What am I doing wrong in this simple pressure calc?

  • Thread starter Thread starter Bluestribute
  • Start date Start date
  • Tags Tags
    Pressure
Click For Summary

Homework Help Overview

The discussion revolves around calculating gauge pressure in a mixed container involving different fluids, specifically water and oil. The original poster attempts to apply the hydrostatic pressure formula but expresses confusion regarding the calculations and the constants involved.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants discuss the application of the hydrostatic pressure formula, questioning the use of gravitational constants and the interpretation of specific symbols like γ (gamma). There is also inquiry about the pressure contribution of air above the fluid at point B.

Discussion Status

Some participants have offered clarifications regarding the use of specific constants and the relationship between weight and pressure in different measurement systems. The conversation reflects a mix of interpretations and attempts to understand the implications of the calculations without reaching a definitive conclusion.

Contextual Notes

There is mention of the lack of constants for air pressure in the problem setup, which some participants note as a potential gap in the information provided. The discussion also highlights the differences between Imperial and SI units in the context of fluid mechanics.

Bluestribute
Messages
192
Reaction score
0
I'm trying to find gauge pressure at multiple points in a "mixed" container. Which, I thought would be equal to ρghliquid 1 + ρghliquid 2 etc. Well, that's not right? And I don't know why?

So for point A, I did: (55.1 * 32.1 * 4) + (62.4 *32.1 * 4). Then I divided by 144 to get it into psi. I got 105. That's not right . . .
 

Attachments

  • 1.jpg
    1.jpg
    23 KB · Views: 410
Physics news on Phys.org
What's "32.1?"
 
Clearly it's g.

Does the extra weight of the column of water @ B add pressure to the column of water @ A?
 
paisiello2 said:
Clearly it's g.
Clearly. And 62.4?
 
Bluestribute said:
I'm trying to find gauge pressure at multiple points in a "mixed" container. Which, I thought would be equal to ρghliquid 1 + ρghliquid 2 etc. Well, that's not right? And I don't know why?

So for point A, I did: (55.1 * 32.1 * 4) + (62.4 *32.1 * 4). Then I divided by 144 to get it into psi. I got 105. That's not right . . .
For the figures in the diagram for water and oil, γ = ρg, so you don't need to multiply γw = 62.4 lbf / ft3 by g ...
 
SteamKing said:
For the figures in the diagram for water and oil, γ = ρg, so you don't need to multiply γw = 62.4 lbf / ft3 by g ...
Wait, that's what that letter means? So it's just ϒh + ϒh?

EDIT: Jeez that would have been helpful to know. Or know to infer . . . Yes, just do that to get psf and convert to psi to get the right answer . . . Wow. Thanks. But what about B? The only thing above B is air . . . and they don't give any constants for air. Should that just be known (because it isn't negligible . . . I tried 0 psi with no luck).
LAST EDIT: Work backwards from A and subtract. Got it!
 
Last edited:
Bluestribute said:
Wait, that's what that letter means? So it's just ϒh + ϒh?

EDIT: Jeez that would have been helpful to know. Or know to infer . . . Yes, just do that to get psf and convert to psi to get the right answer . . . Wow. Thanks. But what about B? The only thing above B is air . . . and they don't give any constants for air. Should that just be known (because it isn't negligible . . . I tried 0 psi with no luck).
LAST EDIT: Work backwards from A and subtract. Got it!
That's one of the things about working in Imperial versus working in SI. In Imperial, you get accustomed to working with weight instead of mass, as in SI.

Fresh water weighs 62.4 lbf / ft3. You can work back to find the mass density in slugs / ft3, which is approximately 2.
 

Similar threads

  • · Replies 7 ·
Replies
7
Views
2K
  • · Replies 10 ·
Replies
10
Views
2K
  • · Replies 2 ·
Replies
2
Views
5K
  • · Replies 4 ·
Replies
4
Views
3K
Replies
18
Views
3K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 27 ·
Replies
27
Views
4K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 13 ·
Replies
13
Views
2K