Recreational Scuba Deco Diving

[scubadada]

My diving had evolved significantly over time. First, on NDL dives, I did a 3 min SS. When I would exceed NDL, I would ascend as soon as I satisfied the light deco requirement. After that, I would do a 5 min SS after dives close to the NDL and would add a 3-5 min SS after light deco. Now, I use SurfGF and extend my SS or my last stop until I am at least in the mid 80s before I surface.

 

[Blackcrusader]

Of course it would be better to set it right in the first place. But....

• Your situation describes someone caught between two levels of knowledge. The person understands gradient factors enough to intuit the meaning and significance of such a change in settings and yet still do a safety stop. The diver who overstays NDLs by 10 minutes is well into deco. When a diver goes into deco, the safety stop is no longer a factor. In that case, the diver would use the decompression information on the computer. The computer would be telling him or her how much of a decompression stop to do, and at what depth.
• A computer like that might well have the capacity to switch GFs in mid dive and keep it as an NDL dive, so if the diver decided to add bottom time during the dive, he or she could switch to 45/95 and continue to follow it.
• If the diver wanted to keep it as an NDL dive and extend bottom time, the diver could (on some computers) look at the GF99 factor and stay within NDLs, regardless of what the settings were.
• The diver with such a computer could do the ascent and ignore the safety stop feature or keep it in tech mode, with no safety stop feature, and instead watch the surfGF feature, ascending when the number looks right.

So, with a computer that has those features, as I said before, it is not using a tool for the wrong job, it is understanding the complete capability of your tool and using it to its full advantage.

Thanks for this reply. I'm a recreational diver who does some deco dives on the odd dive. I bought a Perdix which has all the features you describe. I have spent a lot of time learning about GF, changing that during a dive if I need to, learning about the GF99 and what it is for. Using Surf GF to know can I safely get to the surface, learned about PPO2 and CNS and spent more time learning about oxygen toxicity. My BSAC deco classes were in 1986 - 1988. We did not have dive computers with such information available back then. I redid some courses with PADI again in 2014 2015, My instructor told me he thought it was a waste of time and money but I replied well it's my money and why not refresh some courses and anyway my EFR had to be done again anyway.

I dive in OC TEC mode. Why? I want to have settings that suit my diving. My computer is not a baby sitter and does not have audio alarms. Many people try to tell me I bought a device that is too advanced for my recreational diving. I also bought it as I love the large display and being able to set the main screen to what I want to see on it.

OK so I don't want to explain why I wanted to buy it and have all those nice features and not understand how they are useful for my diving. In fact I setup my safety stop to 6m for 5 minutes. One thing I really like on the Perdix is that when you do a deco dive you get a large clear DECO CLEARED message when you have cleared your deco obligation. Also it won't lock you out if you did not complete the deco stop. CLEARED when finished a safety stop. As I dive a lot at vacation centers if I suddenly start not doing safety stops with other divers it would cause some confusion so I do them. I don't want to be telling people safety stops are not necessary.

 

[Angelo Farina]

He did not exclude a pony or twinset. Many people will do some deco without and there is a risk with that. Generally it is difficult to get much of a deco obligation with a single cylinder, also risk on the dive itself might be unacceptable without at least a pony.

I really do not see much a difference regarding the choice of the tank for these RSDD.
I started using a 10+10 liters steel twin set at 200 bar, with two posts and reserve, in the seventies.
Soon replaced by an Aralu (9+9 liters Luxfer alu at 200 bar, 3600 liters tot).
A few years later I switched to a 15-liters, 232 bars (3480 liters), again with two independent valves and reserve.
I did not perceive any reduction of safety or redundancy, as I use the same two independent regs I was using with the twin set.
Why some people here think that these RSDD are safe with the twins (or a small single plus a pony) and unsafe with a properly-sized single, carrying almost the same amount of gas?
Please note that the valve system used on twins and single are functionally identical (two posts plus reserve), and that the geometry and mechanics of the connection between the two cylinders is intrinsically more prone to failures than the single-piece valve of the single.
In fact, I think that diving with the single is actually slightly safer than my old Technisub Aralu twin set...

 

[EFX]

My diving had evolved significantly over time. First, on NDL dives, I did a 3 min SS. When I would exceed NDL, I would ascend as soon as I satisfied the light deco requirement. After that, I would do a 5 min SS after dives close to the NDL and would add a 3-5 min SS after light deco. Now, I use SurfGF and extend my SS or my last stop until I am at least in the mid 80s before I surface.

So, based on a previous post you are diving a GFHi of 95. Whether you are doing an NDL or deco dive the computer will calculate stop depths and times or an NDL to bring you to the surface at or just below your set GFHi. By waiting at your last stop or SS you are artificially changing the GFHi to 85 by using the SurfGF. You can get the same result by just setting your computer to a GFHi of 85.

 

[rx7diver]

Why some people here think that these RSDD are safe with the twins (or a small single plus a pony) and unsafe with a properly-sized single, carrying almost the same amount of gas?

@Angelo Farina,

For me personally, I decided on two separate cylinders for RSDD diving because I realized that events can happen that will compromise my entire air supply if my air supply is not separated into two separate cylinders.

In the case of putting all my air supply in a single cylinder, some possible compromising events are (1) a tank-to-valve O-ring failure, (2) a burst disk failure, and (3) the inability to shut down a free-flowing regulator (in the case of two regulators connected to a Y-valve).

In the case of putting all my air supply in a set of manifolded doubles, a possible compromising event is any event that could be solved by isolating the cylinders, but there is an inability to isolate them.

Keep in mind, I am doing solo dives, so I have no dive buddy there carrying an emergency, back-up gas supply for me.

rx7diver

 

[rx7diver]

For me personally, I decided ...

Also, for me personally, I decided on back-mounted independent doubles rather than, say, a single back-tank with an appropriately-sized pony, because:

1. Transitioning to back-mounted independent doubles was straightforward for me since I had been diving back-mounted manifolded doubles.

2. The pony system is a stand-by system, and stand-by systems always make me a bit nervous. A stand-by system must work NOW, absolutely, when you need it to work. However, you never quite know if it will work until you absolutely need to use it. With back-mounted indies, you alternate breathing from both regs (say, switching regs every 500 psig or so), so you know, sooner rather than later, whether something has happened to your air supply system--which provides the ability to "always" safely abort the RSDD dive.

rx7diver

 

[EFX]

Thanks for this reply. I'm a recreational diver who does some deco dives on the odd dive. I bought a Perdix which has all the features you describe.

I dive in OC TEC mode. Why? I want to have settings that suit my diving. My computer is not a baby sitter and does not have audio alarms. Many people try to tell me I bought a device that is too advanced for my recreational diving. I also bought it as I love the large display and being able to set the main screen to what I want to see on it.

Same as you I bought a Perdix for its large, bright, colorful display and all the features. I also paid to unlock VPM. As a recreational diver I prefer the rec mode (for version 81, now called air, nitrox, or 3 gas nitrox). I like the automatic countdown for the safety stop. In tec mode you need to start and stop a timer for your safety stop which I though was a PITA. Unfortunately, I have to use tec mode in order to use VPM-B (the rec modes only support Buhlmann with GF's). I found that VPM-B +2 was slightly more conservative than Buhlmann ZHL-16C with GF's set at 70/90.

 

[scubadada]

So, based on a previous post you are diving a GFHi of 95. Whether you are doing an NDL or deco dive the computer will calculate stop depths and times or an NDL to bring you to the surface at or just below your set GFHi. By waiting at your last stop or SS you are artificially changing the GFHi to 85 by using the SurfGF. You can get the same result by just setting your computer to a GFHi of 85.

No, not exactly. I spend more time at depth and then spend the time at my safety stop to lower my SurfGF

 

[Blackcrusader]

Same as you I bought a Perdix for its large, bright, colorful display and all the features. I also paid to unlock VPM. As a recreational diver I prefer the rec mode (for version 81, now called air, nitrox, or 3 gas nitrox). I like the automatic countdown for the safety stop. In tec mode you need to start and stop a timer for your safety stop which I though was a PITA. Unfortunately, I have to use tec mode in order to use VPM-B (the rec modes only support Buhlmann with GF's). I found that VPM-B +2 was slightly more conservative than Buhlmann ZHL-16C with GF's set at 70/90.

You can set a safety stop at different depths and time in OC TEC mode. You do not need to start and stop a timer. Once you reach the safety stop depth the clock starts and once you are CLEAR you get plus minutes. So in this picture I have cleared the safety stop and have another 8 mins at the safety stop depth. Often I am hanging around taking photos on the reef at that depth or shallower.

 

[Ouvea]

You can set a safety stop at different depths and time in OC TEC mode. You do not need to start and stop a timer. Once you reach the safety stop depth the clock starts and once you are CLEAR you get plus minutes.

View attachment 632775

I don't believe you understand the "Last Stop" function. This does not set the depth your safety stop. OC Tec Mode does not have a safety stop feature. The "Last Stop" function sets the depth where you will perform your last decompression obligation. Hence, this is going to be shallowest depth of your decompression ceiling. I'll venture a guess and say that the most of us have set this between either 20 or 10 feet.

The Perdix does provide a safety stop function but per its owner's manual, only in OC Rec, which Shearwater also calls Recreational Nitrox Mode. Page 14 of the Recreational Nitrox Mode manual states the diver can set the length of the safety stop between 3-5 minutes. The Perdix will not automatically require a safety stop. It will add the safety stop if the diver exceeds 35ft of depth. Furthermore, unlike other dive computers which have you hoover at 15feet. The Perdix initiates the safety stop countdown once the diver ascends above 20ft but must remain below 7ft.

I'm hoping you are not confusing a safety stop for a decompression stop. They are vastly different, where omitting a mandatory decompression stop carries far greater risks than omitting an optional safety stop.