18 replies to this topic

[shadragon]

- and it was only 12 feet deep max, but most of our stuff was done 6 feet or less. Not even 1 atmosphere, so how much can the air expand in 12 feet? I don't know the math.....

The math is easy.

(Depth/33) + 1 = ATA (Absolute Pressure) in sea water

(Depth/34) + 1 = ATA (Absolute Pressure) in fresh water

Assume a 12 foot deep pool. So (12/34) + 1 = 1.35 ATA

So if she left the very bottom of the pool with full lungs (1.0) it would have expended to just over one and a third at the surface (1.35). That's enough to hurt.

A 6 foot rise in a pool to the surface would be 6/34 + 1 = 1.176 ATA. Much lower... (Amended - Thanks Racer184...)

Thanks for the math help! I would never have expected that much expansion in 12 feet. Good to put it all in perspective, and I bet all of us on this thread appreciate these types of discussions, even if only as tough reminders of our training......

I wonder if this can be implemented in the OW course. Fill a small plastic bottle at the deepest part of the pool with your octo. Seal it with the cap and let it go. It probably won't explode, but will be quite nicely pressurized, maybe even deformed once it hits the surface. Or a small balloon maybe. Will try that on my next pool session and see what happens. If a class saw that it would give them a visual reference to remember.

Edited by shadragon, 01 August 2008 - 06:20 AM.
Post split from another thread to highlight excellent info regarding pressure changes in shallow water.

[Dive_Girl]

This is an excellent point you remind us of shadragon. I have had a number of divers who after discussing pressure changes/atmospheres and the need for following dive safety rules underwater will say something to the effect of "oh I know, but I don't plan to dive deep just to 40' or so." I always respond that it's those first 33'+ that cause the greatest % of change on our bodies, so it is exactly the depths you plan to "stay within" that warrant some of the strongest respect of and adherence to dive safety rules.

Watch a diver returning from a decompression dive and you will see them INCH up the last 20'.

[Racer184]

A 6 foot rise in a pool to the surface would be (6/34) + 1 = 1.03 ATA. Much lower

6/34 + 1 = 1.176 ATA Still, I wouldnt want to overinflate my lungs 17%

RE: plastic bottles burst at surface.... nope. The typical 2 liter soda bottle can handle 175 psi (about 12 atmospheres).
Its a good idea.... we just need to find something easy to fill 'at depth' that will burst at 1.5 ATA. Maybe a 'ziplock' bag?
I think it would be fun and more dramatic to start with something that looks 'normal' at 60 ft depth and then bring it to the surface than the typical bring the balloon from the surface down to 60'.

[pir8]

When at the quarry I try to get students while sitting on the surface to look at the bubbles rising from the depths and watch how they expand and break apart

[Bubble2Bubble]

Simon

Or a small balloon maybe

I highly recommend the balloon, I have seen it done before and it provides a very nice visual example of the expanding air. I think it would be a great tool to use in a OW course to drill home the physics of diving.

Mike

Edited by Bubble2Bubble, 31 July 2008 - 06:41 PM.

[peterbj7]

I once filled a screw cap water bottle at 130' and carried it up to the surface with me, having told the student that it would burst at some point. Back on the boat I started to unscrew the cap, and the explosion startled the boat captain over the noise of two outboards, and the cap vanished into the sky and disappeared.

I tell divers to make their final ascent very slow, and typically take over a minute from 15' to the surface. It's the most important part of the dive from the point of view of gas expansion.

Some people think you can only get a gas embolism if you hold your breath, or perhaps if you're suffering from congestion (so shouldn't be diving anyway). But that's not the case. Some people have sticky surfaces to the alveoli in their lungs, and without any obvious symptoms may suffer minor and minute gas embolisms as these surfaces adhere to each other. The slower you go the greater the chance that the excess pressure will dissipate harmlessly. Smokers are particularly at risk in this way.

[Landlocked Dive Nut]

Smokers are particularly at risk in this way.

Diving is the reason I was finally able to quit that habit last year! I learned how much more risk smoking caused a diver, and that was all the incentive I needed.

10 months and counting! Never going to backslide.

[shadragon]

6/34 + 1 = 1.176 ATA Still, I wouldnt want to overinflate my lungs 17%

DOH! You are absolutely correct. The math IS easy, but finger troubles entering the numbers into calculators complicate things. Thanks for the correction! Will amend the posts accordingly...

[peterbj7]

You know, the calculation of pressures at different depths is vastly easier if you work in metres. I always advise my students to use metres even if they're American. Simply convert depths in feet to metres at the start of the calculation, and back from metres to feet at the end. Particularly apposite in nitrox calculations but also useful with air. I can do most of these calculations in my head at least approximately, without a calculator or any paper.

Edited by peterbj7, 01 August 2008 - 04:32 PM.

[shadragon]

You know, the calculation of pressures at different depths is vastly easier if you work in metres. I always advise my students to use metres even if they're American. Simply convert depths in feet to metres at the start of the calculation, and back from metres to feet at the end. Particularly apposite in nitrox calculations but also useful with air. I can do most of these calculations in my head at least approximately, without a calculator or any paper.

Yes!!! The evil metric system is making inroads into the last country on earth that still uses gallons and miles. mu-hahahahahahahaha....

That is Step One.

Step Two is to get you to start using more of the letter u. Colour, honour, neighbour, etc...

[Landlocked Dive Nut]

Yes!!! The evil metric system is making inroads into the last country on earth that still uses gallons and miles. mu-hahahahahahahaha....

That is Step One.

Step Two is to get you to start using more of the letter u. Colour, honour, neighbour, etc...

We tried adding Metric to our schools for about 2 minutes here. We're damned stubborn, we Americans. Gotta do things our own way, regardless of how the rest of the world does them! Love the evil laugh.....

[Racer184]

Most colleges will tell you that Thermodynamics is the class that convinces most people that they should not pursue a degree in any kind of engineering. For me it was two semesters. I got an A on every test and I was ALWAYS the first one done with every test. They allowed 3 hours for the final exam; I finished in a little over an hour. So many of the other students were concerned with flunking out and kept asking me how I did it.

How did I do it? The instructor taught everything in American units. On all homework, every quiz, every test I did the same thing.

• Convert the stated problem into I.S.O. units
• Solve the problem
• Convert from I.S.O. units to American units.

I.S.O. = International Standards Organization; also known as "the metric system". What seems to be 'extra work twice' the process of converting from English to ISO and from ISO back to English, was always worth the time. I was ALWAYS the first student done with every test.

If you want to lean thermodynamics in the 'English system' you have to memorize several hundred factors, starting with 231 cubic inch to a gallon, 128 ounce to a gallon, 5280 ft to a mile, 12 inch to a foot, 43560 sq ft to an acre, 1 horsepower= 33,000 ft-pound per minute and that 1 pound (abbreviated 1 lb) is 1/32.2 of a slug.

In the I.S.O. system, you learn ONE conversion factor. 4.18 joule per calorie. And you very rarely have to use that conversion. You have now mastered Thermodynamics.

After we get all the spell checkers on computers to alert people they mispelled colour, honour, harbour etc, we need to properly teach the "ABCs".... 5 year olds get most of it pretty quickly, but even 50 year olds only get the first 25 letters correct; they keep mispronouncing zed.

Edited by Racer184, 02 August 2008 - 09:39 AM.

[uwfan]

A 6 foot rise in a pool to the surface would be (6/34) + 1 = 1.03 ATA. Much lower

6/34 + 1 = 1.176 ATA Still, I wouldnt want to overinflate my lungs 17%

RE: plastic bottles burst at surface.... nope. The typical 2 liter soda bottle can handle 175 psi (about 12 atmospheres).
Its a good idea.... we just need to find something easy to fill 'at depth' that will burst at 1.5 ATA. Maybe a 'ziplock' bag?
I think it would be fun and more dramatic to start with something that looks 'normal' at 60 ft depth and then bring it to the surface than the typical bring the balloon from the surface down to 60'.

Not that I was holding my breath...but I was glad to use one of the Cochran computers from WW on my last dive trip. I kept thinking I was going REALLY slow up as I ascended, but tried to stay in the safe range all the way up. I've since learned that the ascent rate I knew from 12 years ago of 60 ft per minute has been changed to 30 ft per minute. My scuba review didn't bring that to my attention. When did this change?

[Landlocked Dive Nut]

I've since learned that the ascent rate I knew from 12 years ago of 60 ft per minute has been changed to 30 ft per minute. My scuba review didn't bring that to my attention. When did this change?

I was taught 30 ft per minute ascent in May 2006 in my OW SSI course.....so it's been in place at lease 2 years.

[peterbj7]

I teach 60ft/minute below maybe 60ft, 30ft/minute up to say 20ft, and 15ft/minute or slower from there to the surface. It's not about the rate of depth change but the rate of pressure change. I'm not sure when the world other than PADI changed from 60ft/min, but I bought a computer in 2001 that had an ascent rate warning at over 7 mtr/min. It's a long time ago.

Similarly, the PADI tables say that you're clear of excess nitrogen 6 hours after any recreational diving no matter how intensive, whereas most computers still penalise you up to 24 hours and some go to 48 hours or even longer.

PADI's teaching is way overdue for updating.