Buoyancy Issues

[neil]

I have taught several divers to hover. And none by using the utterly wasteful fin pivot. I have then lay on the bottom of the pool, in proper trim position and breathe in to try to raise off the bottom. If they cannot, they add 2 bursts in their BC, and repeat. I do this in 4ft of water. I have taken divers from being unable to gain neutral buoyancy, to netural, trimmed, and working on frogkicks in 2, 3 hour sessions.

Me too. I don't even use the words any more. And this takes 5 minutes for the initial explanation, and then they do everything in that position. I thinks it's important for the first intro to neutral bouyancy to be on the first night of scuba training. I teach reg clear and recovery, mask clearing and then immediately into bouyancy control.

 

[ClayJar]

Together (hehe), we arrived at the conclusion that the larger bubble changed volume in response to changes in his grip pressure much more than the smaller bubble, as it was, well, larger.

That doesn't sound right.
All things being equal, if both Cartesian divers are truly adjusted to the exact same bouyancy, then an equal amount of hand pressure on the bottles should control each one similarly. Double the pressure, halve the volume, as an example, no matter what size the bubble is. But, what do I know

Actually, your conclusion is wrong, while your reasoning is correct.

For example, let's say the lightly-weighted cartesian diver has a 2 ml bubble (displacing 2 grams of water) and the heavily-weighted cartesian diver has a 6 ml bubble (displacing 6 grams of water). The two divers are the same size and shape. The lightly-weighted diver is 1 gram lighter than neutral at 1 ata, and the heavily-weighted diver is 3 grams lighter than neutral at 1 ata.

When you double the pressure by squeezing the bottle, the bubble in each diver is reduced to half its original size. At 2 ata, then, each diver is neutrally buoyant. Obviously, any deviation from 2 ata will manifest as a change in the size of the bubble and the resulting increase or decrease in buoyancy, but as it is a matter of ratios, the buoyancy swing in the heavily-weighted diver (with triple the bubble) will be triple that of the lightly-weighted diver.

If you have each diver neutral, and you then instantaneously release the bottles, dropping the pressure to 1 ata, the heavily-weighted diver's bubble will be buoying it up with a force from an additional 3 grams of displaced water, while the lightly-weighted diver's bubble will be buoying it up with a force from only an additional 1 gram of displaced water. With triple the buoyant force acting on it, the heavily-weighted diver will shoot upward at a significantly faster rate than that of the lightly-weighted diver.

 

[Thalassamania]

That doesn't sound right.
All things being equal, if both Cartesian divers are truly adjusted to the exact same buoyancy, then an equal amount of hand pressure on the bottles should control each one similarly. Double the pressure, halve the volume, as an example, no matter what size the bubble is. But, what do I know? quote]We have two “divers” both are neutral. One has one unit of buoyancy and the other has two. The “diver” with two has more “lead” on. As you squeeze the bottle both divers sink, but the diver with two units of buoyancy will be more negative for a give change in pressure (think about it, if you could squeeze enough to add an atmosphere of pressure diver one would sink be ½ unit negative while diver two would be one full unit negative).

 

[neil]

Thal and ClayJar,

Help me out, I'm not understanding why or IF I'm wrong. Here's a really simple example to play with.


A 64 lb. weight with a 1 cu. ft. balloon attached is neutral in sea water.
A 128 lb. weight with a 2 cu. ft balloon attached is neutral in sea water.

(Let's not nitpick about the volume of the weights, we'll use depleted uranium)

If I double the pressure on wt. #1, the bouyancy will be reduced by 32 lbs. or HALF.
If I double the pressure on wt. #2, the bouyancy will be reduced by 64 lbs. or HALF.


The net reduction in bouyancy ratio is the same, no? They are both neutral in example one, and then both negative by the same ratio.

If what I'm reading from you is correct, then every diver smaller than me should have an easier time adjusting bouyancy than I do because their air bubble is smaller.(?)

 

[ClayJar]

A 64 lb. weight with a 1 cu. ft. balloon attached is neutral in sea water.
A 128 lb. weight with a 2 cu. ft balloon attached is neutral in sea water.

(Let's not nitpick about the volume of the weights, we'll use depleted uranium)

If I double the pressure on wt. #1, the bouyancy will be reduced by 32 lbs. or HALF.
If I double the pressure on wt. #2, the bouyancy will be reduced by 64 lbs. or HALF.


The net reduction in bouyancy ratio is the same, no?

That is indeed correct. As you show, they have both lost half their initial buoyancy.

They are both neutral in example one, and then both negative by the same ratio.

The process by which pressure on a non-rigid container of gas changes volume (and thereby affects buoyancy) is described by simple ratios, but the effects of those buoyancy changes (i.e. floating toward the surface or dropping like a rock) are related to the value itself, not any ratio.

If what I'm reading from you is correct, then every diver smaller than me should have an easier time adjusting bouyancy than I do because their air bubble is smaller.(?)

Not exactly. If you're both weighted to be neutral at the end of the dive, a diver smaller than you and wearing the same model wetsuit (in a smaller size) will indeed be dealing with buoyancy changes of a smaller magnitude than you. However, being larger, you could generally assume that your lung volume is greater, which means you have a much larger "window of neutrality" in which you can function merely by adjusting your breathing.

Of course, a small, overweighted diver would be suffering on both counts. By being small, they likely have smaller lungs, reducing the window in which they can control buoyancy just by breathing more toward the top (or bottom) of their vital capacity, and by being overweighted, they have a larger air bubble to deal with. If they were also wearing considerable neoprene, they would have that to deal with as well.

In general, however, the smaller the bubble you have to carry, the easier buoyancy control will be.


Counterpoint:
If you're diving with no neoprene or other compressible gear, the amount of weight you have to carry is only related to your body's buoyancy (which doesn't change) and the tank you're diving. A small person diving the same tank as you will have to use the same amount of BC air to maintain neutral buoyancy. Logically, it would require identical effort to maintain neutral buoyancy, right? Well, that would be the case if lungs weren't involved, but since they are and yours would have more capacity, you would have the easier time, as you could fine tune your buoyancy throughout a larger window. (When you're handling buoyancy well, you don't need to keep it on the knife's edge of perfection. You only need to keep it within the "window of neutrality" in which you can use your lungs to fine tune it to neutral. If you start bumping the top or bottom of the window, you just dump or add a small amount to put yourself back inside.)

 

[neil]

...... we arrived at the conclusion that the larger bubble changed volume in response to changes in his grip pressure much more than the smaller bubble, as it was, well, larger.

Now I understand what you are getting at. Of course I understand all this, I was just aguing the explanation to your friend. The larger bubble does not change volume differently because it's larger. That's what I was getting at. The values of the bouyancy are different, clearly, but that's not what you said. So there.

 

[ClayJar]

...... we arrived at the conclusion that the larger bubble changed volume in response to changes in his grip pressure much more than the smaller bubble, as it was, well, larger.

Now I understand what you are getting at. Of course I understand all this, I was just aguing the explanation to your friend. The larger bubble does not change volume differently because it's larger. That's what I was getting at. The values of the bouyancy are different, clearly, but that's not what you said. So there.

It is not incorrect to say the larger bubble changed volume much more than the smaller one. The formula for change in volume is simply:

deltaV = V₂ - V₁

Now, was I very precise when I explained to him how to apply Boyle's Law and the relationships between the ratios of ideal gas laws and the effects of the magnitudes of the values on buoyancy? Certainly, but I was not intending to teach that entire segment here.

You needn't worry that I am *excruciatingly* precise when actually teaching physics to people. (It's a standard fault of people with chemical engineering degrees.)

 

[neil]

It is not incorrect to say the larger bubble changed volume much more than the smaller one. (It's a standard fault of people with chemical engineering degrees.)

I think that's only partially true purely from the point of semantics, but that's STILL not what you said. I have a couple of degrees too, in music. I don't tell my students that the bass sounds lower because it's bigger. That's a gross oversimplification and only part of the story. I could discuss Pythagoras with them but it wouldn't help. Anyway, it's been fun. What WAS the original topic?

 

[ClayJar]

I think that's only partially true purely from the point of semantics, but that's STILL not what you said.

You do realize that it's impossible to say that what I said was partially untrue and expect me to be able to drop it, eh? I'm certainly smiling as I type this, of course, as I know that I am proving the point that people with engineering degrees are quite often somewhat... er... retentive.

Now, I *was* using plain English to describe the concept, which means I was opening myself up for someone to arrive at an imprecise conclusion; however, I fail to see where anything I said was wrong. As I've indicated, I neither did nor would explain the process and results using a single line about "bigger", but is there anything in what I typed itself that is incorrect? (Note: saying it is correct, but only by splitting semantic hairs, is an affirmation that it is indeed correct. To engineering people, semantic hairs are perfectly valid.)

If I said something that was incorrect, I need to correct it, but the fact that my short "summary" was not sufficient to properly convey the entire conversation is irrelevant. It was never intended to be a substitute for the segment of my conversation which it denoted.

(In the interest of full disclosure, I have never been diagnosed with obsessive-compulsive disorder, nor do I expect that to change. Still, I do possess certain obsessive-compulsive tendencies in varying ranges of moderation. )

 

[neil]

Now, I *was* using plain English to describe the concept, which means I was opening myself up for someone to arrive at an imprecise conclusion; however, I fail to see where anything I said was wrong. As I've indicated, I neither did nor would explain the process and results using a single line about "bigger", but is there anything in what I typed itself that is incorrect? (Note: saying it is correct, but only by splitting semantic hairs, is an affirmation that it is indeed correct. To engineering people, semantic hairs are perfectly valid.)

I may not have an engineering degree, but I know back-pedaling when I see it!!

we arrived at the conclusion that the larger bubble changed volume in response to changes in his grip pressure much more than the smaller bubble, as it was, well, larger.

I still think you gave your friend the wrong conclusion from the Cartesian diver example.

From a scuba instructor's perspective:
If I said what you originally did in a class and I had some wise-guy-paying-attention physics professor in the back row call me on it (yes, it's happened), I'd have to explain it more precisely. Mr. Professor doesn't know squat about controlling his buoyancy, but he knows EXACTY what happens to that bubble and why. My job is to connect the skill with his knowledge of what's happening. Not difficult.
At the same time we have Neil, who can’t add 2 and 2 without a calculator, in the front row. Neil needs to understand the concept sans math, at least at first. Instructor ClayJar just told Neil that larger bubbles are harder to control than small ones. Neil is worried. Neil weighs 200 lbs. Neil IS a big bubble. Neil wishes he had chosen his parents more wisely. Golf sounds nice.
What was the point of telling the class that it’s easier to control the smaller bubble? I’m not sure it’s pertinent, to be honest. In fact, it’s not really true. If we have a 100 lb. woman and a 200 lb. man learning to control buoyancy, it makes no difference. The rates of change are still the same, as we know. They’re going to be adding or dumping air in about the same ratios, with about the same rate of “difficulty”. Don’t nitpick about lungs and wetsuits.

And to make matters worse, I think you need to re-evaluate your methodology: I doubt the smaller Cartesian diver was easier to hold down because it was smaller. Were the eye droppers exactly at the same buoyancy? Was the water the same temperature, the same air content? Were the rubber portions of the droppers equally pliant? Did you weigh the droppers in and out of water, with and without water in them? Did you calibrate your hands? Ha!