B Hydrostatic pressure vs. gravity

[CherryB]

TL;DR Summary

When I opened the atmospheric inlet the bellows extended only when there was water inside. I think that the weight of the water is causing that, but why?

 

[hmmm27]

How heavy are the bellows ?

 

[CherryB]

The bellows are very light, but the valve on the bottom weighs about a pound

 

[CherryB]

Also, the diameter of the bellows is 2.5"

 

[Baluncore]

What are the maximum and minimum depths from the surface to the bottom valve 2.

 

[kuruman]

Is the valve attached to something that prevents it from moving relative to the water? If so, there would be a buoyant force that compresses the bellows when there is no water in the valve. This explanation assumes that the bellows is extended when it's out of the water.

 

[CherryB]

What are the maximum and minimum depths from the surface to the bottom valve 2.

the top valve (Valve 1) is about 12 inches from the surface, while Valve 2 starts out 20" away and ends up 32" away, falling a total of 12"

 

[CherryB]

Is the valve attached to something that prevents it from moving relative to the water? If so, there would be a buoyant force that compresses the bellows when there is no water in the valve. This explanation assumes that the bellows is extended when it's out of the water.

The valve is attached to the bottom of the bellows and the bellows extend and contract, so the valve can move. There was water already inside the bellows, both valves were closed and the bellows were in their retracted position. The valve itself is denser than water, so it readily sinks, and when the top valve is opened to the atmosphere, the bellows descended and sucked air in as they fell. I did not think they would fall, and that's what is puzzling to me. I thought the water would push against the bottom of the bellows and would not allow them to fall

 

[hmmm27]

There's really not a lot to consider : displacement of the apparatus vs. weight of the apparatus, and any springiness the bellows have,

 

[CherryB]

There's really not a lot to consider : displacement of the apparatus vs. weight of the apparatus, and any springiness the bellows have,

But isn't the water pressure from below stronger than the atmospheric pressure from above?

 

[kuruman]

There's really not a lot to consider : displacement of the apparatus vs. weight of the apparatus, and any springiness the bellows have,

I agree. Suppose one fills the bellows with water while compressing them relative to their relaxed position and then closes the top valve. When released, the springiness of the bellows would cause them to extend a bit but that would create a partial vacuum inside which would prevent the bellows from extending fully. When the top valve is opened, the inside and outside pressure are equalized as air is sucked in.

@CherryB: Can you tell us under what conditions the top valve was closed?

 

[Dullard]

Sum forces on the bottom plane of the bellows (ignoring bellows weight):
-Weight of the valve (down)
-pressure difference (buoyant force - up)
-weight of any water inside bellows (down)
-force required to extend bellows (up)

The bellows will extend when the net force is 'down.' it will stop when the net force is '0'. It will retract when the net is 'up.' It sounds as if the buoyant force exceeds the weight of the valve for your apparatus - adding water (effectively) increases the weight of the valve.

 

[Chestermiller]

What is the pressure in the chamber at the upper valve before it is opened (a) with no water present and (b) with water present.

 

[CherryB]

I really appreciate all the feedback! I don't know the internal pressures, Chestermiller, as I did not have gauges in use. The bellows have a pretty strong spine that would keep them from being crushed by the hydrostatic force, but would allow them to be compressed through their length, so I would assume that the pressure inside would have to be pretty close, if not equal, to the pressure outside when full of water, but I am not sure about that pressure when full of air, because I was only allowing atmospheric air to sucked in vs pumped in.

 

[CherryB]

You reminded me about material density (displacement v weight). PVC is quite a bit denser than water, and it is the material of the lower valve, and the skin of the bellows, while the spine of the bellows are made of stainless steel, so that the bellows present just a little bit of resistance to being compressed.

 

[Chestermiller]

I really appreciate all the feedback! I don't know the internal pressures, Chestermiller, as I did not have gauges in use. The bellows have a pretty strong spine that would keep them from being crushed by the hydrostatic force, but would allow them to be compressed through their length, so I would assume that the pressure inside would have to be pretty close, if not equal, to the pressure outside when full of water, but I am not sure about that pressure when full of air, because I was only allowing atmospheric air to sucked in vs pumped in.

Is it possible that there was partial vacuum in the heat space above the water in the bellows?