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Go Back   SailNet Community > General Interest > General Discussion (sailing related) > Need answer to something I've always Wondered About
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Thread: Need answer to something I've always Wondered About Reply to Thread
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Topic Review (Newest First)
07-26-2013 08:56 AM
smurphny
Re: Need answer to something I've always Wondered About

Thanks for your explanation Minnewaska. I think most us us non-trained divers have considered cleaning the boat bottom or fixing underwater gear with some sort of breathing device. I, for one, never appreciated the possible dangers in so shallow a dive. Will stick to snorkel/holding breath method.
07-26-2013 08:32 AM
Minnewaska
Re: Need answer to something I've always Wondered About

I don't have any desire to make anyone look bad, I just don't want to see anyone get hurt.

Air at the surface is under pressure, which is the reason that air is moved into solution (blood) in your lungs. That pressure is approx 15 lbs per square inch. It's actually the weight of a 1 inch column of air that extends from sea level to the top of the atmosphere that is pressing down on your head. We call that pressure, 1 Atmosphere.

Since water is significantly more dense than air, it only takes approx 33 ft of water to weigh the same 15 lbs, therefore, pressure will double at 33ft beneath the surface. It becomes 2 atmospheres.

When pressure doubles, gas volume halves and when pressure halves, gas volume doubles. Boyles Law.

Therefore, air that is compressed by the surrounding pressure at 33 ft (not the scuba tank) will double as you ascend to the surface. Gas compressed at 6 ft will expand 18%, which can be plenty to cause damage if you start with a full lung and don't exhale as you ascend. Although, not so much that you will likely feel it along the way.

This is not a problem with snorkeling, as the air you inhale at the surface is first compressed as you dive down and then re-expands to it's original volume, as you ascend. However, even breathing through a hose that extends to the surface will draw compressed air into your lungs at depth. This is both why it would require more exertion to inhale and how one could get hurt if they don't properly exhale as they ascend.

There a lot more to it, but I'll stop there for the sake of this discussion.
07-26-2013 03:12 AM
Fstbttms
Re: Need answer to something I've always Wondered About

Quote:
Originally Posted by Hartley18 View Post
Well, I'm glad they don't say that to tourists taking part in shallow-water "reef dives" available along most parts of the NQ coastline.. the resultant panic would shut down the entire industry.
Wow. The ignorance runs deep in this thread.
07-26-2013 02:05 AM
Classic30
Re: Need answer to something I've always Wondered About

Quote:
Originally Posted by Fstbttms View Post
You can embolize coming up from just a few feet. It doesn't matter if you are scuba diving at 100' or cleaning a boat bottom. It's a real danger when breathing compressed air (regardless of the source) and if you don't know what you're doing then you probably shouldn't be doing it.
Well, I'm glad they don't say that to tourists taking part in shallow-water "reef dives" available along most parts of the NQ coastline.. the resultant panic would shut down the entire industry.
07-26-2013 12:32 AM
Fstbttms
Re: Need answer to something I've always Wondered About

Quote:
Originally Posted by Hartley18 View Post
Hang on there just a cotton-pickin' minute.. We're not (and I'm most certainly not!) talking about scuba diving - we're talking about cleaning someone's hull. I don't know many people at all whose boats draw more than around 8ft - and if they do, they can certainly afford to pay a professional diver to do it.
You can embolize coming up from just a few feet. It doesn't matter if you are scuba diving at 100' or cleaning a boat bottom. It's a real danger when breathing compressed air (regardless of the source) and if you don't know what you're doing then you probably shouldn't be doing it.
07-25-2013 09:30 PM
Classic30
Re: Need answer to something I've always Wondered About

Quote:
Originally Posted by Minnewaska View Post
By the time you actually feel the pressure in your lungs, you've long passed the critical point.
Quote:
Originally Posted by scratchee View Post
From my own experience holding my breath from the surface (so no danger of lung damage,) I feel quite a bit of pressure just from swimming, working (scraping the prop, for example,) and just using up my oxygen. Think about that "bursting" feeling you have at the end of a long breath hold while swimming. I'm not at all convinced that I'd notice a dangerous pressure change before it's too late. Those 60 people in 2008 probably didn't either.
Hang on there just a cotton-pickin' minute.. We're not (and I'm most certainly not!) talking about scuba diving - we're talking about cleaning someone's hull. I don't know many people at all whose boats draw more than around 8ft - and if they do, they can certainly afford to pay a professional diver to do it.

The posts I've made here are based solely on my own personal experience and not in any significant depth of water. As long as you're not stupid about this, yes, you simply, routinely, exhale when (frequently) coming up to the surface.. from 6 feet down. If you doubt me, go try it in a swimming pool.
07-23-2013 10:00 PM
scratchee
Re: Need answer to something I've always Wondered About

Quote:
Originally Posted by hopcar View Post
"You can feel the pressure building in your lungs as you rise and you automatically exhale."
I agree with you, it's a danger but not a big one.
Please understand that I'm not just speculating about a theoretical risk. The dangers of lung damage due to breath holding are very well understood and are taught and re-taught during all training having to do with underwater breathing (scuba, surface supplies, etc.)

The Divers Alert Network reported about 60 cases of lung damage from over expansion in 2008 (the last year I could find a report for.)

From my own experience holding my breath from the surface (so no danger of lung damage,) I feel quite a bit of pressure just from swimming, working (scraping the prop, for example,) and just using up my oxygen. Think about that "bursting" feeling you have at the end of a long breath hold while swimming. I'm not at all convinced that I'd notice a dangerous pressure change before it's too late. Those 60 people in 2008 probably didn't either.
07-23-2013 09:17 PM
Minnewaska
Re: Need answer to something I've always Wondered About

Quote:
Originally Posted by hopcar View Post
.....The danger is that if you breath from the bag and then rise to the surface with out exhaling, your lungs will burst.
Quote:
Originally Posted by Hartley18 View Post
........You can feel the pressure building in your lungs as you rise and you automatically exhale.
By the time you actually feel the pressure in your lungs, you've long passed the critical point.

Gas(air) expands as pressure decreases when you ascend, but is not generally a risk of bursting your entire lung like a balloon. It's the little sacs at the end of the system, where the blood-air exchange takes place, the alveoli, that are the concern. These can rupture with virtually no warning feeling whatsoever and introduce infinitesimal unsaturated air into the blood stream, which will not pass through the small capillaries and, therefore, block blood flow. If this happens in your finger, you probably lose feeling in it. If it happens in heart tissue or the brain, you may die. You can't choose where the air embolism occurs.

Air will not likely expand sufficiently from 2 ft below the surface to cause this, but theoretically could. It is a real risk from as shallow as 6 or 8 ft, without proactive exhalation. I didn't want to leave anyone the impression that they would be okay by automatically feeling the need to exhale. In fact, its a natural reaction when you are scared to hold your breath. It has to be trained out of you.

........ former deep wreck rescue dive instructor.......
07-23-2013 12:52 PM
hopcar
Re: Need answer to something I've always Wondered About

"You can feel the pressure building in your lungs as you rise and you automatically exhale."
I agree with you, it's a danger but not a big one.
07-23-2013 02:00 AM
Classic30
Re: Need answer to something I've always Wondered About

Quote:
Originally Posted by hopcar View Post
I also tried this as a teenager. I couldn't inhale at much more than a foot. Tube diameter has nothing to do with it. It's just pressure differential.

"As kids snorkelling on the Great Barrier Reef, we sometimes used a large tough clear plastic bag that was held open at the mouth with a wire frame weighted at the corners and sunk a few feet to the bottom, to provide a few lungfuls more air before we needed to head back to the surface."

This works because the air in the bag is pressurized to the same pressure as the water. The danger is that if you breath from the bag and then rise to the surface with out exhaling, your lungs will burst.
True enough.. but in practice, unless you're a Darwin Award candidate, it doesn't happen. You can feel the pressure building in your lungs as you rise and you automatically exhale.
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