physics-why upside down breathing is hard

[halocline]

If I understand correctly, in use there is always air on one side of the diaphragm (inside) and always water on the other. (outside) Enough air enters the case to equalize (along with the spring force?) against the ambient water pressure on the diaphragm. When you inhale, you lower the inside air pressure, which allows the water to push in the diaphragm, opening the lever, etc...

Okay, so why isn't the explanation like this?
When swimming normally, the water is on the underside of the diaphragm, and the buoyant air is above. When you inhale, the air flows upward away from the diaphragm. But when you're inverted, looking up at the surface, the buoyant air is underneath the inside of the diaphragm, pushing up, which makes it more difficult to pull the diaphragm "down" to open the valve.

I'm not sure I understand the influence of the exhaust valve on this, as what I've experienced is an increase in inhalation, not exhalation. I'm sure I'm oversimplifying!

 

[rndboulder]

Correct.

The pressure delta between the second stage diaphram and the diver's lungs is of no consequence... unless the diver has an unusually long neck.

The pictogram from 1962 in the link above is misleading.

As I understand it, this is true for the performance of the regulator by its self but if you look at the diver and the regulator as a single system the position of the regulator(Air Chamber) in relation to the lungs does matter. The regulator equalizes to the pressure of the surrounding water. So ostensibly the lungs being under a different pressure than the air in the regulator would, I believe, affect the work of breathing. If the regulator is above the lungs it is harder to inhale and if the regulator is below the diver then inhalation is easier. The lungs after all work based on pressure differential to begin with. It takes more work to inhale from a low pressure vessel than it does from a high pressure vessel. Is this not the case? -Ryan

 

[Greg Barlow]

As I understand it, this is true for the performance of the regulator by its self but if you look at the diver and the regulator as a single system the position of the regulator(Air Chamber) in relation to the lungs does matter. The regulator equalizes to the pressure of the surrounding water. So ostensibly the lungs being under a different pressure than the air in the regulator would, I believe, affect the work of breathing. If the regulator is above the lungs it is harder to inhale and if the regulator is below the diver then inhalation is easier. The lungs after all work based on pressure differential to begin with. It takes more work to inhale from a low pressure vessel than it does from a high pressure vessel. Is this not the case? -Ryan

DA Aquamaster's explanation of case fault geometry is correct. Manufacturers tend to ignore this important aspect of regulator design. Given the CFG designs of most regs, I believe that we have pretty well reached the zenith of reg performance. Sure, there will be slight modifications, but the design parameters have pretty well been developed to their potential.

Your ideas about the pressure differential with regard to second stage diaphragm depth as related to the lungs is also right. This is the primary reason why double hose regs do not offer the inhalation work of breathing that a high performance single hose model can provide. I collect and ocassionally dive with double hose regs and can attest to this act of physics. While in the horizontal swimming position, the pressure sensing diaphragm is in shallower water than that of the centerline of the lungs. This creates an increase in the effort. If one rolls onto one's back, the reg now goes into a positive pressure mode. When one considers that a foot of water excerts around 0.45 pounds of pressure this is a point that can't be ignored.

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

 

[halocline]

I'm not sure I understand the influence of the exhaust valve on this, as what I've experienced is an increase in inhalation, not exhalation. I'm sure I'm oversimplifying!

Never mind, I figured it out. The pressure required to open the exhaust valve is what determines the pressure on the inside of the diaphragm to a certain extent. IOW, the interior pressure in the second stage is not EXACTLY equal to the ambient water pressure. Looking down, it's a little lower, looking up, a little higher.

 

[miketsp]

On the original question about breathing in all positions, there is a US Divers Octopus in the form of a yellow disk (about 1 1/2" thick, 2 1/2" diameter) with both the hose and the mouthpiece on the edge of the disk, which is preferred by several wreckies I know. They tell me that they change to their Octopus when navigating tortuous passages in wrecks because it doesn't matter what attitude they get into, the breathing force hardly changes.
I don't know what the model name is but it's quite a common octopus and many people have it without being aware of this feature.