Problem with back-inflate bcd

[Colliam7]

To start my descent from the surface, I'm upright in the water left shoulder a little higher the right one, push the deflate button, exhale completely and start sinking. But only like about 2 ft, then my descent stops. In order to get any deeper, I have to flip over and actively swim down. Once I get beyond let's say 12 ft, everything is fine and I have no further problems during the dive. I can even easily stay at shallower depths than 12 ft. It seems as if I just have to get beyond this point once. . . . Also when ascending at the end of the dive, I'm not popping out of the water.

Three things come to mind: 1) I presume you are diving a single cylinder. The 911 has 65 lbs of lift, FAR more than you need for any single cylinder. What that translates to is the possibility of small amounts of air being trapped in a rather HUGE bladder. Sure, you may love it, but you are dealing with an enormous bladder, and the possibility of a moving bubble of air that makes precise buoyancy control next to impossible; 2) there is nothing wrong with having to use a little swimming to get from 2ft to 12 ft, IF - and you say this isn't an issue - you have no problems on the ascent, holding a safety stop, etc.; 3) DON'T add more weight. Yes, it may help the issue you describe, but it is completely unnecessary. You might find, with a less 'floaty' BCD, that you don't need anywhere near the weight you currently use.

Is there any kind of 'special move' I can make to make sure there is no air trapped in the bcd?

YES, get a more appropriate single cylinder BCD, with a much less lift, that doesn't have such a big, baggy bladder.

 

[mnjhuz76]

First of all thanks for all those helpful informations!


I'm having no problems with buoyancy as soon as I once was below 12 ft. After that I can easily keep level even at 6 ft and go back down from there without any difficulty. Being able to control buoyancy during the dive that easy was the major point why I fell in love with this bc in the first place.


It's just when I'm going down from the surface (where I have quite a filled bc in order to stay afloat conveniently) I'm struggeling to get beyond a certain depth.


Thing is, I couldn't explain it to myself from a physics pov. When I deflate my bc, exhale and then go down, how can it stop me from going further down at a couple of feet if I don't inflate my bc? Well, the answers I got here led me to a good starting point: Unconsciously breathing/kicking/skulling during descent (most likely a combination of all three).


I also will try the different approach to descent by going heads down finning and using the butt valves, just to see if I suits me more and if it does, why not stick to it?


I'd keep adding slightly more weight in the back of my mind as a last resort.


And only if all of that doesn't work (I have pool-time on Sunday and will report afterwards ), I will return my beloved 911 and replace it with some acme bc


Thanks again folks, you rock!

 

[gypsyjim]

Good luck. It sounds like you have the positive attitude, and are on the right track.

 

[Doc]

You've gotten the best mechanical advice... Understand where air might be, orient various dump valves "up", and pull the cord.

....Since that (stride) is my main way to get into the water, that's one more thing I will check.

As you evolve in equipment, so must you evolve your technique. Time to do some changes. Your "entry method", per-se is irrelevant. What matters is what you do next...how do you then orient yourself to perform all necessary purges of trapped air. (as you were previously advised in the above posts) +1 to Ayisha who boiled that down.

I find that ALL such BC like devices are identical, they are carriers of weight and buoyancy. I find no distinction in my $1600 so-called PSD rig versus my $15 1980 "vintage" Cousteau BC. For this discussion, they're both "bags of air". The driving of our Acura NSX versus the Honda Accord? You're still driving a car. Like accomplished pilots say, "Fly the plane (that you're in)". Adapt through competency of technical expertise.

....Once I get beyond let's say 12 ft, everything is fine and I have no further problems during the dive. I can even easily stay at shallower depths than 12 ft. It seems as if I just have to get beyond this point once.

Yup. Sweet mysteries of life. After 4500+ dives, I have no ability to explain this, but I got D grades in Physics, although I've been judged to have phenomenal buoyancy by my mentors. I dunno.

 

[The Chairman]

It sounds like you have the positive attitude,

Perhaps, he needs a more neutral one!

 

[mnjhuz76]

Perhaps, he needs a more neutral one!

Most definitely

 

[Storker]

But if I were underweighted and it shows already right at the beginning of the dive, wouldn't I have more and more problems the longer the dive takes and especially towards the end shoot into orbit like a missile?

If you're just a little underweighted, you won't do the Polaris missile impersonation if you otherwise manage your buoyancy right. That outcome needs a little more incompetence or carelessness.

When I was breaking in my new DS, I lost control at 18m from being underweighted. Loose fins prevented me from swimming downwards, so all I could do was to exhale as much as possible, ride the elevator and hope for the best. It was an uncontrolled ascent, sure (I couldn't stop it, so it was, by definition, uncontrolled), but the ascent speed never tripped my PDC's alarm (and I use a Suunto, which is known to be pretty anal about ascent speeds, skipped safety stops and short SIs). The only thing that was hurt was my ego

 

[george_austin]

I had the chance to dive a Zeagle 911 for the first time a couple of weeks ago. Right away I fell in love with the concept and bought one.


I'm still struggeling with one thing, though. To start my descent from the surface, I'm upright in the water left shoulder a little higher the right one, push the deflate button, exhale completely and start sinking. But only like about 2 ft, then my descent stops. In order to get any deeper, I have to flip over and actively swim down. Once I get beyond let's say 12 ft, everything is fine and I have no further problems during the dive. I can even easily stay at shallower depths than 12 ft. It seems as if I just have to get beyond this point once.


Also when ascending at the end of the dive, I'm not popping out of the water. I can maintain a perfectly slow ascend-rate. Once on the surface, trying to get down again would result in the same problem as described above.


Before the 911 I used to dive a jacket-style bcd and didn't have that problem. I was told to add 4 lbs with the 911 which I did (otherwise I probably wouldn't start descending at all).


It feels as if there's some air trapped in the bcd that prevents me from sinking any further during my initial descent. Once at a certain depth the water pressure seems to take care of that problem (at least that's my only explanation for not shooting up to the surface at the end of the dive).


Is there any kind of 'special move' I can make to make sure there is no air trapped in the bcd?


Thanks in advance for your help!

Maybe you're more relaxed at the end of the dive and your breathing has stabilized? - plus all the entrained air is out of your suit and stuff?
You didn't say if you're boat or beach diving but what most of us active divers do here when we're doing a GS from the stern is just hit the water and head for the bottom in a nice long purposeful exhale. - using your momentum to counter any floaty issues at the beginning. Granted, you may not be doing this if it's a line descent or something and (caveat) I don't know what the training agencies have to say about this but I'm cheap with my air and at my age, I don't like to waste momentum and work the bigger muscles which take more O2 to operate.

 

[Ayisha]

I will return my beloved 911 and replace it with some acme bc

I'm sure you will grow to love your new BC, whatever it may be. Single tank diving usually calls for a BC with about 30 pounds of lift, so look for something along those lines. 40 pounds and up is for doubles, etc.

All that extra fabric of the 65 pound lift bc just creates more positive buoyancy and traps more air, and makes dumping the air much more complicated than it needs to be. You may think that you don't have any air in your bc, so you're underweighted, but you may just not be able to vent it all that easily. You could reach back and feel it if you can, and you can ask someone to look carefully at how empty your wing is and where the bubble is. We are pretty certain that you're not underweighted because you can stop at 6 feet, as you said, at the end of the dive and still get back down. So we can pretty much narrow it down to trapped air, which is a common issue with huge bladders.

Once you figure out your new bc, you will likely love it more since it won't cause you frustration. Just my 1 cent worth.

 

[NAM001]

Ive not looked to see what the wing lokks like.

try laying face down raise your butt and use the butt vent to start decent. when you feel a steady depth increase occuring, stop venting.