Safety stops - depth, duration and trim

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Deefstes

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This is probably no big deal but I'm starting the thread to find out:D

We've been taught that safety stops should be made at 3m for 1 minute. This is according to the Buhlmann tables. That said, I know many divers who's been trained to do safety stops at 5m for 3 minutes. At first I struggled a bit to remain neutrally buoyant at 3m and my instructor told me not to worry and rather do the stop at 5m for 3 minutes then. Now that I have my buoyancy better under control I actually like the 3m stops better. Do you guys have any opinions on this one way or the other?

My more important question is this though. Is there a compelling reason why you should be in horizontal trim during your safety stop? I remember reading once that if you're in an upright position there is a pressure differential across your lungs from top to bottom which has an adverse effect on your off-gassing. WHAT?!?!? Could that really be? 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?

But I'm getting in to deep. What I really want to know is, what are the pros and cons of being in an upright / horizontal position while doing your safety stop. Personally, and I know this is probably wrong but it works well for me, I am in an upright position. I carry 4kg of weight which makes me slightly negatively buoyant. On my safety stop I stay in an upright position and can then either slightly inflate my BC to stay neutrally buoyant or I can purge all air from my BC and just give a gentle frog kick downwards every 10 seconds or so.

Now accepting your inputs:wink:
 
a instructor i use teaches to be horizontal for the reason you mentioned. i also like it becuase it is just a little more practice with buoyancy

brett
 
a instructor i use teaches to be horizontal for the reason you mentioned. i also like it becuase it is just a little more practice with buoyancy

OK, I agree with the comment about buoyancy practice but seriously, I'd like for someone to explain to me how much of a difference a pressure differential of 0.03 Bar across your lungs are going to make and whether (as per my balloon example) it actually even exists.

I'm all for safe diving and good diving principles but I've got no appetite for bad science. :shocked2: Did I just say that? Now I'm gonna get it.
 
I do my safety stops in a horizontal hover. What is the difference between horizontal and vertical? Put simply, buoyancy is dynamic. With each inhale, your buoyancy increases; with each exhale, your buoyancy decreases. "Neutrally buoyant" is basically managing your buoyancy so that the average buoyancy through your breaths cancels out to zero.

If you are hyperventilating at 120 breaths per minute, your inertia will be sufficient to damp out the buoyancy oscillations, but that would not be the case if you were breathing normally. With each inhale you will ascend somewhat, and with each exhale you will descend. If you're taking calm, slow, deep breaths (as is often the case on safety stops), this effect could be magnified.

Now, how does that relate to the horizontal vs. vertical question? Your buoyancy only *averages* out to neutral, oscillating between positive and negative as you breathe. When you're vertical, you slice up and down through the water with each breath much more than you would if you're horizontal -- lying down in the water, you present much more area to retard vertical motion. Add to that the fact that buoyancy excursions create positive feedback loops (i.e. losing some buoyancy and descending makes you even less buoyant, making you descend faster, et cetera), and it's readily apparent why horizontal stops are so nice.

I can make vertical stops if I need to, but I have found that holding a precise depth takes significantly more management than when I'm in a horizontal hover. I can't help but wonder if I would still extend my safety stops as much as I do if I had to work at them. (It's trivial to just hover for 10 minutes or more watching others' bubbles when you're not expending any effort.)
 
I don't buy that pressure differential thing for one second. Since they are three-dimensional, whether you are horizontal or vertical, there is always a pressure differential across your lungs. Does it matter within the realm of reasonable returns? I don't see how.

As for vertical vs. horizontal, I find it's much easier to control depth when I'm horizontal. This is primarily because I present more area to the water in the direction of motion, and thus there is a greater drag force resisting my motion (either up or down).

Furthermore, I don't have to rotate my body should a situation arise that requires me to swim (could be current, could be a toxing or panicking or buddy otherwise in need of aid, etc.). I'm already in my swimming position, so I just go.

Furthermore-er, on the note of current, if I were vertical, not only could I not swim against it, but it would have a stronger pull on me.

I've heard people say "you should be vertical so you can easily look around." I don't know about them, but my head allows me to look around regardless of body position. I can't look straight up, but I can turn my head to see above me. I find that to be a bogus argument.


In my experience, all things that matter point to horizontal.
 
Aha, there's a compelling argument - "Presenting a bigger surface area to the direction of movement with buoyancy change". That makes much better sense than this pressure differential thingy.
 
I was told once to keep the majority of your body at the 15-20 ft depth for the 3 minute duration, and that by being horizontal, you accomplish that...may be BS, but it sounded good!
 
We're trained here to do safety stops for 1min at 6m if following BSAC tables. In a rough sea 3m can be extremely unpleasant.

Most computers will allow them anything from 3m to 6m so generally these days as just about everyone is on a computer they're done between 5m and 6m and for 3 mins. (on planned mandatory deco dives though i set my last stop depth to 5m).

I dont buy the pressure differential thing either and not found any real research to back up the claims it has an effect. As for trim though, vertical people from my experience TEND to be vertical because their buoyancy isnt neutral - most of them are finning slightly to maintain depth whether they realise it or not.

As the original topic here is safety stops which aren't mandatory at all i really dont think any tiny theoretical benefit of horizontal vs vertical will make even the slightest real world difference.
 
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.......
 
I don't buy the pressure differential argument, either, but one's ability to move quickly to a buddy is far better when one is horizontal. Since ascents and descents are times when people have problems, this is a good argument to me.
 

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