Why is it necessary to start this experiment with the water level at 400mL?

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Aurelius120 said:
Then as long as water level is at the level of upper rim of the bell or lower, we have to blow against atmospheric pressure since downwards pressure of water is zero. So the amount of exhaled air is same in 400mL or 0mL.
I do not know what you are trying to say here. It is the difference in water level between the outside of the bell and the inside that tells you whether you are exhaling at atmospheric pressure, above atmospheric pressure or below atmospheric pressure.
 
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There is a reason that a leaky internal seal in an unloaded hydraulic cylindar will cause it to fail in extended position. The closing force is less because the area of the connecting rod does not see hydraulic oil with high pressure. Therefore the "closing" force is less than the "opening" force because opening pressure acts only on the annulus exposed to the oil and not the entire circular face. If the tube seals to the beaker face in our apparatus, the areas will not be the same and can produce error.



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jbriggs444 said:
I do not know what you are trying to say here. It is the difference in water level between the outside of the bell and the inside that tells you whether you are exhaling at atmospheric pressure, above atmospheric pressure or below atmospheric pressure.

What I meant is that
1000000068.jpg

I have to blow against atmospheric pressure but
1000000066.jpg

But here I have to blow against water pressure.
I think I am making a mistake but I can't figure it.

Sorry for the late reply.
 
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DaveC426913 said:
How do you know blowing air will make it float? In order for that to be true, the Bell jar must already be neutrally buoyant, which it is not, because it is resting on the bottom.
Was he trying to say something different than the pictures I posted?
I think I have made a mistake.
 
OK. let's look at your diagrams for a moment.

  • The bell is resting on the bottom of the reservoir.
  • How much weight is it applying there? What if it weighs - say, for the sake of argument - five pounds?
  • If the patient blows into the tube, they're going to have to compensate for five pounds of weight before the bell even lifts off the bottom!
  • If they can only manage four and a half pounds of pressure, the device will remain registered at zero.
Implications:
  • The bell must be nuetrally buoyant.
  • It must float somwhere in the middle - 400mL is as good a place as any, and it abides by a universal standard
 
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Oops.
Aurelius120 said:
What I meant is thatView attachment 357419
I have to blow against atmospheric pressure
Yes this was my mistake.
DaveC426913 said:
OK. let's look at your diagrams for a moment.

  • The bell is resting on the bottom of the reservoir.
  • How much weight is it applying there? What if it weighs - say, for the sake of argument - five pounds?
  • If the patient blows into the tube, they're going to have to compensate for five pounds of weight before the bell even lifts off the bottom!
  • If they can only manage four and a half pounds of pressure, the device will remain registered at zero.
Implications:
  • The bell must be nuetrally buoyant.
  • It must float somwhere in the middle - 400mL is as good a place as any, and it abides by a universal standard
The second diagram I drew as the source of error like you were saying.