Manometry problem

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 got (C)

MikeGlass1969 said:

I got (C)

Click to expand...

This is correct.

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

Slay the P.E. said:

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.
 
 

Click to expand...

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

cfm03 said:

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.

Click to expand...

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).

cfm03 said:

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!

 

Click to expand...

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

cfm03 said:

Time : under 5 minutes.

Click to expand...

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? 

Vel2018 said:

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

Click to expand...

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.

Slay the P.E. said:

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.

Click to expand...

Ok