Difficult to breathe upward

[jonnythan]

Originally posted by reefrat
Bob3 is right about the old twin hose regs.
In fact they were the opposite of modern two stage units to breathe off ie. horizontal face down = hard to inhale, horizontal face up=easy to inhale.
The reason is simply that face down the diaphram of the modern reg second stage is slightly lower (deeper) than your lungs and face up the diaphram is slightly higher (shallower).

Then why is it so much harder to breathe horizontal face up than when vertical in the water? Seems, in my experience, to be a combination of the orientation of the regulator, not the water pressure difference due to different depths... it's very easy to breathe when I'm ina head-down veritcal position (head much lower than lungs). It's slightly harder to breathe in a head-up vertical position, but not nearly as hard as when horizontal with my face towards the surface.

I'm generally good with physics but at a loss to explain this... Perhaps it has something to do with the difference in depth between the diaphragm and... the valve? I don't know..

 

[reefrat]

I take your point about the orientation of the regulator second stage!!

The only other thing I can think of that might influence ease of inhalation in various orienations is this-

Any air contained in the second stage is going to exert force up toward the surface of the water (bouyancy). When then diaphram of the second satge is in a vertical position (ie. normal horizontal face forward) this is force would not affect movement of the diaphram one way or the other.

When facing upwards the diapham of the second stage would be positioned horizontally and air inside the reg would push the diaphram up toward the surface and away from the demand lever.
The diver would have to inhale harder to pull the diaphram downwards until it contacted the demand lever.

The opposite would happen if the diver is positioned face down!

Whaddya think of that?

 

[landlocked]

That would explain what I have experienced with a number of different regs.

 

[jonnythan]

I'm not quite sure I buy that explanation... when it's horizontal, there will be no force of air pushing either way on the diaphragm, and it doesn't freeflow. When the diaphragm is pointed downward, there's no air pushing up on it from underneath, yet it freeflows.

For this to work, the reg would need to *normally* freeflow, but be prevented when diaphram-up by air inside, since there is no air on the water side of the reg to push up when diaphragm-down.

The reason the reg will freeflow when inserted diaphragm-down into the water is that the interior is still filled with air at atmospheric pressure while the mouthpiece is out. The water pressure on the other side of the diaphragm will be higher, and hence it will depress and cause a freeflow.

This doesn't explain why it happens at depth, though. With the reg diaphragm-down, there should be equal pressures on either side of the membrane, right? Surely the water pressure from a 1mm difference isn't enough to crack the reg.

So what is making it freeflow?

 

[DivingDoc]

I have a new SP MK25/S600. Tried it out in a SW pool yesterday. Breathes great! Tried out a few different positions underwater -- did summersalts, looked up -- left-right -- swam horizontally face up for some 10 feet or so.

The thing I was happy about was that it breathed dry in all positions, which I understand can be a problem with some regs. But I did notice that it breathed harder when looking up and when I was horizontal face-up. Not really hard -- still breathing no problem, but not quite as effortless as in more normal positions.

 

[DPhantomas]

Well, I own this system and tried the same face up position. It breaths totally dry, but that's true, it breaths harder on this position. I tries it at 10, 20 and 30 metres.

 

[landlocked]

Originally posted by jonnythan
I'm not quite sure I buy that explanation... when it's horizontal, there will be no force of air pushing either way on the diaphragm, and it doesn't freeflow. When the diaphragm is pointed downward, there's no air pushing up on it from underneath, yet it freeflows.
...So what is making it freeflow?

(If I am following this thread correctly.)Lack of air pushing the diaphram out (reg diaphram down with air rising to the mouthpiece) makes it easier to breath and easier to free flow. Air pushing up on the diaphram (reg diaphram up with air under it trying to rise) would mean that you have to inhale sufficient to pull the diaphram in and to overcome the pressure of the air trying to rise.

 

[sharpenu]

The air contained inside the reg does not try to rise. In order to rise, the gas and it's container must be lighter than the water it displaces. Remember that the gas is contained in the reg.

To answer this, refer to the excellent drawings on this page:

http://www.howstuffworks.com/scuba1.htm

The intermediate pressure is set by the first stage at a value about 135 to 165 psi above ambient. When you breathe from the second stage, the pressure change gives you the air.

The reason it is harder on your back is the relationship between pressures. When you are on your back, the difference between the high and low point of the lungs is around 5 inches. When vertical, it is around 13 inches. That changes the pressure differential. That is how it was explained to me, but if you want more detail, it will have to wait for my day off. (When I have more time)

 

[landlocked]

Originally posted by sharpenu
The reason it is harder on your back is the relationship between pressures. When you are on your back, the difference between the high and low point of the lungs ..... but if you want more detail, it will have to wait for my day off. (When I have more time)

If i had anything to do with the position of the lungs in relationship to the lungs then there would be a large difference in the breathability of the reg in the vertical vs. horizontal positions. My experience dosen't support that. It seems to me that the reg becomes harder to breath anytime the diaphram is pointed upwards. It dosen't matter whether my lungs are above or below. Next oportunity, I will do some specific field tests to be sure.

 

[DMAlan]

Like srkdvr I have a Scubapro MK25 with S600 and never have difficulty breathing at any angle. It really is a top class reg.