last stop at 3 or 6m?

[Valery]

I know there might not be a correct answer for this question but I'd like to have an instructor point of view on the following:

Why should I set 6m for the last stop rather than 3m?
Well after speaking with my instructor he insistes that more and more tech diver are now doing there last stop at 6m. I told him that it must be because they are doing there last stop with O2, but he kept telling that it would be better for me to do my last stop at 6m even with air.

What do you think? Some argue that the 3m stop shouldn't be replaced with the 6m and other says that you off gas better at 6m so what's the real story???

 

[Divor]

Do what the tables tell you to do.

 

[ScubaDiverRyan]

I Recommend 6m

 

[fdog]

There are all kinds of reasons to do the last stop at 6m, especially if breathing Oxygen. Some others include surface conditions, swell, current, keeping the team together, etc.

There are lots of reasons to to do your last stop at 3m. Some include: it's what the model tells you to do, reduction in gas supplies, temperature (thermocline), shape you need, etc.

Pick the reason that's highest priority for that dive and use it. Getting locked in to a particular strategy is not good.


All the best, James

 

[H2O 70]

TSandM suggested an excellent book that covers your question. It is called Deco For Divers by Mark Powell.

In short, it depends upon what algorithm you, or your computer, prescribe to. Some want you to stop closer to the 6m and some towards the 3m.

 

[Bubbletrubble]

Do you know why the safety stop was initially implemented by dive training agencies?

I'm a little disturbed that your instructor couldn't give you a better reason to do the stop at a depth of 6m vs. 3m. fdog listed some excellent reasons to choose one depth over the other. Many dive computers will allow the user to conduct the 3 minute safety stop in a depth range of 10 - 20 fsw. (This is my way of saying it probably makes little difference which depth you select.)

With a lot of up-and-down movement in the water column, it can be helpful to do the stop at 6m, since a 3m safety stop + the swells can easily cause unintentional surfacing.

Rather than fixate on doing the stop at 6m or 3m, probably a good practice is to extend the 3-minute safety stop to 5 minutes or longer.

Hope this helps...

 

[TSandM]

Are you talking about recreational diving, or technical diving?

If you are doing staged decompression diving WITHOUT accelerating your decompression, gradient is the only factor driving offgassing. Therefore, stopping deeper means slower removal of N2. If you are on 100% O2, gradient is irrelevant except as reducing ambient pressure allows bubbles to grow, so it makes sense to do your O2 time as deep as is safe, to keep bubbles small for better offgassing, and to avoid any turbulence from surface conditions.

For recreational diving, the main reason for a shallow stop is to decrease ascent rate, and the precise depth at which you do the stop is probably not very important, so long as it is shallow enough to have some gradient driving nitrogen elimination.

As everybody has said, if you are cutting tables for staged decompression diving, the algorithm you are using will prescribe the depth and time of the shallow stops, based on the assumptions made in the programming.

 

[Mr Carcharodon]

gradient is the only factor driving offgassing. .

That is correct as long as you keep the gas in solution. But once you bubble if you grow the bubbles past the point where they cannot get into lungs gas exchange will be inhibited. You probably have a better number for the size of the pulmonary capillaries than I do (20-50 um?). Anyway deeper stops reduce the gradient on ascent, as do slow ascent rates. But to answer the original question it may not matter -- usually.

 

[Valery]

Well I'm talking about both tech and recr diving.

I know that on O2 I should stay at 6m.

Now decoing with 50% or with air (most of our dive are small deco with air) and knowing that I can set my computer last stop at 3 or 6m, should I care about the gradient and do my last stop at 3m? My instructor state that you off gaz better at 6m while another 1 state that your not doing any good by staying at 6 and he can't find any good reason not to do my last stop at 3m, and that I'm doing no good in terms of off gasing by staying at 6m.

Still can't make up my mind on what's best for me.

 

[AndyNZ]

Another strategy is to do your last stop at 6m, then a 1m/min ascent up from 6m to the surface as additional stops. So you plan for your last stop at 6m, but then do additional stops.

The final ascents tend to be 15s from 6m to 5m, pause for 45s... 15s up... pause etc.

It's that top 5m where you get the most (relative) pressure change, so it's where you want the slowest and most controlled ascent.


Personally, I think this is the best of both worlds - you can, if needed, blow those last stops if conditions dictate but they add additional conservatism.