Feedback on recent two-tank and dive limits

[Blackcrusader]

If you are surprised to find you have an unexpected decompression stop at 21 meters, you made a super serious NDL violation to get a first deco stop that deep, even with the lowest of the preset GF low settings. I would be surprised if an NDL diver with an AL tank could make that happen.

Yeah I'm pretty decent on gas and have never thought I could do that on an AL80/100

 

[Centrals]

I never ever had any deco issue in making FIVE dives a day with my trusted 28yrs old Aladin Air/Pro-nitrox donkey years ago.

Accidental deco obligation is for inexperienced diver or those who do not pay any notice to the dive computer. PERIOD.

 

[old frogman]

So you are warning people that if they use GFs, they need to watch their computers to make sure they don't go into decompression. Are you saying that if they use another system, they don't need to watch their computers to make sure they don't go into decompression? What's the difference between Buhlmann with GFs and any other system?

If you are surprised to find you have an unexpected decompression stop at 21 meters, you made a super serious NDL violation to get a first deco stop that deep, even with the lowest of the preset GF low settings. I would be surprised if an NDL diver with an AL tank could make that happen.

I am not saying that they are going into decompression. The Low GF they select (which sets the first stop), that programs the computer may tell them they are going into decompression, as opposed to a safety stop. In reality they may be well within the NDL.

And, if they accidentally exceed the NDL, the ascent decompression profile calculated by the computer, may increase exponentially in a short time. Hence the 21m or perhaps 15m first stop.

One could argue that it is not probable, but it may be possible. We are talking about recreational divers who may not have much experience.

 

[old frogman]

Let's review the original post of the diver who was rushed out of the water by the dive guide. We will use the average depth as a reference point.

Dive 1 52ft for 45 min, SI 35 min, Dive 2 45ft for 53 min

The ascent profile that his computer on GF 40/85 produced may have been as follow:
Dive 1 - 30ft 1min, 20ft 1 min, 15ft 3min.
SI - 35 min.
Dive 2 - 30ft 1min, 20ft 2 min, 15ft 3min.

These ascent profiles look to me like stage decompression not safety stop. Stage decompression is commensurate with technical diving.

Had he been using GF100/100, which is pure Buhlmann, the ascent profile may have looked as follows:
Dive 1 - 15ft 3 min.
SI 35 min.
Dive 2 - 15ft 3min.

3min at 15 feet looks to me as a genuine safety stop commensurate with recreational NDL diving.

Regardless of algorithm or GF, if he had surfaced directly to the surface without completing any stops, he would probably be OK. Sure, he may feel a bit tired (after Dive 2) with elevated microbubbles but generally OK.

My point is the relevance of GF in recreational diving.

The exception is the UK and Europe where some recreational divers are trained in decompression procedure and execute dives to 50m on air. In this instance GFs may be relevant.

 

[jborg]

Dive 1 52ft for 45 min, SI 35 min, Dive 2 45ft for 53 min

The ascent profile that his computer on GF 40/85 produced may have been as follow:
Dive 1 - 30ft 1min, 20ft 1 min, 15ft 3min.
SI - 35 min.
Dive 2 - 30ft 1min, 20ft 2 min, 15ft 3min.

These ascent profiles look to me like stage decompression not safety stop. Stage decompression is commensurate with technical diving.

I doubt it. Those dives are within NDL at 40/85.

 

[34109411]

Apologies for the headache. But GFs can be a headache for the recreational diver. I agree that if you are using a GF on your computer for NDL dives you need to monitor the duration of your bottom time. When you select a particular GF, it will reduce your bottom time not only on your first dive but even further on your second dive and even further on your third dive (common on dive resorts and liveaboards). If not monitoring your computer (on the subsequent dives), your computer may direct you to complete a multi-stop ascent with the first stop at 21 metres. This surprise may be very challenging for an inexperienced recreational diver.

In reality you may not need to do the multi-stop ascent. However, you selected the GF and now you have to decide to either follow the computer on a long multi-stop ascent or just do a 5 min safety stop at 5 metres. What would you do?

What i do is that first of all i have a good understanding of the good old dive table and know the do and don't of ndl, surface interval and so on.

By mastering the concept of dive table you have a better understanding of computer, most diver don't have a clue of the dive table and don't use them. I've done my OW in 2005 and it was with dive table. I don't know how they do it now but i tend to believe newer diver stay away of them when training is complete

Diver where diving for years without dive computer and before they didn't had all those technical data on their wrist and everything was fine. You add data you add complexity of understanding what they mean.

So short story long i like to Kept it simple.

Be safe

 

[JMarc]

I doubt it.

Yesterday I tried to reproduce the deco profiles given by @old frogman, I succeeded for none and finally decided against the tedious answer hoping the thread'd die or be split outside of basic scuba. Their affirmations have to be fact checked before used as a starting point for an answer.

 

[Blackcrusader]

Apologies for the headache. But GFs can be a headache for the recreational diver. I agree that if you are using a GF on your computer for NDL dives you need to monitor the duration of your bottom time. When you select a particular GF, it will reduce your bottom time not only on your first dive but even further on your second dive and even further on your third dive (common on dive resorts and liveaboards). If not monitoring your computer (on the subsequent dives), your computer may direct you to complete a multi-stop ascent with the first stop at 21 metres. This surprise may be very challenging for an inexperienced recreational diver.

In reality you may not need to do the multi-stop ascent. However, you selected the GF and now you have to decide to either follow the computer on a long multi-stop ascent or just do a 5 min safety stop at 5 metres. What would you do?

I do 60 - 75 minute dives using GF 45/95 and have never had my Perdix to do a first stop at 21m. Also if the guides are using Sunnto they are the ones with shorter NDL on repetative dives. I'm a recreational diver and so are several of my dive buddies. We all use GF 45/95 when we dive together. We want the longer NDL times on Nitrox or air that we get. Even when I have gone into deco I have never been required to do a stop at 21m from deeper depths.

 

[Mobulai]

I had this stuck in my throat and debated if I should say it or not (since, for the millionth tim, this is basic scuba)
21m sounds oddly specific to 50% deco MOD/Ceiling — a computer will suggest it most probably because it was made available to it as a gas; something sounds wrong here

If someone doing an NDL dive on air/nitrox and „slips into deco“ their computer will for sure prioritize RBT (remanning bottom/gas time) over deco optimization (GFs or MB levels are to be escalated to the next level all the way to the least conservatism)

50% isn’t a recreational gas (ok maybe at DT academy but that’s a whole other can of worms)

Yes, a „rec“ algo/computer might suggest intermediate stops (like Scubapro PDIS; but that’s a feature that the user activates not on by default); but they’re not mandatory

And for the stop to be „efficient“ at 21, you must have really loaded some slow tissues — suggesting that the diver went WAY out of depth/time limits

Plan the dive, Dive the plan, read the manuals, the best computer is between your ears, and stick to your limits

 

[Blackcrusader]

If someone doing an NDL dive on air/nitrox and „slips into deco“ their computer will for sure prioritize RBT (remanning bottom/gas time) over deco optimization (GFs or MB levels are to be escalated to the next level all the way to the least conservatism) Yes, a „rec“ algo/computer might suggest intermediate stops (like Scubapro PDIS; but that’s a feature that the user activates not on by default); but they’re not mandatory

And for the stop to be „efficient“ at 21, you must have really loaded some slow tissues — suggesting that the diver went WAY out of depth/time limits

Plan the dive, Dive the plan, read the manuals, the best computer is between your ears, and stick to your limits

How so? My Perdix does not know my gas consumption on the dive and does not tell me to priorotize anything. I do not use air intergration as it is not necessary for me.