BCD management under an overhang - unintended rising

[CapnBloodbeard]

Hi all,
Not sure if this is the best section of the boards for this query, but here goes.
2nd post regarding my trip to Komodo National Park, Indonesia over Christmas, and there were 2 scenarios I found problematic here. I'll post them separately
I'm a PADI OW diver, some 35ish dives.

One particular dive, there was a large rocky overhang we were exploring underneath for some time. I did find now and then that I'd seem to start rising and I'd become worried that I'd crash into the overhang and I'd be desperately dumping all the air from my BCD - I don't recall if I had been adjusting my BCD at this point; I do know that I sometimes seem to find that by BCD goes from neutral to negative, and I guess I don't have the experience to know if that I'm doing something (ie unintentionally finning up to make me think it's neutral when it's not)....

of course if I had put air in my BCD and 30s later I was about to headbut the o/hang that's self explanatory, but say I wasn't adjusting the air.....any advice in this sort of situation? would the variation of even a metre of two in depth exploring the area be enough to cause the expansion of BCD air causing an unexpected rise, or am I doing something else wrong here, or is there some way to manage these environments that I'm not aware of?

 

[Diving Dubai]

Two main contributing factors. Both biological.

1. Fight or Flight reflex. If anxious (even subconsciously) yoru diaphragm "drops" to give you a greater lung volume (in which to flee - the second part of the reflex is adrenalin) Basically you become more buoyant. So when we say to newer divers that as they become more relaxed in the water they will use less weight - this is one reason


2. Bad breathing technique. This also is a subconscious thing Imagine your lungs as the fuel gauge on a car. the proper range is between 1/4 and 3/4 with a "normal breath" at the 1/2 mark.

Again when stressed you'll breathe in the top half of the lungs - your brain is okay with this - until it forgets, and goes back to normal in which case you're suddenly "negative"


So on descents take time to relax your breathing - Get yourself "neutral and then concentrate on where you are in the exhale. if you make a relaxed exhale - your lungs should be 1/2 full then set you're buoyancy to that.

You'll be surprised how little you need to adjust yoru BCD when breathing properly - for me, I might dump a little gas maybe twice during a dive, and a final tweak at the SS

I'll guess that because of this see sawing more weight was added which exacerbates the issue since you have a bigger bubble to manage

Rule of thumb - with a full tank and a normal breath, the top of your head should be a hands width below the surface. So easy to do a fast weight check before the start of a dive. Your lungs are really good buoyancy compensators so weighting "in the ballpark" is good enough

 

[Subcooled]

You can quickly and accurately compensate for small buoyancy changes with breathing. This will give you more time to fiddle with BCD buttons.

A very common problem is to inflate or vent too much too quickly without waiting for the effect to kick in.

 

[TMHeimer]

All great advice. As my OW instructor said 15 years ago, "small bursts in & out of BCD". And yes, breathing with mostly full or mostly empty lungs. Kick when you need to or want to.

 

[chillyinCanada]

I do my best to dive with little to no air in my bcd. After I have dumped upon descent, I don't add air to my bcd again unless going quite deep within the context of recreational dive parameters. I use my breathing to deal with my bouyancy.

I don't expect you to be able to do that at 35 dives but offer it here as something for you to consider and strive for.

 

[oly5050user]

Do you own this bcd or rent? I ask as a rental does not usually get rinsed properly post dive and salt and mineral deposits can cause inflator button to stick.

 

[Doc Harry]

Never let your attention wander from your breathing.

In flying, the priorities are as follows (even in emergencies):
Aviate (fly the damn airplane)
Navigate (watch where you're going)
Communicate
Checklists

People crash airplanes when they get distracted and allow "aviate" to drop from the forefront of their attention.

In scuba, buoyancy control is the equivalent of "aviate." Never let your attention wander from your breathing. Buoyancy control is all about breathing, not your BC. You will use less gas if you focus on controlling your breathing, and you won't have the problems that you describe.

The caveat is that you have to be weighted properly for your buoyancy to be controlled by your breathing. So focus on getting your weight dialed in perfectly.

 

[boulderjohn]

So this is relatively shallow water, right? (It makes a difference.)

The first thing you want to ask yourself is how much air was in your BCD. With shallow NDL diving, there should be very, very little. If there is more than that, then you are compensating for being overweighted. My guess is that is the heart of the problem. For every pound (0.5 K) of weight that you do not need, you have to have about a pint (~.5 L) of air to balance it. If you ascend a little, just a little, that air expands, and when it expands, it starts pulling you to the surface.

The immediate solution when you feel yourself rising unexpectedly is to exhale sharply. That should stop the ascent and even start a descent. That should be all it should take.

You have two buoyancy control devices in play at the same time--the BCD and your lungs. On an NDL dive with a light wetsuit, a properly weighted diver will have very little air in the BCD, so the lungs are the primary buoyancy control device. Such a diver should not need to do much of anything with the BCD--the lungs should do the trick. If the diver is overweighted, then the compensating air in the BCD becomes the primary buoyancy control device, and the expanding and contracting air in it might be too much for the lungs to overcome. In that case, you need to go add or dump air to control buoyancy.

In OW classes, I do the following demonstration for students in the deep end of the pool (about 3.5 M). I visibly exhale and sink to the very bottom of the pool. I then inhale until I rise off the bottom. I then control my inhaling and exhaling until my head breaks the surface. I then make a big exhale and reverse the process until I am all the way to the bottom. I never touch my inflator. I can go all the way from bottom to top and back down again without using the BCD even if I am overweighted by a few pounds.

In contrast, when I practice technical diving in the same pool with heavy steel doubles, I am very much overweighted, and my BCD requires a bunch of air. In that case, I can only control my depth by breathing over a few feet. Any more than that, and I need to go to the inflator and add air or dump air.

 

[TMHeimer]

I do my best to dive with little to no air in my bcd. After I have dumped upon descent, I don't add air to my bcd again unless going quite deep within the context of recreational dive parameters. I use my breathing to deal with my bouyancy.

I don't expect you to be able to do that at 35 dives but offer it here as something for you to consider and strive for.

Admit that is impressive. I find that I will probably add BCD air at 20-30 feet depth on a shore dive, figuring it makes life easier, I guess. What do you consider "quite deep" recreationally? How deep do you go using lungs only?

 

[chillyinCanada]

Admit that is impressive. I find that I will probably add BCD air at 20-30 feet depth on a shore dive, figuring it makes life easier, I guess. What do you consider "quite deep" recreationally? How deep do you go using lungs only?

Easily 90