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

[Dan]

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?

From Buoyancy - Wikipedia

buoyancy = weight of displaced fluid.

So buoyancy = the total volume (internal cavity of 11 liters + tank wall of say 5 liters) x the density of the water.

If the internal volume is 10 liters & the tank wall say 4 liters, the buoyancy = 14 liters x 1 kg/liter = 14 kg, regardless flooded or not.

What I’m trying to say is that there are some misconceptions in this thread about the weight of the flooded tank underwater would be. It is not equaled to the weight of the water in the flooded tank. You have to account the weight of the tank itself (downward due to gravity) that is in the opposite direction to the buoyancy force (upward). If the tank weigh say 16kg, then negative buoyancy or the weight of the flooded tank underwater would be 16 - 14 = 2 kg, not 14 kg.

That’s what I learn from high school physics.

I neglected the weight of 10 liter of air at 2 psig, which would be about a whooping 0.01 kg.

 

[johndiver999]

From Buoyancy - Wikipedia

buoyancy = weight of displaced fluid.

So buoyancy = the total volume (internal cavity of 11 liters + tank wall of say 5 liters) x the density of the water.

If the internal volume is 10 liters & the tank wall say 4 liters, the buoyancy = 14 liters x 1 kg/liter = 14 kg, regardless flooded or not.

What I’m trying to say if that there are some misconceptions in this thread about the weight of the flooded tank underwater would be. It is not equaled to the weight of the water in the flooded tank. You have to account the weight of the tank itself (downward due to gravity) that is in the opposite direction to the buoyancy force (upward). If the tank weigh say 16kg, then negative buoyancy or the weight of the flooded tank underwater would be 16 - 14 = 2 kg, not 14 kg.

That’s what I learn from high school physics.

I neglected the weight of 10 liter of air at 2 psig.

View attachment 512296

You make it tough, i presented the most simplistic scenario with a 10 liter internal volume tank and you talk about 11 liters and copy wikipedia. what is the answer to my question?

 

[Dan]

You make it tough, i presented the most simplistic scenario with a 10 liter internal volume tank and you talk about 11 liters and copy wikipedia. what is the answer to my question?

Buoyancy force deals with the external volume, not internal. So what is the external volume?

 

[johndiver999]

LOL, when the tank with 10 liter capacity floods, what is the reduction in the actual displacement of the object? There is no distinction between internal and external when the tank is opened .. maybe that helps?

 

[Stoo]

There's so much discussion of flooded scooters, and in my experience (I have a LOT of friends that use them) I've never heard of one flooding. Obviously it happens, but it seems like many here are dwelling on a somewhat unlikely occurrence, even though there seems to be pretty universal agreement that a competent diver like Cam could have ditched a problematic DPV easily.

 

[Dan]

LOL, when the tank with 10 liter capacity floods, what is the reduction in the actual displacement of the object? There is no distinction between internal and external when the tank is opened .. maybe that helps?

What do think the negative buoyancy (weight underwater) of the flooded tank then?

It’s not equaled to the weight of the water in that flooded tank (10 liters) or 10kg. It is the weight of the tank empty - 10 kg. If the weight of the empty tank is 16kg, then the weight of the flooded tank in the water is 16 - 10 = 6 kg. So you will have a negative buoyancy of 6 kg when the tank is fully flooded with water, not 10 kg that some people here in the thread are saying.

When you flood that tank, you won’t be dragged by 10 kg dead weight, only by 6 kg. That’s all I’m saying.

 

[johndiver999]

Hint: the tank fills with 10 kg of water so it is going to get 10 kg heavier in the water. The flooding of it reduced the displacement by 10 liters.

 

[Dan]

Hint: the tank fills with 10 kg of water so it is going to get 10 kg heavier in the water. The flooding of it reduced the displacement by 10 liters.

Yes, if you weigh it on dry land. In the water it would be less due to buoyancy. If you do the same in seawater, it would weigh 2-3% less. If you do the same in liquid mercury, it would weigh almost nothing as it’ll float.

 

[michael-fisch]

Yes, if you weigh it on dry land. In the water it would be less due to buoyancy. If you do the same in seawater, it would weigh 2-3% less. If you do the same if liquid mercury, it would weigh almost nothing as it’ll float.

Dan,
are you purposely being dense?
If the tank is neutrally buoyant and you then add 10 Kilos of water to the inside of the tank, you have not caused any change in its displacement but it now weighs 10 Kilos more.

Michael

 

[johndiver999]

Technically, the displacement change IS exactly why it gets 10 kg heavier.

The initial mass or weight or external volume of the tank is irrelevant to the change in buoyant force.

Ships sink because they displace too little water when they fill up with water. Filling of the tank reduces the total displacement.