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

[Captain CaveMan]

I dont know about you but my computer will take time away on my next dive if i dont do a 3/15. so if its you first dive of the day and you want to do a 2nd why not stay the extra 2 min at 15?? its just 2 mins. as long as you got the air I would stay

 

[Charlie99]

You would be offgassing at 99' only if

1) you were fully saturated at 100' and
2) you ignore the fact that O2 is metabolized.

In the real world, when breathing air, you won't be offgassing until you get to something shallower than 79% of max absolute pressure even if saturated. When using nitrox, your offgassing depth will be a bit shallower. For example, saturated at 100' on air loads compartments to (100+33)*0.79 = 105 fsw = 72' depth. 72' will be your deepest offgassing depth, even for a long dive to 100'.

A 100' dive to NDL limit is limited by the faster compartments (5 to 10 minute halftime range). The slower compartments are more lightly loaded than for example, a 60' dive to NDL limits. Hanging at 20', or even 30' will get the fast compartments back well within their limits, while the slower compartments have never been heavily loaded. In other words, hanging in the 25' range works well for deep dives.

Edit -- see LooseBits post below. The calcs above describe the point at which dissolved N2 pressure exceed ambient pressure, rather than the point where ongassing reverse and become offgassing.

 

[drbill]

Realize that when doing a deep dive on an open water wreck you may have to dive a somewhat square profile. One reason I don't particularly like that kind of diving.

I frequently do dives to 100+ feet. When I do, I ascend with a multi-level profile. I may stop for a few minutes at 60-70 ft, then a few minutes at 40-50 ft, stay at ~30 ft for a few more and then swim in at 15-20 ft. Of course this is highly dependent on what I encounter to film along the way, but in general it has served me well. Of course I do most of my dives in areas with a sloping bottom and shore entry/exit so it is fairly easy to accomplish.

By the time I reach the upper levels, I have no deco requirement (IF I stayed at depth long enough to go into deco... I dive with an HP120 so that does occur regularly, especially if I'm filming at deeper water subject).

 

[SurfTheWorld]

Is it not the case that newer computers use algorithms that take into account deeper safety stops? Speaking of which, why is it (purely profit?) that dive computers are not reprogrammable... unlike the many other computers we pay less for in our daily lives? If safety were the manufacturer's main concern, you would think it would be a simple matter at the annual service to insert a new chip with the latest algorithms. It would even be a good incentive to have that annual service done on schedule. My Suunto Vyper is built like a tank. It has years of life left in it (hopefully), but I would feel more comfortable with such a product knowing it also contained the latest science.

 

[loosebits]

You would be offgassing at 99' only if

1) you were fully saturated at 100' and
2) you ignore the fact that O2 is metabolized.

You don't need to ignore the fact that O2 is metabolized. Lets do a 2 ATA version. At 33 fsw (2 ATA total pressure), the ambient ppN2 is 1.6 (lets call ppN2@surface 0.8 ATM just so I can do the math). If you are saturated at that depth the rate of diffusion of N2 into a given tissue is the same as the rate of diffusion of N2 out of the tissue - a net change of 0. If you come up 1', the rate of diffusion of gas out of your tissues remains the same (the tissues are still at 1.6 ATM ppN2) but the rate of diffusion of gas into the tissue decreases - so you offgas.

This is where a lot of confusion of partial pressures of inert gasses in liquids comes into play. There really is no pressure of the gas dissolved in the liquid (it doesn't exert a pressure like air in a balloon) but it just makes a convienient way to talk about concentration of a gas in solution. When we talk about gas in solution with a pp of 1 ATM we mean it would be in equilibrium with 1 ATM of that gas sitting outside of soltution. If you have 1 ATM of N2 disolved in a glass of water sitting in a room of O2 at 10 ATM, N2 will stil come out of the solution just as fast as if it were sitting in the room with 1 ATM of O2.

 

[FIXXERVI6]

Not my understanding...but I'm not a medical professional. My understanding is that offgassing is always happening albeit at a smaller rate....but I can see it being less significant if it's only 1 foot from being maximum saturated. I've heard this from some Medical Professionals, but again don't have the professional experience to discuss either way. Perhaps someone with a medical background such as Dr. Deco would be more knowledgeable in commenting on these topics.

I'm not a medical professional, I'm a diver, and not an instructor.

In the realm of non commercial diving, example non saturation diving, if you ascend 1 foot in the water, you may still be loading gas, just at a slightly slower rate than you were when you were at 100 feet, example:

If I dive to 100 feet for 10 minutes, and make my first stop at 99 feet, guess what, stilll loading gas.

If I dive to 140 then ascend to 68 feet on air, stop for a few minutes, guess what, I'm doing SOME offgassing, and SOME loading.

Again, I'm not a medical professional, or even an instructor, just a diver.

 

[Charlie99]

You don't need to ignore the fact that O2 is metabolized. Lets do a 2 ATA version. At 33 fsw (2 ATA total pressure), the ambient ppN2 is 1.6 (lets call ppN2@surface 0.8 ATM just so I can do the math). If you are saturated at that depth the rate of diffusion of N2 into a given tissue is the same as the rate of diffusion of N2 out of the tissue - a net change of 0. If you come up 1', the rate of diffusion of gas out of your tissues remains the same (the tissues are still at 1.6 ATM ppN2) but the rate of diffusion of gas into the tissue decreases - so you offgas.

True, true, true. I was confusing the point where the "N2 tissue tension/pressure exceeds the ambient pressure" with the point where you will begin to offgas.

 

[kensuguro]

I dont know about you but my computer will take time away on my next dive if i dont do a 3/15. so if its you first dive of the day and you want to do a 2nd why not stay the extra 2 min at 15?? its just 2 mins. as long as you got the air I would stay

Well, that's the thing with assisting classes.. I have to be ready for emergencies, so I can take alternative action based on correct theory if something happens. The whole reason behind my asking is kind of related to my other post of a deep dive incident:
Group is congested at safety stop, there is mild current (only the students are hanging on line for safety). The instructor is at the head of the group up to, AI, and two DM candidate (incl me) are sort of at the bottom end, 24ft or so. We spend a couple of minutes there.

Then, as the congested group shifted itself out, I move up a little just so I won't bend my computer. A minute there, a student comes to me signing 500 psi. Well, that's not too little, but not too much air.. and I was sort of confused if the 2 minutes plus 1 minutes combined meant we're basically done with the safety stop.. Because I thought if we're done, then 500 psi may be alright (and then switch to my alt air).. but if I were to go with the computer, or be conservative and do a full 3 min at 10-20, then 500 may not last another 2 minutes since he was huge, and probably not very calm. So, I had to priortize between: 1. safety stop time 2. safety stop depth 3. the dude's air (and I guess many more whatnots)

In a situation like this, I thought it was good to know what's really going on. But you're right of course, that's exactly what I'd do if I were diving for my own fun. My situation was also kind of extreme so it's not something I'd expect all the time. I'm just reviewing so I understand what a good choice of action would have been. It turns out the low air dude went to the instructor, which probably was the best choice.

 

[MXGratefulDiver]

Then, as the congested group shifted itself out, I move up a little just so I won't bend my computer.

Unless you violate a required stop (i.e. you exceeded no-decompression limits during the dive and violate a mandatory stop), you will not "bend" the computer ... not even by skipping the safety stop altogether.

Most computers will penalize you on subsequent dives for staying less than the recommended 3 minutes, but they will still compute your residual nitrogen based on the algorithm that the computer is using, and compute your subsequent NDL's accordingly.

It's far more important to consider what the ascent is doing to your body than it is to consider what it's doing to your computer ... and in some cases there is a significant variance between the two ...

... Bob (Grateful Diver)

 

[Whatgoesdown]

I don't have any relevant materials to hand at the moment, so this is all preceeded by a big "I THINK THAT..."

Safety stop recommendations are at/around 15fsw (5msw) because that is roughly where we have an extra 1/2 atm of pressure over the surface. Current thinking is that a diver's tissues can accept a supersaturation of 50% without significant bubble formation. So, although a diver may still be taking on nitrogen in some of the slower tissues (which didn't come close to saturation at depth), the fast tissues will be off-gassing. The reality is that these faster tissues can probably accept a greater supersaturation but I'm not volunteering to be that guinea pig! As an interesting (worrying?!) aside, the tissue half-time of the spinal cord is probably around 12.5 minutes! (If you know what that means and can carry out the sums, this is probably not good news!)

I believe that the US navy used to teach that an extra 100% of extra gas could be supersaturated, but then they were playing around with super-fit men who were probably not carrying a lot of extra fatty tissue. They also have ready access to hyperbaric chambers. Haldane was the first to introduce the idea of what we now call a deep stop, he reckoned that the depth/pressure gradient should be halved and then halved again etc.

In a similar vein, if it is very rough on the surface it is recommended that the safety stop should be a little deeper. There has been at least one case of a diver getting bent because he insisted on carrying out a 15' safety stop on a violently bucking line. (As I remember the write-up, his doctor pulled out a soda and shook it to explain to the diver what he had been doing to himself). Anyway, some kind of safety stop is better than no safety stop at all.

Finally, addressing a comment earlier in the thread. The days of not getting bent on one tank are long gone! I have frequently managed 2 hour dives, reaching a max depth of 100' on a single 80 and my girlfriend could probably manage 3 if she didn't get cold! (obviously we are not at 100' for the whole duration!)