Need help with understanding this solution (fluid pressure problem)

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The discussion centers on understanding fluid pressure calculations, specifically at point B in a system involving oil and water. The pressure due to water is considered negative while that of oil is positive, as measurements are taken from the oil-water interface to determine differences in pressure. The method involves calculating hydrostatic pressure as one moves down through oil and then up to point B, with the atmospheric pressure serving as a reference. The calculations can be visualized similarly to a U-shaped manometer, emphasizing the importance of the interface for accurate pressure measurement. Clarifying these concepts aids in grasping the overall pressure dynamics in the system.
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I dont fully understand the solution
So here’s the question (I am only talking about the pressure at point B, the other 2 I can understand.)
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And here is the solution
3C602BA6-4E95-403A-8973-BFE9E6853BB6.jpeg

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Here is what I am not understanding, why is the pressure due to the water negative and the oil positive, and why are all measurements only made from where the oil meets the water?

i must be missing something really obvious
 
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bigmike94 said:
why are all measurements only made from where the oil meets the water?
Because that's how we find the difference between ##p_{atm}## and ##p_B##
Going down contributes a positive ##\Delta p## and up a negative ##\Delta p##.

One can just as well take the bottom as the turning point: the ##\Delta p## of the extra 1.25 m down is cancelled by ##\Delta p## going up the same 1.25 m

##\ ##
 
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BvU said:
Because that's how we find the difference between ##p_{atm}## and ##p_B##
Going down contributes a positive ##\Delta p## and up a negative ##\Delta p##.

One can just as well take the bottom as the turning point: the ##\Delta p## of the extra 1.25 m down is cancelled by ##\Delta p## going up the same 1.25 m

##\ ##
Thank you that helped a lot. I attempt the problems quite a bit after I learnt about that method of finding the pressure. Hopefully I’ll remember it 😃
 
I think of the problem as moving along a path from the external reference surface to the destination, at point B.
1. Start at the surface with atmospheric pressure. Specify absolute or gauge pressure for the reference.
2. Compute the increasing hydrostatic pressure as you move down through the oil, to the oil-water interface surface.
3. Compute the reducing hydrostatic pressure as you move up through the oil to the destination at point B, on the water-air interface.
 
bigmike94 said:
Here is what I am not understanding, why is the pressure due to the water negative and the oil positive, and why are all measurements only made from where the oil meets the water?

i must be missing something really obvious

This arrangement can be calculated like a U-shaped manometer, if you can imagine the bend located just underneath the central partition.

Please, see:
https://pressbooks.online.ucf.edu/osuniversityphysics/chapter/14-2-measuring-pressure/
 
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