gauge p vs. hydrostatic pressure vs. atm--a problem that combines them all

[SaintJude]

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A large cylinder is filled w/ an equal volume of two immiscible fluids. A balloon is submerged in the first fluid; the gauge pressure , Pg1, in the balloon at the deepest point in the first fluid is found to be 3 atm. Next, the balloon is lowered all the way to the bottom of the cylinder; and as it is submerged in the second fluid, the hydrostatic pressure , Pg2, in the balloon reads 8 atm. What is the ratio of the gauge pressure at the bottom of the first fluid to the gauge pressure at the bottom of the second fluid? (Atmospheric pressure = 1 atm).

Does anyone have a drawing for this? I'll try to look for one as well...

 

[SaintJude]

Is this right?

If so, why is the atmospheric pressure at the 1st/2nd interface = 3 atm?

View attachment 18646

 

[milski]

That's mislabeled.

At the surface: Patm=1, Pg=0, Phs=1
At the 1/2 interface: Patm=1, Pg=3, Phs=4
At the bottom: Patm=1, Pg=7, Phs=8


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I am here: http://maps.google.com/maps?ll=47.657927,-122.306192

 

[SaintJude]

Wait..You are saying the ratio (answer to question) is what?

 

[milski]

3/7? Unless they reset the gauge for fluid 2, then it would be 3/4. Am I wrong?

 

[SaintJude]

Yeah, it's 3:4. Yo milski, could please change those values accordingly so I can re-draw & post correct label.

Currently, from top to bottom the atmosphere on the diagram is:
3, 1,4, 3, 8
How should I change those numbers...

Is anyone else still CONFUSED...oi.

 

[milski]

Gauge pressure is just normal pressure with the zero set at a convinient place, normally Patm. The idea is that you want to measure only the pressure of the fluid, not any additional pressure. If they want to reset gauge pressure at the 1/2 surface, you need to change a bit:
At the surface: Patm=1, P0=1, Pg=0, Phs=1
At the interface: Patm=1, P0=1, Pg=3, Phs=4
Now we reset the gauge.
At the interface: Patm=1, P0=4, Pg=0, Phs=4
At the bottom: Patm=1, P0=4, Pg=4, Phs=8

Whatever you do, P0+Pg=Phs

 

[SaintJude]

Is this correct then? Thank you Milski!

Scenario 1:
View attachment 18647

Scenario 2: (when balloon is at bottom) Replace the word label "absolute pressure" with "gauge pressure"!
View attachment 18648

 

[milski]

Yes, the graphs are correct. The blue is ok as absolute pressure or if you want to use the same terminology as the rest of the problem, it's hydrostatic pressure.