High School Hydrostatic pressure vs. gravity

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SUMMARY

The discussion centers on the behavior of bellows in a hydrostatic pressure system, specifically addressing the dynamics of two valves and the buoyant forces acting on them. The bellows, which are 2.5 inches in diameter, are lightweight but are affected by the weight of the bottom valve, approximately one pound. The top valve is positioned 12 inches from the surface, while the bottom valve varies from 20 to 32 inches. The conversation highlights the balance between the buoyant force and the weight of the apparatus, emphasizing that the bellows will extend or retract based on the net forces acting upon them.

PREREQUISITES
  • Understanding of hydrostatic pressure principles
  • Familiarity with buoyancy and weight dynamics
  • Knowledge of bellows mechanics and their material properties
  • Basic grasp of pressure differentials in fluid systems
NEXT STEPS
  • Research the principles of hydrostatic pressure and its applications
  • Explore buoyancy calculations and their relevance in fluid mechanics
  • Study the mechanics of bellows and their design considerations
  • Investigate pressure differentials in closed systems and their effects
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Engineers, fluid mechanics students, and anyone involved in designing or analyzing hydrostatic systems will benefit from this discussion, particularly those working with bellows and valve mechanisms.

CherryB
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TL;DR
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?
Bellows Ex1.JPG
 
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How heavy are the bellows ?
 
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The bellows are very light, but the valve on the bottom weighs about a pound
 
Also, the diameter of the bellows is 2.5"
 
What are the maximum and minimum depths from the surface to the bottom valve 2.
 
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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.
 
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Baluncore said:
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"
 
kuruman said:
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
 
There's really not a lot to consider : displacement of the apparatus vs. weight of the apparatus, and any springiness the bellows have,
 
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  • #10
hmmm27 said:
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?
 
  • #11
hmmm27 said:
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?
 
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  • #12
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.
 
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  • #13
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.
 
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  • #14
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.
 
  • #15
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.
 
  • #16
CherryB said:
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?
 

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