possibility of advancing to advanced nitrox and decompression procedure course, a dive computer such which incorporates GFs would be a serious contender.

Why? Yes, it's true that heading toward the tech route often requires the use of a Buhlmann computer, thus incorporating GFs. I'm just trying to understand your thought processes here.

However, when using the dive computer (with GF) for recreational level NDL dives, I would avoid GFs lower than 70/70. My view is that GFs such as 45/95, 40/85 and 35/75 are designed more for planned decompression dives. If you use them for NDL recreational dives you may find yourself being told by the computer to do unplanned decompression, and there lies the risk.

I don't think you are saying here what I think you are saying. Trying to return this to Basic scuba. Why would running any of those conservatism levels force any diver to do unplanned decompression? If I were doing a rec dive and had my computer set to 15/30, this would not ever force me to do unplanned decompression. Why? Because I've consulted my computer at various points during the dive and watched what my NDL was. I simply ascend before that NDL reaches zero and I have no unplanned decompression stops. Only stop I would have is a safety stop, and every modern computer that I'm aware of, will have a safety stop timer built in.

As a further risk mitigation strategy, I would check at the dive plan screen on the computer before committing to the dive. This reduces the risk for square dives but for multilevel dives the risk may still be unknown.

Agreed here. I'm not a fan of blindly following the DM on a dive. Each diver should check to make sure they are able to make that next dive. However, most dives are not square, so it should be expected that actual dive time will be a bit more. I likely wouldn't splash if my DC was showing 5 minutes at that depth, but if it's showing 20 or so, there is reason to believe I could get a longer dive by just staying off the bottom.

You can safely use a tech computer for rec dives. The inverse is not necessarily true. Just because my computer allows me to customize GFs doesn't mean I need to use it. Until I learned a bit more about GFs and what they mean, I just kept mine in the default conservatism. Whatever you want to call it, it was still using GFs to add conservatism to the raw Buhlmann algorithm.

 

[Mobulai]

Back to Buhlmann. Can you point me to a dive computer running Buhlmann that is not using GFs? That's pretty much how Buhlmann is modified to add conservatism. So, if you have a computer running Buhlmann that is not using GFs, then it is just running to full M value. Or, in GF terms 100/100. I would really not recommend that for a new diver

Scubapro/Uwatec Computers run a ZHL-B ADT where you have control over MB (micro bubbles) conservatism: L0-L9 (in most of them, new Luna has 0-5)
In effect and from reverse engineering (it’s a proprietary algo) they equate to GF levels where GFHi=GFLo, hence only modification of M value happens; which is all an NDL diver needs and can actually have control on (since GFLo has no impact on NDL calculations)
Ref: https://www.divetable.info/skripte/G2_GF.pdf
The only exception in SP computers that uses a “stock” ZHL-C is the G2HUD, and it was done as a custom job and not developed internally by them

I wouldn’t say that’s (the ADT) a better algorithm than a ZHLC as it has modifiers (heart rate sensor for example) that aren’t fully understood by users (also since I have one and it isn’t easy to match to team when planning deco due to the low controls over GF; but again this is basic scuba)
I prefer the transparent ZHLC that’s on shearwater, ratio, and garmin; I use SW for CC but agin — basic scuba; deco and technical diving is out of scope)

 

[Belzelbub]

For a 20 min dive to 30 metres (100 feet). Those GFs change the concept of a NDL dive from a short safety stop in the shallows like about 3 or 6 metres for about 3 mins which is optional, to three or more decompression stops that can range from 3 metres to 15 metres. If you like to hang for 9 min + 3 min (ascent time), total time to surface 12 min, be my guest.

No they don't. For a recreational dive, adjusting GFHi (only number relevant for NDL dives) lower shortens your NDL time. If your computer allows for 15 minutes at that depth, but you do 20 minutes anyway, then you screwed up. If you are a recreational diver only trained to NDL dives, then you shouldn't be planning to do any deco dives. If your computer says NDL is approaching 0, it's time to head up. If you do that, then the only stop is the safety stop.

 

[boulderjohn]

For a 20 min dive to 30 metres (100 feet). Those GFs change the concept of a NDL dive from a short safety stop in the shallows like about 3 or 6 metres for about 3 mins which is optional, to three or more decompression stops that can range from 3 metres to 15 metres. If you like to hang for 9 min + 3 min (ascent time), total time to surface 12 min, be my guest.

Recreational NDL diving - anywhere between 100/100 to 70/70.
Planned decompression diving - below 70/70, for example, 45/90, 40/85, 35/75. These GFs will give you deeper stops and over all longer ascent profiles.

I am honestly having trouble understanding what you are saying. If I misunderstood because of my inability to read the king's English, I apologize.

It appears to me that when you talk about the GFs, you are focusing on the GF low and its impact on NDL dives, but the GF low has no impact on NDL dives. If you have a low GF low value on a NDL dive, it won't make any difference. You won't have to do any decompression stops. On an NDL dive, there is no difference between 70/70 and 35/70.

The GF low only comes into play if you violate NDLs and have to do decompression stops. In that case, it is no longer a NDL dive. In that case, it seems that you are arguing that people doing decompression dives should use higher GF low values than those, in which case I agree with you.