100ft deep dive, safety stop at a little below 20ft

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kensuguro

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This is one of those grey situations.. say your'e doing a safety stop from a 100ft deep, and you have a mandatory safety stop to do. 3 min at 10-20ft.. well, as you all know, 10-20 ft zone is ALWAYS crowded.. So, chances are, you're a little deeper as you wait for the conjestion to clear.

Say you were at 22ft for 2 minutes, and then at 19ft for 2 minute. Obviously, my computer doesn't register the 2 minutes at 22 ft, so it'll tell me I still have 1 minute left. But what about the actual nitrogen content in my body? Has the 2min at 22ft and the 2min at 19ft combined effectively done what the 3 minute saftety stop is supposed to do?

I mean, it's one of those "well, just do the full 3 minutes as a precaution and be conservative" kinda situation, but it's good to know what's REALLY going on in case of an emergency. Like, is it better to be deeper, or shallower if you had to dodge a bulge of people?
 
kensuguro:
This is one of those grey situations.. say your'e doing a safety stop from a 100ft deep, and you have a mandatory safety stop to do. 3 min at 10-20ft.. well, as you all know, 10-20 ft zone is ALWAYS crowded.. So, chances are, you're a little deeper as you wait for the conjestion to clear.

Say you were at 22ft for 2 minutes, and then at 19ft for 2 minute. Obviously, my computer doesn't register the 2 minutes at 22 ft, so it'll tell me I still have 1 minute left. But what about the actual nitrogen content in my body? Has the 2min at 22ft and the 2min at 19ft combined effectively done what the 3 minute saftety stop is supposed to do?

I mean, it's one of those "well, just do the full 3 minutes as a precaution and be conservative" kinda situation, but it's good to know what's REALLY going on in case of an emergency. Like, is it better to be deeper, or shallower if you had to dodge a bulge of people?

Well that's a good question. You have to understand one thing...you are always decompressing as you are going shallower. Every foot that you go shallower, you are offgassing Nitrogen...that's what your computer is accounting for.
Honestly, given that scenario, I would try to stay outside the group of people 15-20 feet in depth and not crowd around the descent/ascent line. I've been in situations where there are a lot of people decompressing and I tend to stay on the outskirts of the group.
Now if that's not a good enough answer, then I would decompress wherever you are safer from running into someone.
 
I'm going to assume that you aren't low on gas and that you don't have a fascist divemaster who needs to out of the water exactly x minutes after the dive started.

Why rush going up to 15 feet to get out? As mars2u said, when you start going up, you start off-gassing. You should be trying to gradually ascend from depth. I think I NAUI came out with some new recommendations that on dives below 60 feet, you should do a 1-minute stop at 1/2 max depth. The deeper stop helps to more gradually release gas from your body. Having said that, it's still worthwhile to make a full 3-minute stop between 10 and 20 feet and afterward proceed slowly to the surface.

If crowding around the ascent line is a problem, you should also try to practice open-water ascents without a line. When you feel comfortable enough, you won't have to hang on the line with the crowd. (Of course, if the ascent is in current, you have no choice but to grab the line).
 
kensuguro:
This is one of those grey situations.. say your'e doing a safety stop from a 100ft deep, and you have a mandatory safety stop to do. 3 min at 10-20ft.. well, as you all know, 10-20 ft zone is ALWAYS crowded.. So, chances are, you're a little deeper as you wait for the conjestion to clear.

Say you were at 22ft for 2 minutes, and then at 19ft for 2 minute. Obviously, my computer doesn't register the 2 minutes at 22 ft, so it'll tell me I still have 1 minute left. But what about the actual nitrogen content in my body? Has the 2min at 22ft and the 2min at 19ft combined effectively done what the 3 minute saftety stop is supposed to do?

I mean, it's one of those "well, just do the full 3 minutes as a precaution and be conservative" kinda situation, but it's good to know what's REALLY going on in case of an emergency. Like, is it better to be deeper, or shallower if you had to dodge a bulge of people?


if your in an area that hanging on to an ascent line is necessary, learn to use a jon-line... In the northeast, I teach this to all my open water students.. You end up with a much more relaxed hang and a more stable hang..
 
You better finish that 3 min stop above 20ft, or your computer will penalize you on the next dive.........as far as diving the tables, you're ok. I fact, I would recommend to stop for one minute at 30ft, another minute at 20ft, and another minute at 10ft. It gives you an average depth of 20ft for 3 min. Learn to control your bouyency so that you don't have to hang on the line like a monkey, now if there is a current, hmmmm, hold onto your buddies fins.
 
mars2u:
Well that's a good question. You have to understand one thing...you are always decompressing as you are going shallower. Every foot that you go shallower, you are offgassing Nitrogen...
That is not true at all... depending on the profile, offgassing may not start for a significant depth above your max depth. The shorter your time at max depth the less your tissues are saturated - offgassing will start at a shallower depth. You would have to be very close to saturated for offgassing to start 1' above your max depth - not likely on a single 80 :)
 
Good post loosebits. Can you even reach full saturation at 100ft on 80cuft? NDL is 20 minutes but the tank would probably be dry by then. VPlanner says off-gassing starts at 68 feet, so you need to rise a fulll atmosphere almost to even begin the deco process by ambient pressure alone.

Kensguro, the idea is not whether it's better to do your deco deep or shallow. The idea is to make your ascent look like a nice curve when you plot time against depth. By shooting from 100ft to 22, you've essentially created a VERY steep curve rather than a more gradual one. The steeper the curve, the more likely you are to bubble and have issues.

If I was coming off 100ft dive, I'd get to 30ft rising at about 30ft per minute to get a nice gradual curve going, then stop for 2 minues at 30ft. I'd then ascend to about 20ft and do another 2 minutes, then 10ft and try to do 2 minutes there, then burn down some tank to maybe 750psi above 10ft if I could. Chance are with that small tank, you won't have much left.

The problem with the NDL tables is that they really don't consider the deco profile at ALL since they don't assume you have any significant nitrogen loading. We now know this isn't really true, but the tables aren't changed to reflect it.

Be forewarned. I am not recommending you do what I just described. That is what *I* would do. But I'm a newbie so don't listen to me...
 
ShakaZulu:
You better finish that 3 min stop above 20ft, or your computer will penalize you on the next dive


Not all computers will "Penalize" you for skipping a stop at 15ft, mine will calculate whatever stop I make at whatever depth. You need to know "YOUR" computer and how it reacts to these situations.


I usually come up last and "Round up the herd" I get stuck at 20ft often, I wait until they surface and hang out at 15ft for my 3 minutes (Usually longer as dives progress during the day) If I go up I bob on the surface and wait for them to get in the boat anyway, might as well do a few more minutes, assuming I have the air available.
 
Interesting question for me, too.
Concerning the extra one minute at half depth, is the idea to do it at half the physical depth, or half the pressure depth? When I was on vacation, I believe it was suggested that we do an extra minute at half pressure depth -- around 30 - 35 feet for a dive at 100'. (There was no concern about reaching NDL, no deco obligation, it was just suggested as a good practice.)
Thanks.
 
The "half" just happens to work out. The idea (by modern methods) is to do the first stop 2 atmospheres above your bottom depth. It just happens that at 99ft, 2 atmospheres is 33 feet. If you were at 300ft, you'd be doing your first stop at about 240.

Essentially, the change in pressure of 2 atmospheres is enough to get the gas (nitrogen) starting to offgas in a slow and controlled manner. Combine that with a nice and easy ascent, with maybe a short stop every 10 feet, you you should get that gas coming out in a very clean way. You spend more time under the water, but you reach the surface with FAR less nitrogen in your system.

Unfortunately, the models most recreational computers are built on will punish you for taking this more conservative approach rather than doing what they should,which is crediting you for dumping nearly all your nitrogen in the WATER instead of letting it all bubble out on the surface. This is why many technical divers don't bother with computers at all. They'd all say the diver is bent, when in fact the diver has offgassed cleanly and will need a much shorter surface interval than the person who rockets to 15 feet for 3 minutes then jumps out.
 

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