Displacement of Scooters at Depth - Spun off from the A&I Discussion about Nothernone

[Storker]

If the tank empty weight is 35 lbs and filled up with 24 lbs (11 liters) of seawater, then the tank would weigh 11 (35 -24) lbs underwater.

No, a little less than 9kg. You forgot the volume of the material making up the tank. 11L is the internal volume, not the total.

I'm pretty amazed. The numbers were posted several pages ago, and you guys are still discussing it? Was the post by some means made invisible?

 

[Dan]

No, a little less than 9kg. You forgot the volume of the material making up the tank. 11L is the internal volume, not the total.

I'm pretty amazed. The numbers were posted several pages ago, and you guys are still discussing it? Was the post by some means made invisible?

You are right. At least I'm not off in the fundamental physics. Just can't resist to point it out.

 

[Storker]

From my previous post:
An empty Al80 has a positive buoyancy of some 2kp, minus the weight og the valve.
A flooded Al80 has a negative buoyancy of a little less than 9kp, plus the weight of the valve
An imploded Al80 has a negative buoyancy of a little less than 9kp, plus the weight of the valve

Give me enough data for a scooter (mass, total volume and internal air-filled volume), and I'll show that it'll behave exactly the same.

If anyone here is able to read something as complicated as simple arithmetic.

 

[michael-fisch]

So, have we wandered off topic long enough for the last 10 pages or so?

A scooter has a volume equal to it's weight.
Since everything in a scooter has a density higher than water, there has to be air in a scooter to keep it from sinking like a rock.
If you replace the air with water the scooter will now weigh much more.
Old gavin magnum scooters weighed well over 60Kg and probably had at least 30 Liters of air in them.
I was unable to lift my flooded 33Ah Gavin off the bottom, using a fully inflated 55 lb wing and drysuit, and magnums were quite a bit larger.
Michael

 

[Dan]

A scooter has a volume equal to it's weight.

I guess you meant volume in liter and weight in kg if you want it to be neutrally buoyant in the water. Your 60kg old Gavin magnum may have overall volume of 60 liters.

If you use volume in cubic feet and weight in pounds, the scooter would have weight of 132.1 pounds and overall volume of 2.1 cubic feet to be neutrally buoyant.

 

[johndiver999]

People tend to forget about the gravity force (i.e., the tank weight) that goes against the buoyancy force. We are still on earth. Once you are in the water, it doesn't mean that the gravity force would disappear. Buoyancy force is up & gravity force is down. If the gravity force (35 pounds) is greater than the Buoyancy force (24 pounds) then you will have negative (down / sink) buoyancy force of 11 pounds.

Buoyancy - Wikipedia

Archimedes' principle:
Apparent immersed weight = object weight - weight of displaced fluid

If you put that tank in a pool of liquid is Mercury with density of 13.6 g/ml (13.6 times greater than water) the buoyancy force would be 13.6 x 24 pounds = 326.4 pounds >> tank weight of 35 pounds. That tank will float as demonstrated by the picture, below (placing a nickel on a beaker of mercury). The coin density perhaps is about 8.9 g/ml < Mercury density of 13.6 g/ml.

View attachment 512285

Another way of estimating whether an object is positively or negatively or neutrally buoyant is by estimating its bulk density relative to the fluid density around it. If the bulk density is 1.00 g/ml (62.4 lbs/cft) then it'll be neutrally buoyant in freshwater. For the case of empty AL80, if its internal volume = 11 liters + 5 liters of aluminum material = 16 liters of outside volume = 0.57 cft. Then the empty AL80 bulk density = 35 lbs / 0.57 cft = 61.4 lbs/cft, pretty close to water density to me. It'll be slightly floating if the nozzle capped, I think.

I can't seem to understand what you are saying about the buoyancy changes. So for clarity, if a scuba tank is filled with air at 2 psi and it is neutral in water and the internal volume is 10 litters, what do you say the buoyancy of the tank will be if the valve is opened and the tank is completely flooded?

 

[dmaziuk]

Review of Archimedes principle: Any object, totally or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.

Fb = ρ x g x V Where Fb= buoyant force; ρ is density g is gravity and V is volume.

Hope this helps.

My brain needed some prodding to remember (or realize) that when you drop a cylinder full of water into a tank, and it's displacing water, you're actually adding water to the system. Whereas when the displaced water can flow back into the cylinder, the total amount water in the system remains the same.

Bad brain, no cookie.

 

[johndiver999]

A flooded scooter would have been pointed at the slope or wall and dealt with by any skilled diver.

There is ZERO evidence that the scooter failed. There is zero evidence that if it did fail, a highly skilled and experienced diver would be unable to adequately address it.

Damage to a sponge is not necessarily evidence that it was caused by Cameron.

The likelihood of two independent events occurring is the product of their individual probability.

So if there is a one in 10,000 chance that the scooter would flood, and there is a one in 100 chance that he would be unable to address before dying, then we have a one in a million (1:1,000,000) chance that this is what happened. Assuming two unlikely events occurred in series is not the way I would approach this.

The only evidence we have been presented with that seems to be "unusual" is that (1) he descended faster than normal over the wall and (2) his bubbles were not risiing and (3) there is also some damage on the wall near or at the dive site.

I would like to see pictures of the damage. Sponges are incredibly tough, I see the effects of anchor and chain damage quite commonly. In my opinion it would be unlikely a diver could unintentionally cause a whole lot of visible damage to a sponge.

 

[michael-fisch]

I can't seem to understand what you are saying about the buoyancy changes. So for clarity, if a scuba tank is filled with air at 2 psi and it is neutral in water and the internal volume is 10 litters, what do you say the buoyancy of the tank will be if the valve is opened and the tank is completely flooded?

Salt or Freshwater?
freshwater would be 10L =(10Kg - the weight of 10L of air at 2psi)
Saltwater (how heavy is saltwater where you dive, for me in the Med saltwater weighs 1030G per liter.
So
Saltwater would be 10L=(10x1.03Kg - the weight of 10L of air at 2psi)

Michael

 

[johndiver999]

Salt or Freshwater?
freshwater would be 10L =(10Kg - the weight of 10L of air at 2psi)
Saltwater (how heavy is saltwater where you dive, for me in the Med saltwater weighs 1030G per liter.
So
Saltwater would be 10L=(10x1.03Kg - the weight of 10L of air at 2psi)

Michael

Oh he can round the numbers if he likes, distilled or seawater.. I don't care.