Manometry problem

[Slay the P.E.]

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.

 

[MikeGlass1969]

I got (C)

 

[Slay the P.E.]

I got (C)

This is correct.

 

[Kloeb222]

I got C as well.

I'm studying for MDM so it took me an extra couple of minutes to stumble through the water density tables

 

[cfm03]

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.



Sent from my Moto G (5) Plus using Tapatalk

 

[Slay the P.E.]

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.

 

[mongolianbbq]

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!

 

[Vel2018]

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

 

[Slay the P.E.]

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.

 

[Vel2018]

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