Originally posted by NetDoc
Stop the analogies...
I will. Analogies are a useful tool to simplify a system somewhat so someone that having a hard time understanding underlying principles can get a beachead on a concept and then more easily get a grasp on the entire issue.
This, alas, has failed. So I will no longer waste my time on analogies. Pure physics from here on out. I no longer care if you understand, but maybe I can enlighten some of our other gentle readers.
Originally posted by NetDoc
Once the diaphragm pulls down on inhalation the chest cavity goes into a relative vacuum... the lungs (using the higher pressure available at the mouth) fill the void to get rid of the vacuum nature so abhors! There is nothing hydro or static about it and the vacuum generated at the base of the chest cavity will be constant throughout the chest cavity...
The fact that you understand this but dont understand the implications is what has me utterly baffled.
Originally posted by NetDoc
There are no orifices in the chest (as there are in the cranium), no ears, no sinus cavities, nothing that enters or exits through the chest walls.
Wow. No orifices in the chest walls. I dont know about you but I have a trachea, which connects external pressure to the inside of my chest. Thats a hole, just like my Eustachian tube is a hole to my middle ear or the holes that are my sinus passages that pass to my sinuses. The problem that occurs with the ears and sinuses (and lungs) is when the hole is blocked and air cannot flow freely to equalize the delta in pressure between the external environment and the internal gas space. But if the hole is blocked, how does the pressure get to the now disconnected gas space in order to squeeze it? Through all your soft tissues that are at ambient external pressure, thats how.
Theres a book that I think you might want to read some day. The book considers this concept so important that after a ubiquitous marketing pitch, in the very first paragraph on page 9, after the marketing pitch, it states: You usually dont feel the airs pressure, though, because your body is primarily liquid, which distributes the pressure equally throughout your entire body.
Please note the words equally and entire.
This book, which Ill assume youve never read, can be found at most dive shops and I recommend EVERY diver to pick it up and read it (or its equivalent) before, and during the process of learning to dive. Its entitled PADI Open Water Diver Manual.
Page 18 also has something else you might find interesting. It states When you skin dive, the increasing pressure compresses the air in your lungs. During ascent, this are re-expands, so your lungs return to approximately their original volume when you reach the surface. So much for your bone and tons of muscle that keeps external pressure isolated from your lungs.
Actually, this would be an excellent thing to look up because the text accompanies a picture, which might assist your understanding.
Youre darn right Pete, at this point I am talking down to you. I dont care if you hate the messenger, I just hope to gawd I can get the message across to you before you pass this kind of logic onto others. Dealing with one person who doesnt understand the physics of diving is bad enough.
Originally posted by NetDoc
As for your sinus squeeze question
Why do you think you do NOT have to equalize the soft tissue (not said derisively) of your brain??? The brain case does not have any orifices in it! None that are exposed to external pressure.
You most certainly DO have to equalize soft tissue including (not said derisively) your brain. This happens to be another intuitive leap that it appears you havent made yet. Though you do understand it at a superficial level.
Why do you do a safety stop?
To decompress (equalize) your soft tissues that took on N2 during the time they were at elevated pressure, due to the elevated EXTERNAL pressure. Because, to paraphrase the PADI Open Water Diver Manual the liquid in your body distributes the external pressure THROUGOUT your body which includes your brain.
Remember, you asked for no analogies, so do try and keep up...
The increased PPN2 in your breathing gas causes N2 to defuse across the alveoli into your blood, which had a PPN2 equalized at your 1 ATA exposure to air (PPN2 of about .79 ATA). As the blood with its now elevated PPN2 travels throughout the body, the tissues (yes, the soft ones), that are now at an increased pressure, have a PPN2 deficit when compared to the blood thats perfusing them. Therefore the N2 migrates into the soft tissues until their PPN2 equals the PPN2 thats in the blood, which matches the PPN2 thats in the gas youre breathing. Understand this and youll understand Nitrox. Understand perfusion and youll understand why short deep dives or fast ascents more commonly cause neurological DCS and long dives cause the classical bends DCS.
Back to the discussion. As you ascend, decompression stops are inserted if necessary to allow the soft tissues to equalize (outgas) at a rate that the body can tolerate. The safety stop, which youve alluded to so I assume you know about, is a just in case action that allows the soft tissues to equalize at a more leisurely rate, adding a margin for error.
Now, before you go non-linear about how this equalization is not like equalizing air spaces, let me state I agree. That was not my point. The point is, and let me shout:
ALL THE BODYS TISSUES, SOFT OR OTHERWISE, ARE ALWAYS AT AMBIENT EXTERNAL PRESSURE. BE IT THE BRAIN, OR THE MUSCLES, BONES AND CONNECTIVE TISSUE THAT MAKE UP THE CHEST CAVITY.
A PADI Open Water student even before their first class (if they read module 1, as they should have) knows this.
Originally posted by NetDoc
While there might be some compression of the rib cage being semi-rigid (not much from a .5 psi drop), it would not be "hour glass"...
I said V shaped. And I was talking about the lungs, not the rib cage. Im starting to think that youre so comfortable with your conclusions youre not even bothering to read the posts.
Originally posted by NetDoc
You can demean my lack of consensus with your theory as an inability to take your intuitive leap...
Let me make it perfectly clear. My annoyance is a result of your inability to understand basic physical concepts. If you understood the physics that effect a SCUBA diver we could then have an informed debate over how (or even if) the physics are applicable to the concept of progressive alveoli collapse under a pressure gradient.
But you have some deep-seated misunderstandings behind absolutely basic diving physics that's taught in the Open Water class. Maybe Ill change your mind. Maybe I wont. But changing your mind is no longer my highest priority (though it still is a priority). Keeping other readers from going down such a horribly incorrect road of reasoning is.
Originally posted by NetDoc
many took that same leap and called the world flat.
I take great pride in that someone somewhere also called Columbus the Old World equivalent of a "stroke"!
Is there anyone else out there that agrees with me even in part? Or is Columbus sailing to the New World with an empty ship???
Now whos using analogies? The ones taking the intuitive leap thought the world was round. Columbus came back from the New World to people like you who still thought the world was flat.
The brain does not have to be equalized because its not exposed to external pressure. Just how much more can you confused over the difference between incompressible and not exposed to pressure can you get?
Go back and read module 1 of your Open Water manual and come back once you truly understand it. Looks like you didnt the first time through.
Roak
