Who believes buoyancy is gained by lung volume???

[ScubaInChicago]

I had an argument with a local legend who is also a tech instructor during a DPV class. He stated that by adding foam inside a scooter it would make it more buoyant in the water. I told him that buoyancy has a direct correlation to the volume of water displaced and by adding weight inside, it would actually make it more negative. I finally sucked it up and let him win his argument so I didn't have to hear it anymore.

Later I started thinking about lung volume in the exact same manner and realized that lung volume doesn't cause displacement. It is the expansion of the chest cavity that is displacing that volume of water causing the buoyancy change. The weight of the gas would stay constant on an inhale at it has transferred. If you made an argument based on weight it should be the opposite of what we know to be a fact (i.e. expelling a breath and the weight of that gas not causing you to become more buoyant).

And on to the last point which is more of a question. Counter lungs??? Common enough terminology within rebreather circles. But aren't they countering the displacement of the chest expansion. Is there a perfect 1:1 ratio of chest expansion to the gas volume exhaled into "counter lungs" to keep displacement equal, or is there a slight bit of MICRO adjustment with each breath?

 

[fbk]

I have no idea what you are actually asking but it is the displacement due to chest expansion that is changing buoyancy. The weight of the gas in the lungs is miniscule (a few grams) and can be disregarded and in any case it is the same molecules being tranferred from tank to lungs.

The scooter housing is rigid, anything you add inside it like foam is not changing the displacement only the mass therefore making it less buoyant (by a tiny amount in the case of foam).

A lump of lead has volume so it's negative buoyancy is slightly less than its mass, by the amount of the water of water it displaces. I'm not doing the maths.

Archimedes' principle - Wikipedia, the free encyclopedia

 

[DevonDiver]

Breathing (open circuit) - changes to the lung volume effect the volume of water displaced. Breathing in expands the chest cavity, but has negligible impact on weight (gas consumed). Gas comes from an inflexible container. This displaces more weight of water, increasing buoyancy. Breath out, vice versa.

Breathing (closed circuit) - changes to the lung volume effect the volume of water displaced. Gas comes from/returns to a flexible container (counter-lungs), so the gas volume variance in the flexible chamber also effects the volume of water displaced. The transmission of gas to/from the lungs to the counter-lungs is generally constant - thus buoyancy changes little (a closed loop). Transmission is not perfect; the diver might loss gas from the loop by, for instance, exhaling from the nose to clear their mask. Constant PPo2 (set point) on CCR accounts for the metabolism of gas in the body however. That gas (O2) is consumed from a cylinder, so some change does occur over time. Likewise, some gas may be flushed to change PPHe or PPN as the dive progresses.

Adding foam to DPV - it depends entirely on whether the compartment was 'wet' or 'dry'. If 'dry' (sealed/inflexible) then the foam provides no extra buoyancy (unless the compartment flooded). The weight of the foam marginally decreases buoyancy. If 'wet' the foam displaced water and increases buoyancy.

 

[Dive-aholic]

Foam in a rigid housing will not make it more buoyant.

As for your lung question, chest expansion makes sense. Lung volume causes chest expansion so in a way lung volume does cause displacement. In a CCR you are exhaling into the loop but you also have O2 being added into the loop to keep the ppO2 at a constant level. So there is going to be some adjustment occurring. It would be interesting to see if the water displacement caused by inhaling is equal to the counterlung displacement caused by exhaling.

 

[Bob DBF]

The volume of air in your lungs is what changes your displacement, your chest would not expand if you could not change the amount of air in your lungs. The chest muscles, lungs and brain work togather to control your buoyancy, however the lungs are the variable ballast tanks.



Bob DBF

 

[Frosty]

I bet the local legend is also a boatie of some sort.lots of foam seems to be the way to keep small boats from actually sinking if the hull is breached.

 

[TSandM]

Since lungs can't expand unless either the chest wall expands or the abdominal wall expands, lung volume is quite a reasonable proxy for what causes increased buoyancy with inhalation.

 

[Wolfie_47]

I'm sure I wouldn't be booking any courses from a "Tech Instructor" who had that poor of a grasp on basic buoyancy.

 

[Diver0001]

I had an argument with a local legend who is also a tech instructor during a DPV class. He stated that by adding foam inside a scooter it would make it more buoyant in the water. I told him that buoyancy has a direct correlation to the volume of water displaced and by adding weight inside, it would actually make it more negative. I finally sucked it up and let him win his argument so I didn't have to hear it anymore.

Later I started thinking about lung volume in the exact same manner and realized that lung volume doesn't cause displacement. It is the expansion of the chest cavity that is displacing that volume of water causing the buoyancy change. The weight of the gas would stay constant on an inhale at it has transferred. If you made an argument based on weight it should be the opposite of what we know to be a fact (i.e. expelling a breath and the weight of that gas not causing you to become more buoyant).

And on to the last point which is more of a question. Counter lungs??? Common enough terminology within rebreather circles. But aren't they countering the displacement of the chest expansion. Is there a perfect 1:1 ratio of chest expansion to the gas volume exhaled into "counter lungs" to keep displacement equal, or is there a slight bit of MICRO adjustment with each breath?

Good question. I don't dive with a rebreather but my feeling is that they must cancel out.

As for your DPV legend. There are two ways of affecting buoyancy, as we all know, by changing the volume of the container (like how a BCD works) or by changing the weight of the container (like how a submarine works).

The DPV is essentially like submarine in that regard. If you fill up the empty (air) space with something like water, which is heavier than air, then it will be more negative than when there is air in it. So if you replaced air spaces with foam, which is slightly heavier than air, then it will *actually* cause the DPV to be slightly more negative as opposed to being more positive. The difference may be very marginal (a fraction of a gram) and hard to notice but in theory since foam doesn't float in air then that's what will happen.

R..

 

[Foxfish]

As for your DPV legend. There are two ways of affecting buoyancy, as we all know, by changing the volume of the container (like how a BCD works) or by changing the weight of the container (like how a submarine works).

It is more correct to say you affect the buoyancy of a body by changing its density. Density = Mass/Volume. You can change the volume of the BCD by filling it with water but it won't float. If the density of the body is less than the liquid it displaces it will float. By adding air to a body that is free to expand the volume increases with only a marginal gain in mass so the density decreases. Hence the BCD floats. In the case of a submarine water is released and replaced by air. The volume remains the same but the mass of the body is reduced so again density reduces.

There is no doubt that a diver neutrally buoyant before taking a deep breath will ascend. Try it if you are not convinced. By breathing in air the divers density is reduced. Their volume increases with only a minimal increase in mass.