Diving without BCD?

[TrimixToo]

I'll just jump in to point out that no-BC diving is best done in warm-ish water and/or shallow depths. The reduction in volume for a thick wetsuit can easily overcome the amount of buoyancy differential one can manage with lung volume alone. At the start of a dive, especially, it could be hard to fin up from very deep. And, dumping weight to get up could result in an unsafe ascent speed.

 

[Storker]

Buoyancy swing is the change in buoyancy during a dive. The individual characteristics of the tank have nothing to do with that.

Allow me to disagree. There's one individual characteristic of the tank which determines its buoyancy swing: The capacity. And, of course, how much you breathe it down.

Apart from that little nitpick, you're of course quite right. If a diver has 80 cubic feet of air in a tank and breathe it down to some 13.5 cubic feet (about 500 psi in an Al80), they have removed some 67 cubic feet of gas from their rig, and the mass of that is some 5 lbs. The buoyancy swing will thus be about 5lbs, no matter whether those 80 cubic feet were stored in an aluminum tank, a steel tank or a fiber composite tank.

 

[RayfromTX]

I dove with a friend not long ago in warm water. He had long practiced diving without a bc at depth and in a rash guard with a 3 lb weight zip tied to the top of his harness. On this dive he had on a big old back inflate bc and some other vintage gear from the 70s. He was using three second stages because his primary was being tested at depth for the first time since 1972. It breathed really nicely btw. When one member of our group went below our planned depth of 135 down to 165, he went down and signaled to them and brought them back up to us and we began our ascent. It was only later on the boat that he showed me the gaping hole in the inflater that let me know that he had used the bc only as a means of securing his tank to himself. He could have added air to it and provided some lift but he did not. He told me in previous conversations that he has a rather large lung capacity and 160 feet is the limit that he could go for a prolonged period because at that depth he was breathing so much at the top of his lungs, to maintain his buoyancy, that CO2 retention became a factor.

Please be advised that this kind of diving is not basic scuba and none of it was actually done without a bc but rather with a bc that was not put into use and he had 1,000s of dives and had training and experience at much deeper depths.

A bc is a necessary tool in my opinion because they exist, are readily available and are often practically given away on the used market. I believe that if a person is asking the question of whether a bc is necessary then the answer is absolutely yes. The real answer; however, is it depends.

 

[doctormike]

Allow me to disagree. There's one individual characteristic of the tank which determines its buoyancy swing: The capacity. And, of course, how much you breathe it down.

Sorry, don't understand. What does tank capacity have to do with the weight of the gas that you consume during a dive? If you consume 60 CUF of gas during a given dive, the buoyancy swing is the same whether it comes from an AL80 or a set of double steel 130s.

 

[RayfromTX]

He is referring in a round about way to the amount of gas that you USE during the dive which is limited by what you are able to carry. Air weighs what air weighs. Using 5 lbs of air is a 5 lb swing but you won't have that much swing if your tank only holds 4 lbs of air to start with. English isn't his first language even if he does use it better than me most of the time.

 

[Storker]

What does tank capacity have to do with the weight of the gas that you consume during a dive? If you consume 60 CUF of gas during a given dive, the buoyancy swing is the same whether it comes from an AL80 or a set of double steel 130s.

Well of course. We don't disagree at all here. Buoyancy swing depends only on the amount of gas you've used during the dive.

My somewhat tongue-in-cheek comment was in relation to the fact that the higher the capacity of the tank, the more gas you can use. So the tank capacity enables you to have a larger buoyancy swing. But it all depends on how much you breathe it down, of course.

EDIT: Ninja'd by @RayfromTX . Thanks, mate.

 

[doctormike]

He is referring in a round about way to the amount of gas that you USE during the dive which is limited by what you are able to carry. Air weighs what air weighs. Using 5 lbs of air is a 5 lb swing but you won't have that much swing if your tank only holds 4 lbs of air to start with. English isn't his first language even if he does use it better than me most of the time.

Don't apologize for his English, he's a smart guy in any language and a great mod (as are you!). But this makes no sense.

If the question is "do tank characteristics affect buoyancy swing during a dive?", then the answer is simply no, with no qualification.

If you are looking for the effect of a variable on an outcome, you can't change another variable for one case and not the other. That would be like saying that marine biodiversity affects buoyancy swing, because if there are more interesting fish to look at on a dive, I'm likely to dive longer and use more gas.

 

[RayfromTX]

I'm definitely not going to get into any kind of argument with Storker about the scientific method or the language used to describe multiple variables. It's kind of his thing and will always be a case of misunderstanding, mostly by me.

 

[Compressor]

To the original OP, when you enter the water, you enter an environment where you are foreign since you are a land mammal.

You can dive without a BC? Yes, but why? Save money ? Not worth it.
You can dive without an SPG and use a J-valve? Yes but why? Save money? Not worth it.
You can dive with one 2nd stage and buddy breath if one of you is out or low on air? Yes but why? Save money? Not worth it.

Most dive equipment today has matured and made diving safer and more enjoyable. The enjoyment you and your dad gets from diving safely and in comfort is priceless. Worth it|? Absolutely yes.

 

[johndiver999]

Now that we have once again agreed that 5 lbs of air weighs 5 lbs; possibly a more relevant issue about tanks and no bc diving could be mentioned. In general the old school diving was done with smaller tanks and those tanks were not near as negative as some large hp modern tanks.

The old school method generally required the carrying of some ditchable lead. If you try to emulate the old school
Practice and use a heavy steel tank, you may be required to eliminate any ditchable ballast. In my opinion, this would be adding a significant risk - that should be avoided if possible.