Where is the buoyancy problem in this situation

[Malpaso]

I encountered a strange buoyancy situation doing a night dive yesterday. A number of variables were in play, and I'm wondering which one or combination caused my problem.

Here are the variables:

Night dive
BP/W - I've only dove bp/w a couple times, first time on a night dive with bp/w. I felt quite comfortable with the bp/w
Towing the flag - I always tow the flag, I'm the noob

Here's the problem:
My buoyancy and trim were fine. I'm getting comfortable with the bp/w. Had to adjust the HOG harness between dives, but that was just really tweaking. The problem happened when I had to adjust the flag line. I was using the yellow plastic spooler, like I almost always do. If I just wound in or out a little, I was fine. A couple of times, though, the line got messed and instead of winding from a horizontal position I had to get somewhat vertical to work it out. As soon as I did, I'd start to ascend. One of the times, it was messed so bad I was spending a while fixing it, and all of a sudden I'm on the surface (only from about 20').

I've never had that happen before. My concern is that it only happened when I was vertical. So my question is, was I having a bp/w issue, or was this likely a night diving issue, or was this a combination of things?

 

[DevonDiver]

When you got vertical.... what were your feet/fins doing?

Lots of novice divers tend to scull with their fins when vertical. That's upwards propulsion you know...it acts just like positive buoyancy

 

[Malpaso]

When you got vertical.... what were your feet/fins doing?

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

 

[ajduplessis]

Once you start with task-loading your breathing changes and this has an impact on your buoyancy. With time and experience this will get better. Try and find a reference point in the water when you engage tasks and focus on your breathing pattern. DD also commented on sculling. Take note of these points the next time you dive. Have fun

 

[boulderjohn]

Without seeing it, anyone would be guessing. Here are some of my guesses.

When you went vertical, you probably kept kicking. that could push you up a little higher in the water column. When you go up in the water column, you become more buoyant. How much more buoyant depends upon a number of factors.

1. The closer you are to the surface, the more you will be impacted. The air in your BCD and the bubbles in your wet suit (or dry suit) expand much much more at shallower depths than at deeper depths. (You can figure this out precisely using Boyle's Law.)

2. The more thermal protection you are wearing, the more it will expand as you ascend. You did not say where this happened, but if it was a dive local to the location you indicate as home, you were probably wearing at least a 7mm wet suit.

3. The more you are overweighted, the more you will be affected. If you have enough weight for the dive, you have enough weight for the dive. Adding more weight only means that you have to add more air to the wing to be neutral--each excess pound requires about 15 fluid ounces of air (two cups) to be neutral. the more air in the wing, the more it will react to changes in depth. Contrary to popular belief, if you add more weight to a diver who tends to pop to the surface as you did, you only make things worse.

4. If you are diving thick neoprene, as in a 7mm suit (or more), it makes things worse. You have no choice but to be overweighted in such a suit. You need the weight to submerge, but as the suit compresses as you descend, you don't need that much weight any more. Unfortunately, you can't get rid of it.

5. Altitude makes a difference, believe it or not. There is a greater buoyancy swing at altitude than at sea level. You probably weren't at altitude, but I just thought I would add this.

Yesterday I was teaching some AOW dives to students wearing 7mm suits in 21 feet of water at Denver's altitude. We were trying to do compass navigation in about 3 feet of visibility. That's about as difficult as buoyancy gets.

 

[Malpaso]

My concern about it possibly having to do with the bp/w was this has never happened with my jacket BCD.

The closer you are to the surface, the more you will be impacted.

Yes, I was only at about 20' at the time

you were probably wearing at least a 7mm wet suit

Correct

The more you are overweighted, the more you will be affected

I actually dropped some weight before yesterday's dives.

 

[TSandM]

It doesn't have anything to do with the backplate.

If I had to guess, I'd guess that either you did continue to scull a bit with your fins, or your breathing changed when you were managing the major tangles. It's also possible that your process of going into a vertical position ended up with your torso at a shallower depth than it had been. But what was really different was that it was night, and you had little or no visual reference to tell you immediately that you were changing depth. As a result, you weren't aware of going up until the process was well underway.

Years ago, I saw someone I regarded as an EXTREMELY solid and accomplished diver, yo-yoing wildly on ascent in back water in Cozumel. He was a fantastic diver, but unused to the lack of visual input that occurs at night.

 

[spectrum]

The dive light blew your trim. One of those 8C light assuming you have alight of that scale held forward can upset the apple cart considerably.

 

[Malpaso]

The dive light blew your trim. One of those 8C light assuming you have alight of that scale held forward can upset the apple cart considerably.

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

 

[supergaijin]

If you successfully descended to 20', then the lights don't seem to have made that much difference.

Something must have changed:

1. a slight fin scull while vertical
2. an extended inhalation (keeping air in the lungs for a longer time than when exhaled), causing you to rise in the water column, becoming Boyle's B!tch
3. a weight dropped out
4. you farted in the 7mm suit.

My money is on a combination of 1,2 and 4.