Recreational Scuba Deco Diving

[KenGordon]

A lot of people have missed the point of this thread and go around mentioning the use of twin tanks or brining a pony.

"Making a planned decompression dive in a non-overhead environment, using open-circuit scuba, and using EANx for both the bottom gas and a--the only--deco gas." (No stage bottles can be used for extending the dive."

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. So the deco might not be the deciding factor for single vs double.

 

[Blackcrusader]

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. So the deco might not be the deciding factor for single vs double.

Yup he also added side mount twins as well. First he wrote. Not many recreational divers using twin side mounts. See the odd one here or there.

"No stage bottles. No travel bottles either
So, yes, I'll go back and revise my definition for RSDD to allow for EAN32/36 as both the bottom gas and the single deco gas."

 

[KenGordon]

What he wrote on page 1. I believe the discussion is about using a single tank and doing a deco dive.

"No stage bottles. No travel bottles either
So, yes, I'll go back and revise my definition for RSDD to allow for EAN32/36 as both the bottom gas and the single deco gas."

How is a pony a stage or travel cylinder? You need it because you might get out of gas at depth and not make it to your buddy in time. That can happen whether or not there will be deco if the dive goes to plan. Frankly I would rather be out of gas at a deco stop and have to share there than at 30m any day of the week.

 

[Blackcrusader]

How is a pony a stage or travel cylinder? You need it because you might get out of gas at depth and not make it to your buddy in time. That can happen whether or not there will be deco if the dive goes to plan. Frankly I would rather be out of gas at a deco stop and have to share there than at 30m any day of the week.

I edited to correct my error you pointed out. Yeah sharing at a deco stop at a shallow depth much better than having to share at 30m depth. My plan for using a single tank recreational dive. Two divers.

OK for me I could plan a 32% nitrox at 25m with 50 min bottom time 2 mins to deco stop and incur 11 mins deco. On a good dive with no complications I would start my deco stop with around 100 bar left on an AL 80 with 210 bar fill. I own an AL100 so we would use AL100 as would have around 2750 liters start with that. Even if I use a sac rate of 10l/m which is more than my normal rate I need 1730 liters to get to the deco stop on a normal relaxed dive. The people I dive with who would do this dive with me are my regular dive buddies and their sac rate one is the same as I am the other less than 10l/min. We dive in warm waters 28c in Asia. We would look at the tide charts and choose a time for least current.

Worst case scenario is a horrid dive with 20l/min sac rate and we abandon the deco part of the dive and shorten the dive to 30 mins and would be at the safety stop at 33 mins nice and safe with plenty of air and no deco obligation.

One dive site to do that is at the Liberty Wreck in Bali. We could even plan for the max depth depth 30m and do a 40 minute bottom time plan on 32% Nitrox and have a nice safe dive. Best thing it's a shore dive and a nice night dive site as well.

 

[scubadada]

...When you put your GFs to 35/75, you're making an agreement to yourself to follow it. If you are willing to accept a higher risk of DCS, the right way to do it is to set your computer to a more progressive mode. From your chart, GF Lo of 45 yields similar bottom time to DSAT, so that's the obvious choice here. Or use a different computer...

NDL is entirely dictated by the GF high, the GF low does not apply until you exceed NDL and go into deco. The GF low then dictates the depth of the 1st stop.

A GF high of around 95 is a relatively good match for DSAT but they are not close to identical. DSAT is generally more liberal on the 1st dive, particularly if it is deeper. Buhlmann tends to become more liberal on repetitive dives, especially shallower. I have almost 800 dives with a computer running DSAT and a second computer running Buhlmann, a Nitek Q and then a Teric.

When trying to match the computers, I would set the Buhlmann anywhere from 99 to 90. I quit messing with it and leave it on 95 now, and dive the more conservative of the two computers.

 

[Brett Hatch]

In making the chart, Tursiops just repeated the settings. He also understands enough to know that the setting of 45 for GF lo is meaningless on a recreational dive. So it is not an obvious choice at all, since it doesn't matter.

NDL is entirely dictated by the GF high, the GF low does not apply until you exceed NDL and go into deco. The GF low then dictates the depth of the 1st stop.

A GF high of around 95 is a relatively good match for DSAT but they are not close to identical. DSAT is generally more liberal on the 1st dive, particularly if it is deeper. Buhlmann tends to become more liberal on repetitive dives, especially shallower. I have almost 800 dives with a computer running DSAT and a second computer running Buhlmann, a Nitek Q and then a Teric.

When trying to match the computers, I would set the Buhlmann anywhere from 99 to 90. I quit messing with it and leave it on 95 now, and dive the more conservative of the two computers.

You are both right. GF high is the relevant number here, not GF low, since GF high determines when the diver's ceiling begins. Thank you for pointing out my mistake and for clarifying, that's helpful.

Except that the computer gives you options, and you can use those options as you see fit, depending upon your understanding of the theory behind it. It is not like using a wrench as a hammer; it is like using a tool to its full capacity, which the novice is unable to do.

The computer section of the PADI course you took is very generic because it has no way of knowing what computers the divers will use. Not only could it not tell what features might be available in the computer you would use at the time the course was written, it had no way of knowing what features would be available in newer computers years later.

I guess my first point was that many rec divers' training does not address mandatory deco stops, other than something like: use your computer to avoid them. Those divers doing dives with deco ceilings is a bad idea. I imagine this point is not very controversial. For GUE rec3, or BSAC, or other agencies, some "rec" divers might have deco training, I have in mind here divers who do not.

The second point about more experienced divers using their computer in this way, also seems like a bad idea to me. Let's say you are rigged for NDL diving with a single tank, and set your computer to 35/75, and splash. And then during the dive, as the NDL is reaching zero, you decide to add 10 minutes of bottom time, because after all, you know that 35/75 is overly conservative for the dive. So you add your 10min BT, ascend normally, do a safety stop, and reach the surface. Are you dead? No. But wouldn't it be simpler and safer to just set the computer to 45/95 in the first place, and then do what it says? That is the point I'm trying to make regarding post #91 . Which, if I misunderstood @tursiops 's point or am being overly pedantic or conservative or am just plain wrong, then I invite your feedback.

 

[boulderjohn]

The second point about more experienced divers using their computer in this way, also seems like a bad idea to me. Let's say you are rigged for NDL diving with a single tank, and set your computer to 35/75, and splash. And then during the dive, as the NDL is reaching zero, you decide to add 10 minutes of bottom time, because after all, you know that 35/75 is overly conservative for the dive. So you add your 10min BT, ascend normally, do a safety stop, and reach the surface. Are you dead? No. But wouldn't it be simpler and safer to just set the computer to 45/95 in the first place, and then do what it says?

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.

 

[Brett Hatch]

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.

In Tursiops's example in post #91. That's where a diver sets their computer to 35/75, where the NDL is 17 minutes. Diver stays the 17 minutes, then decides to stay another 10 minutes. Then does a normal ascent without deco stops, as though they were on a 45/95 or DSAT (since those GF/algo wouldn't have them under a ceiling for that profile). I don't think that this really make sense to do, better would be to set the computer ahead of time.

As you say, even if we take it as a given that the diver did the 27 mins BT with 35/75, many computers would have better alternatives available anyway (switch the GFHi to 95 on the fly, ride the GF99 or the SurfGF). I cannot imagine someone going "ah heck, this computer is too conservative anyway, I know I should be able to get 30 mins on DSAT, so I'm going to stay until then and ignore what it says." Are we in agreement there?

 

[tursiops]

The second point about more experienced divers using their computer in this way, also seems like a bad idea to me. Let's say you are rigged for NDL diving with a single tank, and set your computer to 35/75, and splash. And then during the dive, as the NDL is reaching zero, you decide to add 10 minutes of bottom time, because after all, you know that 35/75 is overly conservative for the dive. So you add your 10min BT, ascend normally, do a safety stop, and reach the surface. Are you dead? No. But wouldn't it be simpler and safer to just set the computer to 45/95 in the first place, and then do what it says? That is the point I'm trying to make regarding post #91 . Which, if I misunderstood @tursiops 's point or am being overly pedantic or conservative or am just plain wrong, then I invite your feedback.

You've hit on the supreme fallacy of having a fancy computer and refusing to use it, trusting to your own fallibility and misunderstanding instead.

 

[boulderjohn]

I cannot imagine someone going "ah heck, this computer is too conservative anyway, I know I should be able to get 30 mins on DSAT, so I'm going to stay until then and ignore what it says." Are we in agreement there?

We are mostly in agreement, but I am not sure to what degree. Maybe totally. It could be that neither of us is doing our best jobs communicating. I think that, in general, if you truly understand decompression theory and the way computers work, your knowledge can allow you to make sound decisions that might make the computer unhappy.

Here is an example unrelated to any other posts earlier in the thread. Some friends traveled to stay and dive with me in Florida last winter, and we made a series of dives with an operator who was doing nothing but relatively shallow reef dives, which we were doing as NDL dives. The operator had a standard policy of a maximum dive time of one hour. We got gas from an operator who charged the same price for any nitrox mix up to 36%, and since 36% worked for the dives, that is what we used. I was using Shearwater computers in tech mode, and they were using recreational computers without the more advanced features.

As we approached our time limit, we would begin the ascent, timing it so that after a safety stop, our heads would break the surface at one hour on the nose. People using a computer with a SurfGF factor (like mine) will usually like to make sure that number is below, say 80, maybe 75, before surfacing, even if it means waiting more than 3 minutes to get that low. Because we were using such a rich nitrox mix, when we first reached safety stop depth, my computer showed a SurfGF of 50 at the most, once as low as 43. That's when we started a safety stop, and their computers would start counting down 3 minutes. Left to my own devices, and with those computers, I would not have done a safety stop. I was not, however, going to impose that upon them. On each dive, we dutifully waited until their computers reached exactly 3 minutes, and then we surfaced.