Buoyancy @ End of Dive-- Why so hard???

[jephre]

Hi All-

So, here I am 15 or so dives in to my diving life (and what a life it is), but yesterday in my bubblings at the Avalon UWP in Catalina I twice found myself losing control of my buoyancy in the last few moments of the dive.

Thankfully, neither occurrence happened from an exceptionally deep depth-- maybe 15 feet, as I was attempting a safety stop in both instances-- but no matter what-- dumping air from BC, kicking down, holding on to kelp for dear life-- I seemed to be floating to the surface.

Here are my stats, in case it might help you better evaluate my predicament:
-Male, 180 lbs, 6;3"-- not much fat on my bones
-Exposure protection: 7mm Pinnacle Elite with a 3mm hooded vest
-BC: Scubapro Knighthawk
-Weights: 3 lbs in each of the BC's non-ditchable pockets, 18 in the ditchable pouches
-Cylinder: I believe I was using a Steel 80, although I may be wrong. If anyone here regularly dives the Avalon UWP, I used the tanks they rent on-site.

Maybe I just need to add another couple pounds in the pockets (supposedly the BC can handle 20 in the ditchable pockets), but it seems to me as though 26 pounds of lead is an awful lot for a guy like me.

At any rate, I know I am opening myself to harsh critique by posting this here, but I have found this board to be exceedingly useful in getting me straight on some other issues, so I would appreciate any constructive feedback, advice, or techniques that I perhaps didn't get in my OW class.

Thanks in advance for any thoughts you might offer.

Jeffrey in Los Angeles

 

[BrianV]

Well I noticed almost everyone/every place in California rents steels. Steels are negatively buoyant when full but they still lose air which has weight so you need to plan accordingly. I had a very similar problem diving a Steel 96 at Malaga Cove in California after I got talked into carrying less weight by the LDS.

I'm similar build to you (6'0", 170) not fat but not rail thin either. Using a 3mm hooded vest + 7mm, I'd be wearing 26 lbs with a steel tank, in fact that's what I did with HP 80's most of the time I dove in California. I did 22 with the steel 96 and I was seriously light with 1300 lbs or so.

I know people say not to be overweight (and that's true), but I tend to always carry an extra 6 pounds to compensate for the lack of air at the end of the dive.

I'm not a particular fan of the way the OW class teaches weighting. They teach it with a full tank, and in my case an aluminum full tank. You need to add 4-6 lbs to that to get an accurate number for empty tank.

Run 26 next time and see if the same problem exists; if it doesn't, try 24 the time after that and see if that makes it worse or not. It takes some dives to dial in weighting perfectly. Diving 4-6lbs overweight for one or two dives isn't going to be a big deal, especially if you're not going real deep. I believe the KnightHawk has pretty good lift power as well.

I've got my weight profiles figured out for fresh water/salt water, 7mm/3mm, alum/steel, but it did take some work and a lot of tough safety stop situations.

Good luck!

 

[ReefHound]

Others will be able to explain better but there may be several reasons. It sounds like you ma be overweighted. That's right, because too much weight and you will have to have a lot of air in your BC to offset. The more air in your BC, the more radically your buoyancy will change with depth changes. At 15 feet at the end of a dive, you should need little air in your BC.

 

[Carolina Diver]

Don't forget that your tank is lighter when empty than when full...

It adds buoyancy. You also have that 7 mil on so you might need a couple of pounds. Experiment a little. I always like to be a little heavier than risk being too light. I can always add air or drop weight, but I don't want a runaway ascent or struggle on the safety stop.

 

[BrianV]

Others will be able to explain better but there may be several reasons. It sounds like you ma be overweighted. That's right, because too much weight and you will have to have a lot of air in your BC to offset. The more air in your BC, the more radically your buoyancy will change with depth changes. At 15 feet at the end of a dive, you should need little air in your BC.

I think you misunderstood his question. He's underweighted, with an empty BC he can't prevent ascent at 15'.

 

[Diveral]

Takes a lot of weight to get those 7mm suits down. I believe the difference in weight between a full and empty AL 80 is about 6 lbs. So if you are neutral at the beginning of the dive you will be positive at the end of the dive.

AL

 

[jephre]

Wow! Thanks for the quick responses... It sounds to me like next time out I'll load up my weight pockets to the max and see how that goes.

Bubble bubble...
j

 

[mack50md]

Don't underestimte the effects of neoprene compression in a 7mm wetsuit. Buoyancy jumps up at 1 ata. Likewise, when you compensate with extra lead you will have to watch your decent rate and use a little more air than you would normally like in your BC at depth. Its a compromise in a suit that thick. Just have to find the happy medium.

 

[BrianV]

Wow! Thanks for the quick responses... It sounds to me like next time out I'll load up my weight pockets to the max and see how that goes.

Bubble bubble...
j

Don't make too drastic of a change, try 26 or 28 lbs and adjust accordingly.

 

[BrianV]

Takes a lot of weight to get those 7mm suits down. I believe the difference in weight between a full and empty AL 80 is about 6 lbs. So if you are neutral at the beginning of the dive you will be positive at the end of the dive.

AL

With a 3mm vest on underneath (10mm on the chest). That's what I dive in So Cal as well.