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REPOST - question about displacement and pressure

Started by Dive_Girl , Mar 23 2006 12:39 AM

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10 replies to this topic

#1 Dive_Girl

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Posted 23 March 2006 - 12:39 AM

Piz posted this last week, and it was a good one, so I am reposting it here! Thanks Piz!!!

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1. When calculating the extra displacement required to lift an object underwater and you are using an inflexable container like a drum, wouldn't you need to include the water displacement of the drum with the water displacement of the object in your calculation to attain neutral boyancy? this should mean that you will require more air in the drum becausr you need to not only lift the object but you also need to lift you drum as well.

2. Why do you have absolute and ambient pressure as definitions in diving when for what I can understand they are the same thing?

3. Are absolute and ambient pressure REALLY the same thing?

I need a better andswer than "I dunno".


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Once in a while, it is good to step back, take a breath, and remember to be humble. You'll never know it all - ScubaDadMiami. If you aren't afraid of dying, there is nothing you can't achieve - Lao-tzu. One dog barks at something, the rest bark at him - Chinese Proverb.

#2 Dive_Girl

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Posted 23 March 2006 - 12:41 AM

I save this topic because I used it to quiz dive_addict. He supplied the following answers. I take no claim, embarassment, or credit for these answers.

"1. You're correct. On the drum, you'll figure out the amount of positive bouyancy it creates by displacement, then subtract it's actual weight to give you the actual amount of positive or negative bouyancy. This holds true for all objects, including the object to be lifted. Even lead weights create an insignificant amount of positive bouyancy. But because lead is so dense (710 pounds per/cubic ft, I'm a dork and know this off the top of my head), the weight of a cubic foot of lead in salt water would be 710-64 or 646 pounds per cubic foot. Or our 5 pound weight we use for ballast would be equivelent to 4.55 pounds in salt water. So basically what I'm trying to say is that any object that displaces water will create positive bouyancy. So in exact calculations this would need to be taken into account.

2 & 3. Essentially ambient and absolut pressure are the same. But lets say you are in an inflexible container such as a submersible. The ambient pressure in relation to your body is 1 atmosphere, while the pressure surrounding the submersible may be greater depending on depth. In my mind the difference between the two, is that ambient pressure is in relation to a human, where as absolute pressure is in relation to an object.

Although, I am wrong .01% of the time :whistle:
-dive_addict"
It's Winter time - you know you're a diver when you're scraping ice off your windshield INSIDE your vehicle...!

Once in a while, it is good to step back, take a breath, and remember to be humble. You'll never know it all - ScubaDadMiami. If you aren't afraid of dying, there is nothing you can't achieve - Lao-tzu. One dog barks at something, the rest bark at him - Chinese Proverb.

#3 Piz

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Posted 23 March 2006 - 12:58 AM

Yep this was my thread

#4 Latitude Adjustment

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Posted 23 March 2006 - 09:04 AM

If you are using 55 gallon drums to lift make sure your lines are protected against chaff. We used six drums to raise an old anchor but when we started towing it in it looked like a WWII movie depth charging scene in reverse with the drums exploding out of the water and going 20 to 30 feet in the air. At some point just outside of the inlet we lost to much lift and cut the anchor free. :whistle:
Some day a historian will be going nuts trying to figure out how a boat big enough for that anchor got so close to shore.
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#5 Dive_Girl

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Posted 29 March 2006 - 05:11 PM

Latitude Adjustment, what kind of drums were you using and what type of venting did they have? We made a make shift drum just to see if it would work, so I am curious what you were using for the anchor! :)
It's Winter time - you know you're a diver when you're scraping ice off your windshield INSIDE your vehicle...!

Once in a while, it is good to step back, take a breath, and remember to be humble. You'll never know it all - ScubaDadMiami. If you aren't afraid of dying, there is nothing you can't achieve - Lao-tzu. One dog barks at something, the rest bark at him - Chinese Proverb.

#6 Latitude Adjustment

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Posted 29 March 2006 - 07:23 PM

Latitude Adjustment, what kind of drums were you using and what type of venting did they have? We made a make shift drum just to see if it would work, so I am curious what you were using for the anchor! :)


We used 55 gallon drums with about 8 I-bolts welded on, the drums were tied on top down, air was blown in through the bung holes and the bung holes left open to vent till the drums surfaced and then plugged.
It seemed like a good idea till we tried towing it in and then the lines chaffed through and the anchor went back down.
I, Latitude Adjustment (insert log in name), do hereby swear, (politely), that I shall not hold SingleDivers, (SD), nor any SD poster, (real or imagined), liable, nor shall I seek legal restitution, (real or imagined), for any perceived, (real or imagined), offenses I may incur, (or Incurrrrrrrrrr on talk like a pirate day), that may or may not be posted on this or any SCUBA related board, (real or imagined), by anyone, (real or imagined), anywhere, (real or imagined). Further, I void any right to privacy, (real or imagined), as it may, or may not relate to any posting, (real or imagined), about me, to me, for me, because of me, all about me, my dog, my cat, my bird, my monkey, my family, (real or imagined), my friends, (real or imagined), or my world, (real or imagined).

By all that is wet, I do hereby swear, (politely), and attest, upon pain of never diving again, (real or imagined), that I understand and affirm, that I agree to the above.

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#7 Dive_Girl

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Posted 29 March 2006 - 07:53 PM

...the drums were tied on top down, air was blown in through the bung holes and the bung holes left open to vent till the drums surfaced and then plugged....

OK perhaps I am slap happy after reading too much securities and investment advisor law today, and I am sure all of those are technical terms - so is it totally immature of me to be giggling right now? :)
It's Winter time - you know you're a diver when you're scraping ice off your windshield INSIDE your vehicle...!

Once in a while, it is good to step back, take a breath, and remember to be humble. You'll never know it all - ScubaDadMiami. If you aren't afraid of dying, there is nothing you can't achieve - Lao-tzu. One dog barks at something, the rest bark at him - Chinese Proverb.

#8 Latitude Adjustment

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Posted 30 March 2006 - 08:14 AM

...the drums were tied on top down, air was blown in through the bung holes and the bung holes left open to vent till the drums surfaced and then plugged....

OK perhaps I am slap happy after reading too much securities and investment advisor law today, and I am sure all of those are technical terms - so is it totally immature of me to be giggling right now? :tears:


On the top of the drum are two holes (bung holes), one I think is 3/4" pipe for a spigot to be threaded in so when the drum is on it's side you can pour stuff out, the other is about 3" which is what we used to put air in them. Back then I had a Conshelf on a long hose as a second (not DIR) and we just put that under the large bung hole and purged. I wish I'd gotten that fiasco on film then you would really be laughing!
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#9 Piz

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Posted 31 March 2006 - 12:16 AM

We used 55 gallon drums with about 8 I-bolts welded on, the drums were tied on top down, air was blown in through the bung holes and the bung holes left open to vent till the drums surfaced and then plugged.
It seemed like a good idea till we tried towing it in and then the lines chaffed through and the anchor went back down.


Thats an interesting way of doing it. I am a bit supprised the lines chaffed through though. Did they chaff through at the drum end or the anchor end? I assume the anchor end, which begs the question, once it is secured to the drums and raised was there a way to stablise it further? I suppose that is really a hindsight question but would be handy to know if I need to try someting like this.

OK perhaps I am slap happy after reading too much securities and investment advisor law today, and I am sure all of those are technical terms - so is it totally immature of me to be giggling right now? :teeth:


OH MY DG!! :hiya:

If you are thinking what I think you are thinking, you have a dirty mind :tears:

#10 Latitude Adjustment

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Posted 31 March 2006 - 07:17 AM

Well the ideal situation would have been to raise the anchor and put it on the boat but it was way to big and heavy for our 45' diveboat, the plan was to tow it in and use the boatlift at the yard. Towing it in the propwash with the drums on the surface probably put a lot of strain on the lines and chaffing gear but once the drums started jumping free of the water our plan fell apart. The chaffing seemed to be going on at both ends so it's hard to tell which end left go first but it was a dismal end to weeks of work cutting big anchor links. Maybe if the drums had been banded or welded together it would have worked.
I, Latitude Adjustment (insert log in name), do hereby swear, (politely), that I shall not hold SingleDivers, (SD), nor any SD poster, (real or imagined), liable, nor shall I seek legal restitution, (real or imagined), for any perceived, (real or imagined), offenses I may incur, (or Incurrrrrrrrrr on talk like a pirate day), that may or may not be posted on this or any SCUBA related board, (real or imagined), by anyone, (real or imagined), anywhere, (real or imagined). Further, I void any right to privacy, (real or imagined), as it may, or may not relate to any posting, (real or imagined), about me, to me, for me, because of me, all about me, my dog, my cat, my bird, my monkey, my family, (real or imagined), my friends, (real or imagined), or my world, (real or imagined).

By all that is wet, I do hereby swear, (politely), and attest, upon pain of never diving again, (real or imagined), that I understand and affirm, that I agree to the above.

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#11 Latitude Adjustment

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Posted 03 April 2006 - 06:54 AM

"calculating the extra displacement required to lift an object underwater" I think these guys need to check their math again!

Posted Image
I, Latitude Adjustment (insert log in name), do hereby swear, (politely), that I shall not hold SingleDivers, (SD), nor any SD poster, (real or imagined), liable, nor shall I seek legal restitution, (real or imagined), for any perceived, (real or imagined), offenses I may incur, (or Incurrrrrrrrrr on talk like a pirate day), that may or may not be posted on this or any SCUBA related board, (real or imagined), by anyone, (real or imagined), anywhere, (real or imagined). Further, I void any right to privacy, (real or imagined), as it may, or may not relate to any posting, (real or imagined), about me, to me, for me, because of me, all about me, my dog, my cat, my bird, my monkey, my family, (real or imagined), my friends, (real or imagined), or my world, (real or imagined).

By all that is wet, I do hereby swear, (politely), and attest, upon pain of never diving again, (real or imagined), that I understand and affirm, that I agree to the above.

_________________________________________(log in name signature)
Signed and Dated
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