Manometry problem

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Slay the P.E.

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Hello TFS test takers. Have some fun with this one. Difficulty level; I'd guess 2 on the scale from 1 to 5. 

SPOILER ALERT: Try to solve it first; then scroll down to see the replies.

well depth.PNG

 
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Hello TFS test takers. Have some fun with this one. Difficulty level; I'd guess 2 on the scale from 1 to 5. 
 
SPOILER ALERT: Try to solve it first; then scroll down to see the replies.
 
 
I solved it by process of elimination....I admit, it is not the most elegant way of solving, but it led me to C.
First, quick look at the problem, the answer A seem to have a suspiciously low value. Also answer D is sketchy.
So, the only possible good answers left are B and C.

Next, I calculated the volume of the entire well:
100ft*(pi/4)*(4ft)^2*(7.48 gal/1ft^3)=9,400 gal. Which emphases again that the answers is lying between B and C. Ballpark check!

Then, I calculated the volume of 'almost' water (knowing that is not correct) (100-20)*(pi/4)*(4ft)^2*(7.48 gal/1ft^3)=7,520 gal. But this is answer B.
Thus the only answer choice left is C.

Time : under 5 minutes.



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I solved it by process of elimination....I admit, it is not the most elegant way of solving, but it led me to C.
First, quick look at the problem, the answer A seem to have a suspiciously low value. Also answer D is sketchy.
So, the only possible good answers left are B and C.
Yes. Choice "A" is actually a trap I created: If you solve the problem using the manometer equation (hydrostatics) you get the distance between the water surface and the tube's submerged end as roughly (2.75 psi)*(2.31 ftH2O/psi) = 6.3525 ft. Then you would get the volume as (80+6.3525ft)*(pi/4)*(4ft)^2 = 1085 ft^3. If you had overlooked the fact that they want the answer in gallons, you would choose (A).

Next, I calculated the volume of the entire well:
100ft*(pi/4)*(4ft)^2*(7.48 gal/1ft^3)=9,400 gal. Which emphases again that the answers is lying between B and C. Ballpark check!

 
Ballpark check, order-of-magnitude, and unit consistency/dimensional homogeneity are fantastic tools.

Time : under 5 minutes.
I love it.

 
Took me way too long. I've been working so hard on cycles, heat transfer, and cycle equipment i completely forgot how to solve the easy basics (static fluids). Once I figured out it was the simplest of manometers everything fell into place. Time to go back to the basics!

 
When you say difficulty 2, are you grading it based on actual exam difficulty or just based on how your own perspective? 

 
When you say difficulty 2, are you grading it based on actual exam difficulty or just based on how your own perspective? 
It's my subjective judgment. I considered the hardest problem I've seen in the exam as a 5 and the easiest a 1. Don't worry about this. This difficulty level is irrelevant and now I'm regretting having put that there.

 
It's my subjective judgment. I considered the hardest problem I've seen in the exam as a 5 and the easiest a 1. Don't worry about this. This difficulty level is irrelevant and now I'm regretting having put that there.
Ok

 

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