Safety stops - depth, duration and trim

[muddiver]

I usually stop around 20 FSW (6 meters, I think) because the change in buoyancy differencial at 15 FSW is so much more pronounced, it is difficult to hold depth in mild to rough surface conditions. Also if you start to ascend at 20 feet you don't move as fast as you do at 15 feet, thus it is eaiser to recover to the stop depth at 20 feet.

 

[Teamcasa]

I'll leave the medical reasons to the scientists to explain or debunk.

For my stops, generally around 18'-20' I remain horizontal for the most part. I'll sometimes end up vertical if for some reason I end up on a line or anchor rode. I don't make a big deal about it one way or the other. After all, we are only there for a few minutes anyway.

To me, the final assent is more important. After my stop, I may take an additional 5 minutes to fully surface, I stay horizontal for all but the last few feet.

 

[ClayJar]

I mean my lungs are probably about 30cm from top to bottom. That's a 0.03 Bar pressure differential on my lungs if I'm in an upright position. Moreover, I'm not convinced my lungs actually do experience that pressure differential as it is a single enclosed space. By analogy, if I take an inflated balloon under water, will the pressure inside that balloon vary from top to bottom or will it have a single internal pressure that is the average of the balloons depth in the water column?

Regarding the balloon...

The pressure inside the balloon will be *practically* uniform. Given perfectly stagnant air inside the balloon, the weight of the depth of air in the balloon will make the pressure at the bottom ever so slightly higher than the pressure at the top. In real life, it's effectively constant pressure on the inside of the balloon (unless it's a *really* big balloon :biggrin.

On the other hand, the pressure on the outside of the balloon is not so nearly invariant. The change in pressure equals the density times the change in depth, and the density on the outside of the balloon is considerably higher than the density on the inside. Since the forces on the balloon must balance out, what else is missing?

Remember the balloon! The balloon material is what balances the equation. On the top, the material is stretched more tightly. The pressure from the water and the pressure from the balloon skin's stretch add up to the pressure of the air inside the balloon. On the bottom, however, the pressure from the water is greater (with the pressure of the air remaining effectively constant), so the balloon's skin does not have to stretch as much to press against the air inside.

Think of a mostly full open-bottom lift bag. Where the air and water meed at the bottom of the air bubble, the air pressure and water pressure are perfectly balanced. (They have to be balanced, since nothing's holding either back.) You can press on the fabric and feel basically no resistance. As you slide your finger up the side of the bag, the bag feels more and more taut. The pressure on the inside is the same, but the pressure of the water is less -- the difference is held back by the fabric, and you can feel that pressure. At the top, the pressure difference between the water and the air in the bag is at a maximum, with the air (still at the pressure of the air and water meeting at the bottom of the bag) pressing quite forcefully against the inside of the bag.​

So, going back to your lungs, the pressure on the inside is effectively uniform (as they are filled with very low density fluid, eg. air), and the pressure on the outside is based on the depth of the fluid in which you're diving (assuming you don't want to go overboard with technicalities). Of course, the magnitude of even that pressure difference is not significant. Although you *can* feel the difference between breathing while upright, face-up, or face-down, the comfort impact of that "work of breathing" is on an entirely different scale from decompression calculations. (Ever tried to use a one meter/yard snorkel? :biggrin

 

[ligersandtions]

I read the pressure differential theory a while ago as well....and came to the same conclusion (could it possibly make that big of a difference? Hmmm, I don't think so....). I do, however, complete my safety stops in a horizontal position as much as possible. This is for the same reason that many others mentioned....there's a lot more area to slow my up/down movement and I can swim to a buddy quickly because I'm already in position.

Also, I'm a new diver, so I like to practice neutral, horizontal buoyancy as much as I can so that it becomes more natural (and less like work) to get in that position when I want.

 

[tgsmith]

On a deep dive(> than 75') I do a deep stop(1/2 max depth for about 2 minutes) then do a 'regular' SS @ 15' for 5 minutes.....for shallower dives, I generally do a 15' SS for 3 minutes....Trim, who cares.......

What's your average dive profile(BT, EAN or air) for dives below 75'? Deep stops aren't really something that's used unless you're going deep(i.e. way outside of "recreational" limits, usually with a helium mixture).

It's not going to be detrimental since you're going to start off-gassing between 60-70 ft for most profiles at 100 ft, but.....a shallower, longer stop and/or very slow ascent from your safety stop to the surface would be much more beneficial.....at least that's the current theory.

Back in the days, way back I'm talking, some were taught to bolt as fast as possible for their first deco stop from the bottom. Makes sense given the knowledge back then......the 60ft/min ascent rate was standard because that's the max rate a navy diver in a suit could be mechanically "reeled" up, so to say, and it just carried over.....today the theory is obviously different.

 

[cooperscuba]

We've been taught that safety stops should be made at 3m for 1 minute. This is according to the Buhlmann tables.

DAN recommend deep stops and 3-5 minute (preferably 5) safety stops at 5.5-6m and, until they suggest 3m, that`s where you`ll find me.

Don`t agree with TSandM`s reasoning for the horizontal positıon (sorry TSandM) as I`m usually within arm shot when diving with a buddy but hey, horses for courses.

 

[String]

What's your average dive profile(BT, EAN or air) for dives below 75'? Deep stops aren't really something that's used unless you're going deep(i.e. way outside of "recreational" limits, usually with a helium mixture).

Deep stops are accepted and used quite readily inside "normal" diving depths. Certainly anything deeper than 30m its worth thinking about.

 

[DBailey]

Regarding the pressure differential topic, it was explained to me a long time ago along these lines:

Decompression theory uses theoretical compartments. These compartments don't line up with any one part of your body. There are fast compartments and slow compartments and some in-between. At 5m, if you are vertical, you will be putting some compartments at a higher pressure and some at a lower pressure. If you are horizontal, the comparments are all closer to the same pressure and a more uniform off-gassing.

Now, does the pressure difference over 2.05 meters (my height) really matter that much for recreational diving?

Regarding the safety stops:

1. I feel they start with a slow ascent from depth. I play the game with my computer that I want to be coming up in the water, but I don't want to see any ascent bars light up. The first bar lights up at 15 feet per minute ascent rate. If there is a compelling reason to come up faster, then 30 feet per minute game starts.

2. Deep stops are used if the dive was deep enough.

3. Safety stop around 15 feet for 3 minutes at a minimum. I will try to stay in longer if the dive plan (and boat plans allow it).

4. The next game starts from here, from 15 feet, I try to take over a minute to ascend. This I feel is the downfall of many new divers that it might take them 15 second to reach the surface.

 

[tgsmith]

DAN recommend deep stops and 3-5 minute (preferably 5) safety stops at 5.5-6m and, until they suggest 3m, that`s where you`ll find me.

Thanks for the read on deep stops. Nice to know my decompression theories instructor is as bad as I thought he was......

 

[Teamcasa]

Also, I'm a new diver, so I like to practice neutral, horizontal buoyancy as much as I can so that it becomes more natural (and less like work) to get in that position when I want.

Probably the best real world reason to stay horizontal.