Holding a safety stop - the effect of breathing/weighting

[Jafo19D]

If you're diving frequently/locally it REALLY makes sense to own gear ASAP rather than rent. Do the math.

The math doesn't make sense in my case until I start diving lots more.

SP Hydros $945
Mk11/C370 $510
SPG $60 (used)
SP Seawing Nova 2 $210
Mask and Snorkel ~$70
SP Shorts w/pockets $60
Titanium dive knife $45
DSMB ~$60
Boots $20

These prices don't even include the import duties they charge here in Colombia.

I'm doing a 5 tank dive this weekend here in Colombia that goes for $236; the guy took $14 off that price because I'm bringing my own stuff. But regardless I'm really happy to be popping the cherry on the BCD, regulator and even the shorts and just having my own gear.

 

[Zef]

I just want to make this point as clearly as possible, since this issue comes up from time to time on ScubaBoard. It seems logical that the rate at which we breathe would have an effect upon both on-gassing and off-gassing. If you are incredibly bored and have loads of extra time due to Covid-19, you could even find a thread I started asking about this 14-15 years ago. In one thread a few years later, a quite famous diver was quite insulting to all those who did not agree with him that breathing rate affects on-gassing and off-gassing.

It turns out, though, that breathing rate has no effect on either on-gassing or off-gassing. If you do find that thread I started so many years ago, you will find that the question was answered that way quite tersely by the man who went by Doctor Deco on ScubaBoard back then--Michael Powell, who was then one of the world's foremost experts on decompression theory.

Just for discussion's sake and to improve my knowledge....so exhaling N2 does not contribute to the gradient that facilitates diffusion of the N2 out of the tissue and into the blood? I would have thought that if the blood had a lower N2 load vis-a-vis exhalation that it would contribute to a gradient that would facilitate further circulation of N2 out of the body.

I am now compelled to learn more about this.

And this is why I put that caveat at the bottom of that post.

-Z

 

[johndiver999]

I don't know, but I suspect that the mass of nitrogen which diffuses from your blood through your lungs is miniscule during a single respiratory cycle. So the "air" in your lungs is not really going to be a higher percentage of nitrogen, whether you breathe fast or slow. So if the actual gradient is the same, then the diffusion rate will be the same.

Another way to think about it is that freedivers also absorb nitrogen during a dive and I think it is assumed to be the same amount as a scuba diver who would be breathing (air) and following the same dive profile.

However, exercise will affect blood flow and would therefore be expected to affect blood supply in tissues and should affect diffusion of gases

 

[NothingClever]

I think it's also important to explain why that is. In a simplified sentence: offgassing is controlled by the rate of inert gas diffusion out of tissues based on saturation and ambient pressure, not the rate of gas going in an out of your lungs.

Bullseye post. Well done.

 

[NothingClever]

Wear a little more lead, deploy an smb from 20 feet and then hang down on it. This will prevent you from moving up and down...

I’m going to say that runs the risk of frustrating @ross9 . I think he’s on a positive trajectory losing weight and that it’s better to get over that hurdle first rather than pack on more weight. The SMB might just end up a frustrating distraction rather than accelerant.

That much said, I wholeheartedly agree with your proposal of (eventually) letting go of his spool/reel to check his buoyancy control.

 

[lowwall]

If you want to practice, let go of the reel//spool and keep the spool right in front of your eyes.

Keep surface conditions in mind when doing this. If it's choppy, don't try to match the bobs

You can still let the spool go, but you want to stay at the bottom of the spool's up and down bobs.

Re, the jacket vs BP/W. I prefer and dive the latter, but it really doesn't make any difference at a safety stop.

 

[boulderjohn]

I don't know, but I suspect that the mass of nitrogen which diffuses from your blood through your lungs is miniscule during a single respiratory cycle. So the "air" in your lungs is not really going to be a higher percentage of nitrogen, whether you breathe fast or slow. So if the actual gradient is the same, then the diffusion rate will be the same.

yes. The issue is the gradient between what is in the blood entering the lungs and the gas in the alveoli. Exhaling has no effect on what you inhale with the next breath. It's not like the N2 goes away and is not replaced to some degree by the next breath.

 

[shurite7]

The one area where I still struggle a little is at the safety stop, particularly when it comes to the effect of my breathing on my buoyancy.

What can I do to help hold the safety stop?

As has been mentioned above, try holding onto a line, if possible. If it is a line that is vertical (hanging from a boat or buoy) lightly hold onto it, then barely hold onto it by using the OK sign with the line in the O part of your forefinger and thumb. If the line/rod hanging horizontal from a boat hold onto it slightly, then let go and place one finger on it. If you find yourself sinking or floating up then grip the line and readjust. Try to remain horizontal; you can skull your fins slightly to keep your feet from sinking or floating.

Another way to work buoyancy and breathing while not moving is to place your ankles onto something, (for example, the rail of a sunken boat, a platform, a log, or anything man-made) with nothing below or above you. Add or release air in your BCD to obtain neutral buoyancy then focus on your breathing. Avoid using your hands to maintain your position, focus on your breathing. I've seen people not want to take the time to do this, but I found it worked quite well with several of my students and a couple of DM candidates who struggled with buoyancy.

Over time with some focus and practice you will be able to ween yourself from having to hold onto the line.

 

[Hartattack]

Yes, I've emptied all my air from the BCD at the safety stop. In the OP when I said I'm adding less air to my BCD, I meant at depth i.e. as a result of getting better at taking a little less weight on the dive.



This is a debate I'm having at the moment. I'm still renting BCDs but planning to get my own gear - haven't decided between BCD or BPW yet. I can see the advantages of BPW, I've just never dived with one. So would like to give one a go before I invest in my own.

If you want to get into tech, this is the way to go. I'm sure you will enjoy it. Cheers

 

[BlueTrin]

I think people have already given a lot of good points but one thing I would emphasise is that, at a shallow depth, you need to be a bit more proactive at managing your buoyancy than at a deeper depth: you should try to correct your buoyancy before you move too far up/down (using your lungs)

This is even more true if you are overweighted.

At 5-6m, if you move 1m up you’ll be much closer to the surface and the final few meters is where the air in your BCD will expand the most.

Especially if it is a bit choppy, you need to use your lungs to counter moving too far up or down. If you wait to be already at 3m before to exhale a bit and make yourself less buoyant you are unlikely to be able to counter the air expanding in your BCD and the momentum.

This is why it will help you to be correctly weighted (as others have said): because you will have less air in your BCD so you’ll be swinging less up and down when you are not neutrally buoyant.

If you practice or just dive more, it will just become easier. I struggled with it as well.