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Offlineteamkiller
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does anyone know physics or mechanical engineering?
    #18762196 - 08/27/13 12:42 AM (10 years, 5 months ago)

how does pressure differential effect force?  give me some formulas. I have 2 known air pressures and a known size opening between them, i want to know how much force is being exerted on porous material between them.


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OfflineKonyap

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Re: does anyone know physics or mechanical engineering? [Re: teamkiller]
    #18762236 - 08/27/13 01:06 AM (10 years, 5 months ago)



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Offlineteamkiller
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Re: does anyone know physics or mechanical engineering? [Re: Konyap]
    #18762291 - 08/27/13 01:41 AM (10 years, 5 months ago)

is this the formula i'm looking for?

http://en.wikipedia.org/wiki/Pressure-gradient_force


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Offlineteamkiller
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Re: does anyone know physics or mechanical engineering? [Re: teamkiller]
    #18769402 - 08/28/13 05:16 PM (10 years, 5 months ago)

Bump for peak hours


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OfflineKonyap

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Re: does anyone know physics or mechanical engineering? [Re: teamkiller]
    #18769404 - 08/28/13 05:17 PM (10 years, 5 months ago)

Einstein


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Invisiblepwnasaurus
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Re: does anyone know physics or mechanical engineering? [Re: Konyap]
    #18769456 - 08/28/13 05:32 PM (10 years, 5 months ago)

You haven't explained your scenario very well.  Maybe try a diagram, or just a better explanation.


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InvisibleBoldAsLove
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Re: does anyone know physics or mechanical engineering? [Re: teamkiller]
    #18769603 - 08/28/13 05:59 PM (10 years, 5 months ago)

In most cases the force exerted by a pressure differential is simply the magnitude of the differential times the area. It gets more complex with non-uniform geometries or pressures, so we need more details to give you a good answer.


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Offlineteamkiller
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Re: does anyone know physics or mechanical engineering? [Re: BoldAsLove]
    #18771709 - 08/29/13 03:04 AM (10 years, 5 months ago)

trying to figure how to adjust an espresso machine.

at sea level with ambient 1 bar pressure (14.5 psi), an espresso machine is supposed to be set to 9(130.53) bar.
here in the mountains the air pressure is around 12.1psi (.83 bar)

what would i have to set an espresso machine to, to get the same amount of force running through the coffee?

the size of the opening between the ambient air pressure and the espresso machine is about 54mm, the mass if the coffee is about 21-24g (lets say 22g).

my intuition says "der set the espresso machine 13% lower" to about 113.53 psi.

I dunno if pressure works that way though.


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OfflineLord_McLovin
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Re: does anyone know physics or mechanical engineering? [Re: teamkiller]
    #18771771 - 08/29/13 03:54 AM (10 years, 5 months ago)

I know now what you mean, that pressure differential stuff was really confusing me. You have to be careful, it's one of those questions where there's more given than you need. The key point is:  the same amount of force!

The answer is actually quite simple. If you apply force to a disk on one side and force to it on the other side directly opposed to it, how do you get the resulting force?

Then, by definition, pressure is nothing more than force per area.


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Invisibleabltsandwich
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Re: does anyone know physics or mechanical engineering? [Re: teamkiller]
    #18771778 - 08/29/13 03:59 AM (10 years, 5 months ago)

Quote:

teamkiller said:
trying to figure how to adjust an espresso machine.




You're overcomplicating it.
Read the directions and test the espresso until it tastes right.
You have to factor in the grind of the coffee in the adjustment.

I did this same process the other day with a new Rancilio.


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OfflineLord_McLovin
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Re: does anyone know physics or mechanical engineering? [Re: abltsandwich]
    #18771784 - 08/29/13 04:03 AM (10 years, 5 months ago)

I don't think that's helping him. :lol:

Btw, OP, your answer is not correct. I would also advise to use SI-units, they save you a lot of hassle in more complicated problems.


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Offlineteamkiller
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Re: does anyone know physics or mechanical engineering? [Re: abltsandwich]
    #18771787 - 08/29/13 04:07 AM (10 years, 5 months ago)

why do i have to factor in the grind when setting the pressure vs setting the pressure then worrying about the grind?

i'm not overcomplicating it, if pressure didn't matter i wouldn't drink espresso from a 20,000$ slayer machine with pressure profiling.


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OfflineLord_McLovin
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Re: does anyone know physics or mechanical engineering? [Re: teamkiller]
    #18771807 - 08/29/13 04:18 AM (10 years, 5 months ago)

:dudewtf:
$
\Delta p := \frac{\Delta F}{A}
\implies \Delta p_S A = F = A \Delta p_M \iff \Delta p_S = \Delta p_M \iff p_{EM} = p_{ES}- p_{AS} + p_{AM}
$

put in here http://www.codecogs.com/latex/eqneditor.php
and try to understand.

:imout:


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Offlineteamkiller
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Re: does anyone know physics or mechanical engineering? [Re: Lord_McLovin]
    #18771809 - 08/29/13 04:20 AM (10 years, 5 months ago)

i wish i was smart.

wait, so am i safe in assuming that there is a linear relationship between the force applied and the pressure differential or does the mass effect things too much?


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OfflineLord_McLovin
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Re: does anyone know physics or mechanical engineering? [Re: teamkiller]
    #18771817 - 08/29/13 04:23 AM (10 years, 5 months ago)

It has nothing to do with smartness, just analytical thinking. You can do it if you only try.


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Offlineteamkiller
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Re: does anyone know physics or mechanical engineering? [Re: Lord_McLovin]
    #18771820 - 08/29/13 04:24 AM (10 years, 5 months ago)

i could probably do it if you'd explain all the variables.


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Invisiblejohnm214
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Re: does anyone know physics or mechanical engineering? [Re: teamkiller]
    #18771833 - 08/29/13 04:35 AM (10 years, 5 months ago)

Quote:

teamkiller said:
how does pressure differential effect force?  give me some formulas. I have 2 known air pressures and a known size opening between them, i want to know how much force is being exerted on porous material between them.





The force exerted through the opening will be (P1-P2)Area of Opening, where P1>P2


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OfflineLord_McLovin
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Re: does anyone know physics or mechanical engineering? [Re: Lord_McLovin]
    #18771836 - 08/29/13 04:36 AM (10 years, 5 months ago)

I won't, because it's standard notation and the meaning of the subscripts is easy to figure out once you understood the problem. Just try it, I gave you more than you need to understand.


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Offlineteamkiller
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Re: does anyone know physics or mechanical engineering? [Re: johnm214]
    #18771853 - 08/29/13 04:45 AM (10 years, 5 months ago)

Quote:

johnm214 said:
Quote:

teamkiller said:
how does pressure differential effect force?  give me some formulas. I have 2 known air pressures and a known size opening between them, i want to know how much force is being exerted on porous material between them.





The force exerted through the opening will be (P1-P2)Area of Opening, where P1>P2




kk


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Offlineteamkiller
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Re: does anyone know physics or mechanical engineering? [Re: Lord_McLovin]
    #18771857 - 08/29/13 04:48 AM (10 years, 5 months ago)

Quote:

Lord_McLovin said:
I won't, because it's standard notation and the meaning of the subscripts is easy to figure out once you understood the problem. Just try it, I gave you more than you need to understand.




i appreciate the response, but i don't know the meaning of those symbols and variables.  the only one i would guess at is f=force.

a might be acceleration but it might be something totally different too i dunno.  maybe its area.


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Offlinelifeiswhatyoumake
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Re: does anyone know physics or mechanical engineering? [Re: teamkiller]
    #18771979 - 08/29/13 06:41 AM (10 years, 5 months ago)

Quote:

does anyone know physics or mechanical engineering?




No.

I know computer science dough


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OfflineLord_McLovin
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Re: does anyone know physics or mechanical engineering? [Re: teamkiller]
    #18772186 - 08/29/13 09:06 AM (10 years, 5 months ago)

You understood it now or shall I explain it to you?


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Invisiblefiddle
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Re: does anyone know physics or mechanical engineering? [Re: Lord_McLovin]
    #18772409 - 08/29/13 10:30 AM (10 years, 5 months ago)

Explain the subscripts to me.
I like to know things.


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OfflineLord_McLovin
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Re: does anyone know physics or mechanical engineering? [Re: fiddle]
    #18772565 - 08/29/13 11:03 AM (10 years, 5 months ago)

Well, by definition pressure $p$ is force $F$ per area $A$. On sea level, you have a certain pressure $p_{ES}$ acting on the coffee coming from the espresso machine. However, you also have the ambient pressure $p_{AS}$ from the other side. Thus the total pressure acting on it is $\Delta p_S := p_{ES} - p _{AS}$. On the mountain you have a pressure of $\Delta p_M :=  p_{EM} - p _{AM}$ acting on the surface of the coffee.
You want the force to be the same, but the area is the same, so the pressure difference is also the same. Some arithmetics and that's it.


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Edited by Lord_McLovin (08/29/13 11:09 AM)


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Invisiblefiddle
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Re: does anyone know physics or mechanical engineering? [Re: Lord_McLovin]
    #18772621 - 08/29/13 11:15 AM (10 years, 5 months ago)

Ah, gotcha. I followed up until you switched from S & M to EM, ES, AM, AS, and then I couldn't figure out what you were trying to represent.


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