Single vs two-hose image questions?

[Mekotronix]

We are not wrong.

Just to make sure there is no misunderstanding, I was decidedly (though perhaps not clearly) NOT claiming all of you were wrong. It was intended as a "I don't really understand why this is the case, but y'all have way more experience in your pinky toe than I have in total so I'll take your word for it."

A snorkel will work for a test but a short piece of garden hose will be better. See if you can breath through it at 6 inches of submersion. You will not be able to draw a breath at 6 inches of water column but just barely and certainly not repeatedly as it would be exhausting. If a regulator had a cracking effort of 6 inches it would not sell very well to say the least.

Like many others here I assume, I did the garden hose to the bottom of the lake when I was a kid. (~15 yo) I went down 12-15' though... never tried it just below the surface. I'll have to give it a go.

Sucking water with your mouth is a little bit different than inhaling air into your lungs.

Try this:

Go to a pool and breath at the surface with you body vertical and your head above water. Then try breathing with a snorkel with your head under the surface with your body still vertical. See if you notice any difference in difficulty inhaling. You may need to hang onto a ladder or something to keep from floating to the surface or becoming horizontal.

I will try this (and the exercises suggested by Nemrod) when I get the chance. Nothing like personal experience to drive a lesson home!

The replies I received really got me wondering about the physics involved here. At first glance, a 6" pressure differential to suck water into a straw should feel the same as a 6" pressure differential to crack a 2nd stage reg. (Assuming both volumes << lung working volume. If you reach the end of you lung capacity trying to crack a reg I imagine perceived WOB goes up.) What factor am I missing?

Is it the fact that your lung are submerged and have to counteract the increased pressure in order to fill? But then, isn't that what the pressurized air is for? To increase the pressure in our lungs (and mouth, reg, etc.) as the ambient pressure increases? It's not obvious to me that explains the drastic changes in perception. Seems to me increasing the ambient pressure would more or less be a wash.

Maybe it's that pressure gradients are so much larger in water than they are in air? 10m deep in water puts us at 2 atm. We'd have to stand in a hole ~5.6 km below sea level to experience 2 atm in air. If my calculations are correct, that 6" pressure differential at 10m underwater is roughly equivalent to 840m pressure differential in air at 2 atm.

Huh... I'm not sure what to make of that. Given no amount of vacuum is going to pull water over 64' high (given 2 atm ambient), I'm either way off base with my line of thinking or very surprised anyone could breathe a reg adjusted to 6" at 10m.

 

[Nemrod]

@Mekotronix an atmosphere is 30 (29.9212) inches of Hg, 33/34 feet of water. We cannot have more vacuum than absolute so 33 feet is maximum suction lift of water at standard atmosphere SL above which would be an absolute vacuum or close enough. The 6 inches of water simulating a snorkel near full submersion is .22 psi or roughly 4 to 6 times greater than what is typically found in a good regulator (1.0 to 1.5 inches of water or .05 psi approx.).

James

 

[GJC]

The replies I received really got me wondering about the physics involved here. At first glance, a 6" pressure differential to suck water into a straw should feel the same as a 6" pressure differential to crack a 2nd stage reg. (Assuming both volumes << lung working volume. If you reach the end of you lung capacity trying to crack a reg I imagine perceived WOB goes up.) What factor am I missing?

Is it the fact that your lung are submerged and have to counteract the increased pressure in order to fill? But then, isn't that what the pressurized air is for? To increase the pressure in our lungs (and mouth, reg, etc.) as the ambient pressure increases? It's not obvious to me that explains the drastic changes in perception. Seems to me increasing the ambient pressure would more or less be a wash.

Maybe it's that pressure gradients are so much larger in water than they are in air? 10m deep in water puts us at 2 atm. We'd have to stand in a hole ~5.6 km below sea level to experience 2 atm in air. If my calculations are correct, that 6" pressure differential at 10m underwater is roughly equivalent to 840m pressure differential in air at 2 atm.

Drawing a couple of tablespoons of water from a cup 6" below your mouth is considerably different than trying to inhale 1 liter of air into your lungs with 6" of water pressure against your chest and abdomen. Sucking from a straw is usually done with your tongue closing off the back of your mouth and moving your jaw down to create negative pressure in your mouth. The lungs aren't involved in this.

Modern regulators are made to maintain the cracking effort they are set to and not change with depth. As the absolute pressure changes, they adjust to allow air to release with the same effort regardless of depth. They also have a venturi effect so that once air is moving, there is less negative pressure needed to keep the second stage valve open and continue the inhalation.

Most people can only inhale through a snorkel from the surface to a depth of 12 to 16 inches. Beyond that, the muscles involved in inhalation are too weak or become fatigued very quickly and they can no longer inhale.

With very deep diving, air becomes dense and there is more resistance to airflow, making it harder to move air through a regulator and into your lungs and increasing the work you need to do a breath.

Hope this helps your understanding.

 

[GJC]

duplicate post

 

[Akimbo]

Drawing a couple of tablespoons of water from a cup 6" below your mouth is considerably different

Just to expand on this concept a little: Surface area matters. The surface area of a straw or small hose is nothing compared to the same hydrostatic pressure acting over the entire surface area of the upper half or your torso squeezing your lungs.

 

[Mekotronix]

Drawing a couple of tablespoons of water from a cup 6" below your mouth is considerably different than trying to inhale 1 liter of air into your lungs with 6" of water pressure against your chest and abdomen. Sucking from a straw is usually done with your tongue closing off the back of your mouth and moving your jaw down to create negative pressure in your mouth. The lungs aren't involved in this.

I had to ponder this for a few minutes but I think it finally clicked. I had been focusing on the pressure differential between my mouth and the ambient pressure without considering how that pressure differential was created. Your post admirably illustrated my error to me.

Yeah, I can totally see how putting 32 lbs on my chest (12"x12"x6" of water) increases the work of breathing.

Most people can only inhale through a snorkel from the surface to a depth of 12 to 16 inches. Beyond that, the muscles involved in inhalation are too weak or become fatigued very quickly and they can no longer inhale.

Like I said earlier, I'll have to give this a try next time I'm at the pool. What really interested me was your comment about the breathing muscles being "too weak." I wonder if strengthening your diaphragm and chest muscles would increase the depth at which you could breathe through a snorkel, and if so, is there a theoretical limit? (I don't expect an answer.... just thinking out loud.)

 

[Mekotronix]

Hope this helps your understanding.

And to further hammer home the point, I just did an experiment with trying to suck water through a straw using only my lungs. With a fully stretched out 25" camelback hose I was just barely able to get some water in my mouth.

 

[Mekotronix]

Just to expand on this concept a little: Surface area matters. The surface area of a straw or small hose is nothing compared to the same hydrostatic pressure acting over the entire surface area of the upper half or your torso squeezing your lungs.

I don't think that's a correct explanation for the effect. The cross sectional area of the water column in the straw doesn't affect the effort required to raise the water 6". I could raise a 10' x 10' square of water 6" just as easily as I could raise it 6" in a straw. (Though it would take many breaths due to the volume of air I'd have to remove.)

 

[Akimbo]

I don't think that's a correct explanation for the effect. The cross sectional area of the water column in the straw doesn't affect the effort required to raise the water 6". I could raise a 10' x 10' square of water 6" just as easily as I could raise it 6" in a straw. (Though it would take many breaths due to the volume of air I'd have to remove.)

The issue isn't the cross section of the hose or straw, it is the force of the water acting to compress your lungs. Sucking on a straw is differential pressure dependent. Total force acting against your chest mussels is the differential pressure times the area.

 

[Akimbo]

@Mekotronix

I reread my description above this morning an realized the context of my comments were incomplete. I had the full context of WOB in mind while I think your comment was more focused on the physics of pulling a relative vacuum with our lungs. Ultimately, WOB is about CO2 buildup, not the differential pressure itself.

Yes, it does feel easy to pull a relative vacuum through a tube with your lungs in the 6-12" range. The difference is "feeling easy" when that vacuum is acting on small area is understandably misleading when apply it to WOB.

Sucking or blowing against a differential pressure that is perceived as insignificant can quickly cause a buildup of CO2, which causes that "air starved" sensation. A case in point are the absurdly long snorkels sold as pool toys in the 1950s.

You could find snorkels that were about 3'/900mm long in sporting goods stores and mail order catalogs (the Amazon of the day). These things were plastic tubes with 180° bends at both ends. One end had a mouthpiece and the other had a small cage with a ping-pong ball that acted as a valve when the end was submerged below the surface. The distance between the two open ends was about 2'/600mm.

When you got in a pool with this thing close the "max operating depth" it felt like an elephant was on your chest squeezing the air out of you. Yes, you could inhale, but not for long before going into CO2-induced "panic mode".