Hard Breathing

[Luis H]

The differential pressure causes by the water column is the important factor here (as described by Captain, Nemrod, etc.).

The buoyancy of air is not relevant. I will try to explain.

A gas like air in itself (for the purpose of this discussion) is not affected by gravity (since we are dealing with small columns of air). Air will fill the container it is in applying the same pressure in ALL directions. This is the case even if that container is underwater (like a regulator second stage). You can rotate that container and the air pressure on all its surfaces (including the diaphragm) is the same.

The air inside a second stage is not aware of what direction is up or down, as I mentioned it pushes in all directions with the same pressure.

Buoyancy is not caused by air trying to go to the surface; it is the forced caused by a denser fluid that has been displaced by a lighter object, fluid, or gas. If you notice an air bubble rising in the water, it does not have the same tear drop shape as a water droplet falling in mid-air. If you truly understand the physics behind the two different geometries, you may know were I am heading with my explanation.

The air in an air bubble is being pushed up by the surrounding displaced water. The air inside a flexible lifting bag will take the shape of least resistance provided by the bag and the forces generated by the displaced liquid. These forces also involve the pressure differential due to the column of water from the bottom of the bag to the top of the bag.

Water pressure on the bottom of the bag is higher than the top of the bag. All the air inside a lifting bag is at the pressure of the lowest point of air (as in the highest pressure in the water column surrounding the air bag). Therefore there is a pressure differential at the top of the bag between the inside air and the outside water. The balance of the forces is provided by tension forces on the bag surface and that is how the lifting bag will influence the shape.

Even inside a lifting air bag the air pressure is the same in ALL directions. The air is not pushing any harder on the top surface of the bag than the bottom surface. It is the displaced water pushing the bag up and perhaps the bottom surface of the air if the bag is open at the bottom.

The air inside a regulator second is not exposed to the water directly. The regulator is essentially a rigid container with one flexible surface that can only move in and out. To complete the volume of air associated with the second stage ambient air chamber you need to include the wind pipe and lungs as part of the system. The lungs are not just an air bag since they are surrounded by muscles but the complete air system is still at a single pressure and it is driven by the location of the second stage diaphragm in the water column.

Pressure in liquids and gases is non-directional. That means that it is applied evenly in all directions. If you have a surface at 33 ft of s.w. it is exposed to 2 atm no mater if it facing up, down, to the left, right, etc. That surface can be the surface of the second stage diaphragm. It doesn’t matter in which direction it is facing, the only thing that matters is the elevation in the water column and the pressure at that point of the water column.

All single hose regulators get harder to inhale when you are vertical facing up because of the elevation difference from the regulator diaphragm to your lungs. It doesn’t mater if the regulator diaphragm is facing up (like in most conventional regulators), facing to the side (like in a Poseidon), or facing down (like in an old Scubapro Pilot).

 

[pescador775]

Very good, Luis. Just one quip: the lungs are not "surrounded by muscles". They are enclosed in a flexible chest wall with the muscles situated on the bottom. That is why the lungs are so fragile and why a 2 psi gauge pressure will rupture them. Nature has protected them from external forces but not the opposite. Other than that, good job! However, I don't expect for one moment that this will quell the debate. Another thread, same subject; and around it goes.
Pesky
PS Captain gets honorable mention. Thanks for pitching in.

 

[do it easy]

However, I don't expect for one moment that this will quell the debate.

This is a new angle for me, but I remain skeptical. I think the way to test Luis' argument would be to "measure" the breathing difference from a single hose reg while the diver is vertical, upright and facing forward ("standing up") and while the diver is vertical, inverted and facing forward ("standing on his head"). I think this would isolate the relative difference between the lungs and the reg diaphragm. Would this be a fair test?

I have a feeling that these things are obvious to divers who have used double hose regs.

 

[pescador775]

Would this be a fair test?

Been there, done that. The subject question was fair and I am surprised, as Nemrod alluded, that this aspect of diving physics is not necessarily part of a curriculum. I don't believe the answers which were offered emphasized enough how much harder it is to inhale in the vertical and how much easier to exhale. More so in the "prone" position. If the differences were due to a quirk in the regulator second stage, then these phenomena would not be explainable. However, body position relative to the second stage does explain it, up to a point. I'll offer a WAG, that the human physiology could shed some information on this. The lungs are very flexible as they are lightly suspended in the chest cavity. My guess is that they change shape slightly due to the physical principles so clearly described by Luis. When the diver is vertical, the lungs bulge upward slightly, and when the diver rolls on his back the lungs bulge upward and flatten downward. Apparently, it is more difficult for the diaphragm (a muscle) to draw vacuum under these conditions. The change in shape of the lungs is possible, nay, probable, because the body is composed of fluid which transmits pressure differentials, and the lungs act like a balloon suspended in liquid, (think lift bag). Also, there is the matter of gravity. In the vertical position, gravity causes the human organs to droop slightly offering more room for the diaphragm to flex. This would make breathing easier in the vertical position. These physiological effects are small and probably unnoticable compared to the physics of the pressure gradiant between the lungs and regulator. However, apparently, somebody noticed it.

 

[pescador775]

Common sense:
In the 17th century, Isaac Newton published the "Laws of Motion". One of the tenets had to do with motion and energy. Although unsupported by mathematics, Newton presumed that an object dropped from twice the height would impact with twice the energy. Although Friedrich Leibnitz objected to this and published a refutation, the world at large, including all of its scientists (except for that famous mathematician), believed in the "common sense" of the law. Eighty years later, in a series of stellar experiments, Madame du Chatelet proved that the energy expended on impact was not twice, but four times as great proving Leibnitz correct.

Fifty years ago, Scuba instructors were teaching the "common sense" that the human body was kind of like a box, and that hydrostatic pressure would crush a free diver. However, as free divers continued to push deeper, naturally, more common sense notions replaced the former. The truth is that the human body, including bones and flesh, is transparent to hydrostatic pressure. That is, with exceptions-- the air spaces within the body. Pressure transmitted through the skull travels instantaneously through bone and the soft tissues of the sinus, etc. That is where the buck stops. The pressure in the soft tissue can not go farther, it can only be opposed; so, unless equalized, the tissues swell and bleed causing pain. The pressure which transmits to the lungs has a different effect, compressing the soft organs. The reduction in lung volume causes a change in shape of the internal space forcing some soft organs to be squeezed into the chest cavity. However, this causes no harm. The air spaces inside a Scuba diver have approximately the same pressure as the hydrostatic. However, depending on the attitude of the divers body, the gauge pressure in these spaces, including the lungs, varies a bit form top to bottom. This could cause a change in shape of the lungs.

By expanding on this subject, hopefully, some questions have been answered before asked. As to the physiological explanation of the perceived variations in regulator breathing effort, you will have to decide whether the previous hypothesis is reasonable, or not.

 

[halocline]

Here's the problem; IME it's not the position of the 2nd stage in relation to the lungs that changes the way the reg breathes, it's the angle of the 2nd stage with regards to the surface. For example, I could be vertical head up or upside down, feet up, and my reg will breathe more or less the same. In other words, if I were to "spin" (like a cartwheel, sort of) around the axis of the 2nd stage, the reg's performance would not change much, although the relative depth of the reg vs my lungs would be changing considerably. However, anytime I'm looking straight up, meaning the diaphragm is horizontal in the water, with the dry side underneath and the wet side above, the reg gets alot harder to breathe. It's easiest when facing down.

With regards to the pressure of air against the water being equal on all sides, that one either is too counterintuitive for me to understand or it's wrong. Here's why I think so: if the pressure of the air is the same in all directions against the water, that must mean that the pressure of the water against the air must be equal from all directions, correct? And if that were so, why would bubbles rise to the surface? The only way they can rise is if there is greater pressure pushing them up (the weight of the water they displace) than there is pressure pushing them down.

Or, for the sake of argument, I'll agree that the pressure exerted by the air against the water is the same in all directions, and that's why you say the bubbles are round and not tear drop shape. (although I'd be willing to bet that bubbles rising are not exact spheres) But, there must be another force acting on the bubble to make it rise or it would just stay where it is. This same force would cause a different pressure against the top of a diaphragm when the dry side is down vs the bottom when the dry side is up. At least in my simplistic perception of the situation that's what makes sense to me.

 

[Luis H]

Very good, Luis. Just one quip: the lungs are not "surrounded by muscles". They are enclosed in a flexible chest wall with the muscles situated on the bottom. That is why the lungs are so fragile and why a 2 psi gauge pressure will rupture them. Nature has protected them from external forces but not the opposite. Other than that, good job! However, I don't expect for one moment that this will quell the debate. Another thread, same subject; and around it goes.
Pesky
PS Captain gets honorable mention. Thanks for pitching in.

Thanks for the correction. As I mentioned on another thread of similar subject, human physiology is not one of my strong subjects. I am always trying to expand my medical knowledge, but I am actually an engineer (my specialty is basically applied physics). I may have some opportunities to expand my knowledge of human and/or animal physiology as I am exploring a career change as an engineer in the medical or veterinary instrumentation industries.

Correct me if I am wrong, but isn’t the rib cage covered by muscles? Also the rib cage is a semi-rigid structure.
I understand the lungs are basically flexible sacs filled with millions of sacs (alveoli) and the interconnecting bronchi. Therefore, the flexible lungs have to be suspended in the thoracic cavity by a vacuum (between the lungs and the pleural membrane) to hold their shape inside the cavity. That is why a penetrating wound into the chest (pleural) cavity will break this vacuum and allow a lung to collapse: causing the so called “sucking chest wound”.
According to one of my EMS books: “the respiratory system includes the diaphragm, the muscles of the chest wall, and the accessory muscles of breathing, which permit normal respiratory movement.”
I am also just a basic EMT so the extent of my knowledge of physiology and its interaction with basic physic just about stop here.

I guess my point is that the basic physic are relatively simple compared to the human physiology.



do it easy
Any attempts to a truly significant “measurement” will require instrumentation. It is a well known fact among engineers and scientist that humans are the worst possible measuring device around. Among many other handicaps, our perception is heavily influenced by many sensations that we are not capable of isolating from the desired measurement. That is not to discard the importance of our sensation. After all that is often the bottom line: “does it feel right or not” may be the most important criteria at time.

BTW the comparison mentioned above of the three types of second stage geometry (diaphragm up front, diaphragm on the side Poseidon, and diaphragm facing the chin Scubapro Pilot) is my interpretation based on my memory of actual test reports that I have read in the past.



Pescador775
“However, I don't expect for one moment that this will quell the debate. Another thread, same subject; and around it goes.”

I totally agree with that statement.

 

[Luis H]

Here's the problem; IME it's not the position of the 2nd stage in relation to the lungs that changes the way the reg breathes, it's the angle of the 2nd stage with regards to the surface. For example, I could be vertical head up or upside down, feet up, and my reg will breathe more or less the same. In other words, if I were to "spin" (like a cartwheel, sort of) around the axis of the 2nd stage, the reg's performance would not change much, although the relative depth of the reg vs my lungs would be changing considerably. However, anytime I'm looking straight up, meaning the diaphragm is horizontal in the water, with the dry side underneath and the wet side above, the reg gets alot harder to breathe. It's easiest when facing down.

With regards to the pressure of air against the water being equal on all sides, that one either is too counterintuitive for me to understand or it's wrong. Here's why I think so: if the pressure of the air is the same in all directions against the water, that must mean that the pressure of the water against the air must be equal from all directions, correct? And if that were so, why would bubbles rise to the surface? The only way they can rise is if there is greater pressure pushing them up (the weight of the water they displace) than there is pressure pushing them down.

Or, for the sake of argument, I'll agree that the pressure exerted by the air against the water is the same in all directions, and that's why you say the bubbles are round and not tear drop shape. (although I'd be willing to bet that bubbles rising are not exact spheres) But, there must be another force acting on the bubble to make it rise or it would just stay where it is. This same force would cause a different pressure against the top of a diaphragm when the dry side is down vs the bottom when the dry side is up. At least in my simplistic perception of the situation that's what makes sense to me.

I never said that bubbles are spheres. They are not.

I did say that there is a differential pressure in the water column. We all know that the pressure increases directly proportional to depth in a fluid/water column. But the pressure inside an air filled underwater habitat is the same in the entire habitat (to the opening at the moon pool at the bottom) even if it is several floors in height (as some of Cousteau's Conshelf habitats).

Please reread my original post more carefully. I understand that my explanations of physics are not always the most clear (my wife reminds me of that at times), but in general they are based on basic physics and for the most part I can normally back it up if I really needed to. A message board is not the easiest tool to try to explain something.

 

[pescador775]

Yes, Luis, the rib cage expands by means of the intercostal muscles. So, muscles are needed to create a vacuum for inhalation purposes, the diaphragm and the flexible rib cage. I guess I left you hanging there. The details, however, are irrelevant except to someone who might have knowledge of how shape change affects the interaction between the lungs and these muscles.

It can be easily demonstrated that water pressure is transmitted directly through the rib cage as if it were not even present as a physical object. It is a known fact that the human body is transparent to sound---in water. Sound is a pressure wave. Whether muscles or bone are present is not open to question, just beside the point. An underwater explosion can destroy the lungs of a diver but have no particular effect on the rib cage. What happens is that the lungs are battered like the clapper inside a bell.

 

[Greg Barlow]

Luis is correct in that the pressure differential between the reg's diaphragm and the lung tissue is the primary reason why regs breathe the way they do in varying positions. This is the reason why it is so tough to design rebreathers that offer both easy inhalation and exhalation.

I am very familiar with the engineering ability of Luis. He has done a great service to vintage divers by developing his Phoenix system for yesterday's double hose regulators. One of the lesser known benefits of this unit is that it places the pressure sensing diaphragm closer to a diver's lungs. This, in of itself, reduces the pressure differential and permits easier breathing.

When DA Aquamaster has discussed "case fault geometry", this is in reference to the position of the exhaust valve in reference to the center of the diaphragm. If the measured distance between the pressure balanced diaphragm and the exaust valve opening is 1.5", then the cracking effort of the poppet valve cannot be less than that or the valve will leak when the valve is at the highest point in the water.

I did have one full year of university level human anatomy and physiology. I also worked in an urban ER for three years. This only made me realize how utterly complex the human body truly is. While I understand the respiratory system's basic physiology, I cannot intelligently comment on such matters. I can tell you that even lung tissue must obey the laws of physics, and water pressure differential is the MAJOR reason why regs operate as they do.

Greg Barlow
Former Science Editor for Rodale's Scuba Diving Magazine