Calculating this gauge pressure reading

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Homework Help Overview

The discussion revolves around calculating gauge pressure using a U-tube manometer in the context of fluid mechanics. The original poster is attempting to solve a problem involving trapped air in a tank and its relation to the pressure indicated by the manometer.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants discuss the relationship between the trapped air pressure and the liquid heights in the manometer. Questions are raised about how to set up equations based on the pressure readings and whether certain variables, like height x, are necessary for the calculations.

Discussion Status

Some participants have provided insights into the pressure relationships and the relevance of certain variables. There is a recognition of the need to establish equations based on the pressure at different points, though not all participants agree on the necessity of height x.

Contextual Notes

There are mentions of unknowns in the problem, specifically the length of height x, which may affect the ability to solve the problem directly. The discussion reflects a mix of understanding and uncertainty regarding the setup of the problem.

Bolter
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Homework Statement
See below
Relevant Equations
pressure = rho x g x h
specific weight = rho x g
Hi everyone!
How do I go about solving this problem?

Screenshot 2020-09-30 at 13.04.21.png

Screenshot 2020-09-30 at 13.05.09.png


I tried working out the gauge pressure using this but I have a few unknowns which won't make this possible such as what is the length of x which I labelled in the figure

IMG_5299.JPG


Any help would be appreciated! Thanks
 
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The U-tube manometer is telling you the pressure of the volume of air trapped in the tank.
The pressure gauge should be telling you the static pressure at that height of liquid, which surface is being pressured down by the trapped air.
 
Lnewqban said:
The U-tube manometer is telling you the pressure of the volume of air trapped in the tank.
The pressure gauge should be telling you the static pressure at that height of liquid, which surface is being pressured down by the trapped air.

Ok I get that, but I am not entirely sure how I can form an equation with that statement. Do I begin off by setting up an expression on the left and right side of the figure then equate them?
 
The trapped pocket of air in the tank is simultaneously in contact with the surface of both liquids, having a unique pressure.
I would establish relations for each side around that common pressure.

I believe that height x is irrelevant, do you agree?
 
Last edited:
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Lnewqban said:
The trapped pocket of air in the tank is simultaneously in contact with the surface of both liquids, having a unique pressure.
I would establish relations for each side around that common pressure.

I believe that height x is irrelevant, do you agree?

Thanks I got the right answer from making relations on both sides and then equalling those 2. Gauge pressure turned out to be 33.5 kPa

IMG_5300.JPG
 
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You are welcome. :smile:
Your result is correct.
 
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