Ascending/Decending/Buoyancy

[crofrog]

This is a "Basic Scuba" forum; technical diving is not the rule here, and discussing the needs of a technical diver in a basic forum can lead to a basic diver using technical techniques inappropriately. Keep this in mind when you post here.

I think the same should be said for describing how things were done a half-century ago...

And also as advice for newer divers, I think it can lead to problems when they don't use their BC enough to get buoyant, but rely to heavily on their breath, especially if they're overweighted, but also if they're not.

I've seen new divers experience CO2 build up when using their breathing for buoyancy vs adjusting their BC to obtain neutral buoyancy with a normal tidal breathing in the mid-range of lungs vital capacity. When they're constantly at the end-range of the breathing and are either doing a full-inhale and hold to prevent sinking, or a full-exhale and hold to prevent rising.

 

[johndiver999]

Buoyancy control is balancing an unstable equilibrium. It requires a certain feel, that is why it takes time to learn. It is important to breathe efficiently, so excess manipulation of the breathing cycle is not desirable. Where those boundaries lie between two opposing goals of breathing efficiency and buoyancy manipulation are fuzzy and personal and dependent on work load and also duration.

If anything, new divers fiddle with the bc more than necessary not less.

 

[steinbil]

If anything, new divers fiddle with the bc more than necessary not less.

When that is the case, I would guess it is because of over-compensation by not anticipating the delay in effect. In other words, it's not that they adjust correctly too often, but they fill the BC too much and dump too much when they adjust.

 

[Doc Harry]

My point is this:
If you're neutrally buoyant at any point in your breathing cycle (let's say half-empty) at 30m/100ft, you will not be neutral at 15m/50ft with the same amount of gas in your wing. Which means you have to change your breathing cadence significantly to compensate, unless you vent gas from the BC.

You're not reading. From my original post:

From there, I spend the rest of the dive dumping small amounts of air from my BC as I consume air and my aluminum tank gets lighter. I am not using the BC to go up or down, just to compensate for the swing in the tank buoyancy. It is, after all, called a "buoyancy compensator."

Once my BC is empty, I'm neutrally buoyant throughout the water column because I'm weighted properly. But then again I don't dive in a thick wetsuit, in which case you're going to be quite overweighted throughout the dive in order to stay down at the safety stop. Which is why I never dive in a thick wetsuit.

 

[johndiver999]

When that is the case, I would guess it is because of over-compensation by not anticipating the delay in effect. In other words, it's not that they adjust correctly too often, but they fill the BC too much and dump too much when they adjust.

If you’ve ever seen a new diver swimming around with the inflator in their hand you know exactly what I mean. Plus too many of them have more lead than needed, consequently a larger bubble to manage and are extra unstable.

 

[steinbil]

You're not reading. From my original post:

From there, I spend the rest of the dive dumping small amounts of air from my BC as I consume air and my aluminum tank gets lighter. I am not using the BC to go up or down, just to compensate for the swing in the tank buoyancy. It is, after all, called a "buoyancy compensator."

I fear that we're really discussing semantics and not actual technique here (and that might be my fault). I don't think you, or any experienced diver, would really try to go from 30m to 15m (or the reverse) without making adjustments to the BC (unless as you say it is empty at the end of the dive). The only thing I tried pointing out is that what you wrote taken to the extreme might sound like "the only adjustment you should ever make to the BC is to account for gas usage". A beginner might get the wrong idea from that, and not grasp that the BC needs different amounts of air to compensate for the same gas weight at different depths. And as you say, small depth changes does not require BC adjustment - but it also doesn't hurt to always have the BC perfectly dialled in, and it's good practice. So I will frequently make small adjustments to the BC whenever I ascend or descend more than 1m/3ft, and I don't see any drawbacks to this.

Once my BC is empty, I'm neutrally buoyant throughout the water column because I'm weighted properly. But then again I don't dive in a thick wetsuit, in which case you're going to be quite overweighted throughout the dive in order to stay down at the safety stop. Which is why I never dive in a thick wetsuit.

Agreed. Again the only thing I would point out is that if your BC is empty at the halfway point, the tanks will get lighter towards the end of the dive, and while you can compensate for that by using less of your lung volume, I don't see any reason to. I find it more comfortable knowing that I can use my full lung capacity even at the end of my dive, and it also gives more room for some air in the drysuit for warmth if that applies.

@steinbil , I think people agree with what you said but if you follow the thread many people pointed out that the OP is a new diver and that we are in the basic forum.

We agree that if you have two stages and dive with twin 18L you may need a bit more gas or even a bigger wing but I don’t think that’s what @Jfinch is asking

I know you mentioned you took OW in twinset. I think most bsac divers will have single cylinders for a while and are likely to start with a single 12 steel (or maybe a 15l)

I know this is answering a comment from quite a few months ago, but since the thread is revived I wanted to clarify for posterity. My point in this thread was not about technical gear or techniques. The only thing I tried pointing out is that the quote below is illogical.

If you have to dump a lot of air to break surface, you are overweighted.

Excess buoyancy at the surface says nothing about weighting. It might say that your BC has an unnecessary lift capacity for your gear. Or it might say that your carrying a lot of gas weight (as in the case with technical gear). How much you have to dump to start descent says something about excess buoyancy, not weighting. Actually, the opposite might be true: If you start sinking very quickly by dumping a small amount of gas from your BC, you are probably overweighted. Especially in a single tank rig, if you sink too easily, without having to exhale and keep your legs still, you're probably overweighted.

 

[Doc Harry]

A beginner might get the wrong idea from that, and not grasp that the BC needs different amounts of air to compensate for the same gas weight at different depths.

Wow. You are seriously misguided. A tank is not compressible, its buoyancy doesn't change with depth. I think it's time for you stop offering bad advice.

Again the only thing I would point out is that if your BC is empty at the halfway point, the tanks will get lighter towards the end of the dive, and while you can compensate for that by using less of your lung volume, I don't see any reason to.

Suit yourself. I only have a couple thousand dives, and I find it so easy to control my depth with breath control that I don't even think about it. You seem to have this bizarre idea that I am huffing and puffing and taking maximal inhalations and exhalations. Nothing could be further from the truth. I'm just breathing normally with only minimally variations in my respiratory pattern.

At the end of the dive, with an empty BC, I go from 70 feet to the safety stop to the surface effortlessly using only breath control. That's the benefit of proper weighting and a balanced rig.

If you can't do that with ease, then it sounds to me like you're overweighted.

 

[steinbil]

Wow. You are seriously misguided. A tank is not compressible, its buoyancy doesn't change with depth. I think it's time for you stop offering bad advice.

I think you need to re-read what I wrote, and think on it. Or better yet, let me state it clearly, so there are no more misunderstandings:

- The gas weight is constant regardless of depth
- The buoyancy needed to compensate for said gas weight is constant regardless of depth
- The amount of air needed in BC to create said buoyancy varies with depth

And just to prevent any potential cheap shots:
If the amount of air needed in BC to compensate for said gas weight is 0 (because the tank is almost empty), you do not need any additional buoyancy, and since there is no air that can expand, it will not vary with depth.

Suit yourself. I only have a couple thousand dives, and I find it so easy to control my depth with breath control that I don't even think about it. At the end of the dive, with an empty BC, I go from 70 feet to the safety stop to the surface effortlessly using only breath control. That's the benefit of proper weighting.

If you can't that with ease, then it sounds to me like you're overweighted.

I really tried to be humble and nice in my comments to you. From the misunderstanding I quoted above I think I'm right that we're probably talking past each other and really are more in agreement then it seems, and I meant it sincerely when I said it might be my fault that we ended up arguing over semantics. I don't know why that makes you feel threatened, but just to make it clear: You are much more experienced then me, and probably a better diver. All of my comments are talking about BC control when there is air in the BC. I agree, if you're neutral with no gas spaces to expand, there is nothing to adjust as you change depth. However, for most of the bottom time there is some gas weight that needs to be compensated for in some way. I have been taught, and would advise other beginner divers, to use the BC for doing this, and to adjust it any time you change depths. That's all.

 

[Doc Harry]

However, for most of the bottom time there is some gas weight that needs to be compensated for in some way. I have been taught, and would advise other beginner divers, to use the BC for doing this, and to adjust it any time you change depths. That's all.

When properly weighted, no one needs to use their BC for routine depth adjustment.
You only need to add air to the BC on initial descent to keep from cratering into the sand.
And you only need to release air from the BC to compensate for the swing in tank buoyancy from gas consumption, and to compensate for wetsuit and air expansion on ascent.

Period. Anything else is bad advice, which you should stop offering.

 

[Eric Sedletzky]

When properly weighted, no one needs to use their BC for routine depth adjustment.
You only need to add air to the BC on initial descent to keep from cratering into the sand.
And you only need to release air from the BC to compensate for the swing in tank buoyancy from gas consumption, and to compensate for wetsuit and air expansion on ascent.

Period. Anything else is bad advice, which you should stop offering.

@Doc Harry
Have you ever tried no BC diving?
You’d be good at it.

A BC actually creates some buoyancy problems on it’s own. Let’s examine why.

Take a diver in warm water with no BC and no wetsuit, swimsuit only. The only thing that will create buoyancy swing is the weight of the air in the tank and the air at any given time in the lungs. Let’s also say that the tank (aluminum 80) is on a simple basket harness or on a non compressible support like a plastic backpack that is neutrally buoyant. The only thing you will need to adjust for is the 5 or so lbs. of air that goes missing during the dive. This is easily done just by lung volume control, without even thinking about it.

Let’s add a BC.
The jacket style BC’s have a lot of inherent buoyancy in padding, which has to be compensated for with extra weight to be able to get down. This will affect buoyancy deeper as the padding compresses (like a wetsuit) at depth. So the BC in this instance has created it’s own problem which it has to compensate for by itself.
Also, getting all the trapped air out of the bladder in virtually all BC systems is impossible with folds and hiding places that it creates another layer of buoyancy problems that also have to be compensated for with more added ballast. So between these two things the diver is adding weigh which would not have to be there if there was no BC.
So the BC really creates a lot of it’s own drama as far as added ballast and compensation.
A lot of people say “Just don’t put air in your BC and see if you can do the dive”.
It’s not the same thing.