Where is the buoyancy problem in this situation

[DevonDiver]

Not moving. That's one of the things that concerned me.

You'd be the 1/10,000 that didn't...

Unconscious (i.e. you're not doing it deliberately, or even aware you are doing it) fin movement is very common in novice divers - especially so when they are task loaded and/or otherwise distracted by doing something.

 

[Malpaso]

I was able to recreate this in the pool last night, although from only 10' deep, but it makes sense. While moving from horizontal to vertical, I took a fairly medium to deep breath, but kept my feet still (actually crossed them to be sure). Bam, right for the surface. Wasn't able to deflate the wing fast enough (no weights on, just the 6# backplate). Tried it a couple more times, and was only able to keep down if I started deflating almost before the maneuver.

Someone suggested that the size of the wing (40#) and the movement of all the air in it to the top may have caused not only a change in center of gravity, but also a momentum shift of the air pocket (physics here) that actually helped push upward to some extent.

Bottom line, lesson learned. I need to be cognizant of my breathing when making a body shift such as this.

 

[Agility]

Just another guess: you might be slightly head down when towing the flag. That means to prevent you from sinking you might have slightly too much air in your BCD. As soon as you stop finning downwards and get vertical you slowly start to drift up. This combined with deeper breathing due to task loading ...

 

[mistafong]

Don't rule out the possibility that your buddy added air to your wing with his hoseless remote powerinflator!

 

[tracydr]

4xAA PrincetonTec, and I had it boltsnapped to the spool. Trying to offset the deficiency of only having two hands.

Actually, if the dive light is not part of the daytime equation, this could be part of the problem. Just a bit more negative weight means a little more air in the wing. Plus, the lack of visual reference at night and then get to 20 feet with a oversized wing that's slightly over-puffy from a little extra weight that is not usually carried during the day, get a little bit stressed and take a big gulp of air in the lungs. That's all it takes. I remember once being surprised when I ended up on the surface from about 10-15 feet while I was clearing my mask. It was back when I was having a lot of stress clearing my mask and my mask flooded. I took a large breath while I tilted my head,( swimming in a horizontal position) pop, I was on the surface!
That was right about the time that I realized that I needed to really do a lot of mask clearing, no-mask swimming and spend a lot more time learning to breath through my reg without my mask because it caused enough stress that I was not stable in the water column while swimming and clearing at the same time.
You may want to consider a smaller wing. 40 lbs is a pretty large air bubble to manage and not really necessary for single tank diving.

 

[Malpaso]

You may want to consider a smaller wing. 40 lbs is a pretty large air bubble to manage and not really necessary for single tank diving.

It is necessary when I've got 36 pounds of weight, 30 in weights and a 6 pound plate. I'm not overweighted either because it takes 36 pounds to sink a 7mm farmer john, shorty, hood and gloves.

 

[halocline]

I don't know if anyone mentioned this, but if you have a sizable amount of air in your wing, and you go vertical, I guess the air bubble will be a couple of feet closer to surface, unless by going vertical your feet go down instead of your head going up. So if the air bubble rises with the change in attitude, it's now bigger and more buoyant.

I was going to say that you're almost certainly finning a bit without knowing it, but if you really crossed your legs, that usually does the job.

I've been diving for many years, been a DM for several of those, and assisted with many peak performance buoyancy classes. Those helped me to learn about about my personal weighting. Even after this experience, it seems every time I really take time to fine tune my weighting it turns out I'm a bit overweighted and diving just a bit negative, maybe a lb or so. Most recreational divers I see are substantially overweighted and substantially negative when diving, and this ingrains a habit of constantly working just a bit to stay neutral. I think there's a bit of basic human security in feeling a slight pull of gravity.