Cylinder type and proper weighting (SPLIT FROM 'Pull Dumps - Lose them')

[Diving Dubai]

I do see why people get confused with buoyancy, because they forget the magic word. Displacement.

Surface area is the big factor. A normal 1kg lead weight will sink, because it has a small surface area and thus cannot displace a volume of water great enough to support its weight. However if you flatten the weight out and increase its surface area, it will float as it then can displace enough water despite it still being 1kg of lead.

Similarly people assume aluminum tanks are lighter than steel. This is generally untrue. for a given gas volume a steel tank on land will be lighter than an aluminum one. Even though aluminum is less dense you need a greater amount of it because it's weaker/

However in water the aluminum cylinder will be more buoyant because it's physical size (bigger more material) will displace more volume (thus weight) of water.

 

[boulderjohn]

I do see why people get confused with buoyancy, because they forget the magic word. Displacement.

Similarly, many people have the idea that when they add air to the BCD or inhale into the lungs, they have made the BCD or the body lighter. They have actually made the BCD or body very slightly heavier. As they did, they made it larger in volume. It now displaces more water volume without adding much weight, so the net effect is greater buoyancy.

 

[grf88]

Surface area is the big factor. A normal 1kg lead weight will sink, because it has a small surface area and thus cannot displace a volume of water great enough to support its weight. However if you flatten the weight out and increase its surface area, it will float as it then can displace enough water despite it still being 1kg of lead.

I think you have the date wrong, April 1st. was yesterday. If you flatten it out to a foil you could probably get surface tension to hold it up but add a surfactant and it would still sink. You would have to change the shape from totally flat to get it to float.

 

[johndiver999]

I do see why people get confused with buoyancy, because they forget the magic word. Displacement.

Surface area is the big factor. A normal 1kg lead weight will sink, because it has a small surface area and thus cannot displace a volume of water great enough to support its weight. However if you flatten the weight out and increase its surface area, it will float as it then can displace enough water despite it still being 1kg of lead.

Similarly people assume aluminum tanks are lighter than steel. This is generally untrue. for a given gas volume a steel tank on land will be lighter than an aluminum one. Even though aluminum is less dense you need a greater amount of it because it's weaker/

However in water the aluminum cylinder will be more buoyant because it's physical size (bigger more material) will displace more volume (thus weight) of water.

Surface area is irrelevant. The definition of density is (mass per volume).

 

[rsingler]

I'm loving this. It's like David Attenborough narrating "Connections", where he explains how the invention of the atomic bomb could be traced from the preferred shape of Attila the Hun's saddle decoration through the invention of ice cream to the Manhattan Project.
I'm learning a lot.

I want more pull dumps!

 

[BRT]

I do see why people get confused with buoyancy, because they forget the magic word. Displacement.

Surface area is the big factor. A normal 1kg lead weight will sink, because it has a small surface area and thus cannot displace a volume of water great enough to support its weight. However if you flatten the weight out and increase its surface area, it will float as it then can displace enough water despite it still being 1kg of lead.

Similarly people assume aluminum tanks are lighter than steel. This is generally untrue. for a given gas volume a steel tank on land will be lighter than an aluminum one. Even though aluminum is less dense you need a greater amount of it because it's weaker/

However in water the aluminum cylinder will be more buoyant because it's physical size (bigger more material) will displace more volume (thus weight) of water.

Please tell me how far you have to flatten out a 1 lb lead weight before it will float? Surface area has nothing to do with buoyancy. Volume vs weight has everything to do with buoyancy

 

[grf88]

I'm loving this. It's like David Attenborough narrating "Connections", where he explains how the invention of the atomic bomb could be traced from the preferred shape of Attila the Hun's saddle decoration through the invention of ice cream to the Manhattan Project.
I'm learning a lot.

I want more pull dumps!

Maybe you are thinking of James Burke.

 

[rsingler]

Maybe you are thinking of James Burke.

Yep, you're right!
See, I'm always (re)learning something!

 

[Dan]

I know exactly what density is do you?

Since you know density, let’s talk about it, in relation to AL80 cylinder. You mentioned in earlier post that it would be positively buoyant (floating) even if it is full of air (at 200 bar). From physics, if an object has density greater than water, it will sink. If its density is less than water, it would float. So, let’s calculate the density of AL80 full with air at 200 bar.

Say we pick the spec of Luxfer 80 from Scuba Cylinder Specification Chart from Huron Scuba, Ann Arbor Michigan
Outside diameter (d) = 7.25” = 18.41 cm
Cylinder height (H) = 26.06” = 66.19 cm
Dome height = half of outside diameter (r) = 1/2 d = 18.41/2 = 9.21 cm
Cylinder section height (h) = H - r = 66.19 - 9.21 = 56.98 cm
Weight empty w/o valve = 31.38 lbs = 14.25 kg
Valve weight = 2.5 lbs = 1.14 kg

Let’s calculate the weight of 200 bar air in 11.1L AL80 cavity. From ideal gas law, the air density (D) is calculated from the following equation:
D = MP/RT,
where:
D = air density in g/mL
M = molecular weight of air = 28.96 g/mol
P = air pressure = 200 bar
R = gas constant = 83.14 (mL.bar)/(K.mol)
T = ambient temperature = 25 C = 298 K
Plugging in the number into the equation,
D = 28.96 x 200 / 83.14 / 298 = 0.233 g/mL = 0.233 kg/L
W air = 0.233 kg/L x 11.1 L = 2.59 kg

Total AL80 full = empty AL80 + valve + air = 14.25 + 1.14 + 2.59 = 17.98 kg

Total outside volume of the AL80 = cylinder part + dome part.
Cylinder part = (Pi) (r) (r) (h) = 3.14 x 9.21 x 9.21 x 56.98 = 15176 mL = 15.18 L
Done part = 4/6 (Pi) (r) (r) (r) = 4/6 x 3.14 x 9.21 x 9.21 x 9.21 = 1635 mL = 1.63 L
Total AL80 outside volume = 15.18 + 1.63 = 16.81 L

Density of AL80 full = 17.98 kg / 16.81 L = 1.07 kg/L, heavier than water (1 kg/L).

Therefore, it will sink.

 

[Umuntu]

@Dan_T & @mac64 , Luxfer's data for their AL80 is -0.64 kg full.