BCD, weights, and buoyancy control questions

[blac86]

So I've adapted a freediving weighting philosophy to all my diving. I am weighted so that I am neutral with a full breath, empty bcd and empty tank at 15 ft/5m. This makes me just barely positive on the surface with a full tank. I use my freediving duck dive to descend and once I break about 10ft/3m I am negative with a full tank. This means even with an empty tank I am able to maintain neutral bouyancy at my safety stop depth. I use about 2/3s of the weight I was using before adopting this weight philosophy.

If I have to do a negative entry I increase my weight for that one dive only, but I find doi g a duck dive and straight down swimming I can descend at apace equal to someone who is weighted negative at the surface with an empty bcd.

 

[Neilwood]

The way I try to look at it is that the weights and BCD are the "rough elements" in becoming neutral and the breath control is the fine tuning.

In rough terms the weights are to offset the buoyancy of your kit (and possibly you), the BCD is to compensate (hence the name) for the weight of the air you will use and the compressibility of your wetsuit (or air in your drysuit).

Once the weights are "dialled in" then your lungs become the fine control. The problem with this (and also the good bit of this) is that using your lungs has a delayed effect on buoyancy.

Why is that a good and bad thing?
Bad: because you need to learn that in order to move with breath you need to allow for a delay between the breath and the movement. Depending on the volume of the breath and speed of the breath that could be a second or two.
Good: once you get the hang of the delayed movement, you can use that to flatten out the up/down motion. You learn how to time it to minimise the vertical movement. That is why you see the better divers moving so little - they are making the adjustments in breathing ahead of when the buoyancy change happens.

The more you dive, the more you will find that you take weight off your gear as you become better at adapting with your lungs.

 

[stuartv]

The whole idea is to be neutral as you describe, with an empty wing.

and a nearly empty tank.

An AL80 holds about 6# of gas, so with a full AL80 and perfect weighting, you'll be about 6# negative (with an empty wing).

So I've adapted a freediving weighting philosophy to all my diving. I am weighted so that I am neutral with a full breath, empty bcd and empty tank at 15 ft/5m. This makes me just barely positive on the surface with a full tank.

The standard weight check is full breath, full tank, empty BCD, and be at eye level. If you are using the same criteria and just barely positive, then you must be carrying more weight than you would be with a scuba diving weighting philosophy. Or I have misunderstood you...

 

[blac86]

and a nearly empty tank.

An AL80 holds about 6# of gas, so with a full AL80 and perfect weighting, you'll be about 6# negative (with an empty wing).



The standard weight check is full breath, full tank, empty BCD, and be at eye level. If you are using the same criteria and just barely positive, then you must be carrying more weight than you would be with a scuba diving weighting philosophy. Or I have misunderstood you...

Most divers are very negative at the surface. That's why they but air in their bcds. They are neutral at the end of a dive which means they are negative until their tank is empty, and are never positive. I am neutral at 15ft at the end of a dive which means I'm positive on the surface at the end of a dive. I'm like - 0.5lbs negative with a full tank at the start of a dive instead the typical 6-8 or so negative pounds a scuba diver wears. Throughout the entire dive I have less carried weight than a typical diver using "neutral at the surface at the end of a dive" procedures.

 

[stuartv]

Most divers are very negative at the surface. That's why they but air in their bcds. They are neutral at the end of a dive which means they are negative until their tank is empty, and are never positive. I am neutral at 15m at the end of a dive which means I'm positive on the surface at the end of a dive. I'm like - 0.5lbs negative with a full tank at the start of a dive instead the typical 6-8 or so negative pounds a scuba diver wears. Throughout the entire dive I have less carried weight than a typical diver using "neutral at the surface at the end of a dive" procedures.

I assume you meant 15ft/5m (not 15m). And this is based on what exposure protection?

If I'm understanding you correctly, then if you were diving in board shorts and a t-shirt, with a near-empty tank, at 15', you would be around 5# positive.

 

[blac86]

Oh

I assume you meant 15ft/5m (not 15m). And this is based on what exposure protection?

Oh no you are right, that's a typo on my part. I meant ft.

Wet I normally dive a one piece 3mm, or two piece open cell 3mm.

Even dry though using a duck dive descent I've been able to cut almost half my extra weight. I was using 24 lbs plus my rig weight, I'm now using 14 extra pounds.

 

[Gary Nease]

I am certified AOW and PADI Bouyancy, among others. My issue has always been how to readjust buoyancy and trim following changes to hardware (or body fat). I don't have a regular dive buddy, and don't want to delay everyone else's dive when I go next time while they wait for me to dial it in. For example, right now I need to adjust for switching back from heavy rubber split fins to my old blades, hopefully compensating for the lighter drysuit feet by moving my tank down. I replaced my stock 30" LP reg hose with a 36" Miflex to allow for moving the tank down. I am in a class a week from today and want to be properly weighted and trimmed when I start the class.

I've been unable to find a pool nearby that will let me bring in my gear for adjustment, without an instructor having rented a session. My best option seems to be to go down to the most popular dive site in the area where there's always divers getting in or out, and do my adjustments solo. That's been the confidential recommendation from three instructors, and was told people do it all the time. But getting that advice was not easy, and it has been a concern for years, and I assume other divers have had the same issue. So I thought I'd bring it up here for what it's worth.

 

[Diver0001]

TL;DR: Is it normal/preferable to have a fully deflated BCD near the surface? Should I feel confident being able to control my buoyancy with my breathing with it deflated?

Thanks!

The only thing you need to be is in control. All the finesse stuff comes after that.

As your course progresses you will (I assume) gain more control but also be coached so that you can use the BCD and your breathing together. What I usually tell students is that control over your buoyancy is done by using BCD for the "rough cut" and using your breathing for "fine tuning".

Try to think of it like that.

R..

 

[Diver0001]

I recently just made a video about being properly weighted, which in time, you will notice your weighting needs will decrease. This of course is dependent on what exposure suit you wear. Your instructor will be a bigger help than any of us will be,because he will be there with you one on one in the pool, and can help you adjust your buoyancy as needed. In a nutshell, you need to be able to hold neutral buoyancy at any stage of the dive, and preferably horizontal trim (though I would focus more at first on just getting neutral buoyant, then you can shift weight to get the trim), no matter the depth and no matter how much air you have. Here is the video we just uploaded showing you how being properly weighted will allow you to ascend and descend at will. Sometimes you will hear it referred too as having a balanced rig.

Thanks for sharing that video, Bryan. Just one follow up question for you.

I'm not clear how you briefed this before the demo. It's quite clear to me what you're doing but what did you tell the students they were supposed to learn from this? Was it about the effect of trim and buoyancy or was it about the effect of breathing and finning? ... or maybe all of that in one?

You're pulling a lot of different elements into one fairly technical demo, particularly if we're talking about novice divers. What is clear is that you were demonstrating that you can control your position in the water column to a pretty wide degree without using the BCD. I think that's a very valuable lesson and something that, in particular, novice divers need to become aware of.

However, you did it using a stabilizing kick that is normally used for station holding and as a starting point for teaching reverse frog kicking. I have to admit that I've had few OW students over the years who could have reproduced that kick with enough control to do what you were doing. Was it your expectation that the students were to reproduce the result or were you just trying to get the point across about how important breathing is to your buoyancy control?

I'd like to hear your thoughts on this. I like to see stuff like this because there's always something I can "borrow" to improve my own methods (if you don't mind).

R..

 

[TMHeimer]

Triad, I would say it is not normal to have a fully deflated BCD at the surface. I have some air in it, but only enough to keep my head out for a surface swim. The more air in there the more of you that is out of the water, which can slow you down. Fins completely submerged is best for a surface swim.