Re-Simplified: Mixed up between Hydraulic & Atmospheric Pressure on Fluids?

• posted

There's 2 web links on the subject where as:

#1: One equation uses the density of the fluid (times gravity)

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#2: and the other equation doesnt use the density

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So I do not understand why both equations are not the same to calculate the travel distance of the fulid since both are related to the total distance travel after opposite ends of a fluid have pressure applied on them.... Can anyone clarify what's my mistake?

In the Web links: the Height ( h ) of part #1 should equal d1 + d2 of part#2 shouldn't it ?

#1 says P1 = density * h * g + P2 (or F1/A1 = density * h * g + F2/A2)

but #2 says Work1 = Work2 thus F1*D1 = F2*D2 (no density of the fluid is used to determine the heights D1 and D2 (D1 + D2 = h)???

• posted

h is the difference in elevation from one point to the other. The work problem (#2) assumes the heights are the same or the difference is negligible (see P1 = P2).

The pressure equations can be derived/simplified from Bernouli's equation or the more complicated Energy equation.

• posted

I think one variable that may be a little too transparent in the calculations is the compressibility of air vs that of hydraulic fluid. Fluids in general are MUCH less compressible than is air, but hydraulic fluid in particular is less compressible than most liquids.

'Sporky'

" snipped-for-privacy@hotmail.com" wrote:

• posted

Sorry, but you need to revisit the definition of a fluid as defined in any fluid mechanics text. Any gas is a fluid. To state otherwise is just to confuse the issue.

-- Ed Ruf Lifetime AMA# 344007 ( snipped-for-privacy@EdwardG.Ruf.com)

• posted

• posted

Equation #1 already is Bernouilli since velocity = 0

thus P1 + pgh1 = P2 + pgh2 P1 = P2 + pg (h2-h1) P1 = P2 + pgh -> h = h2 - h1

in #2: D1 + D2 = h

as you said P1 = P2 (P1 = P2 + density(g)(h) thus P1 = P2 + 0 ) and both must be ten fold more than the weight of the fluid thus density * g * h is probably negilible?

in #2 P1 = P2 thus F1/A1 = F2/A2 and F1*D1 = F2*D2

thus F1/F2 = A1/A2 = D2/D1 but this seems different from Equation#1 where h = h2-h1...could it be the fluid weight is much bigger than the applied pressures which makes the difference where as in #2 it's vice versa ?

• posted

-- That IS a simplification.

-- More simplification.

-- Nope. h = 0 for #2 since P1 = P2.

As I already stated h is related to elevation in Bernoulli's eqn. d1 and d2 are the piston displacements based on a work equation. h and the ds are 2 unrelated things. That's the main thing you're missing.

That's all I have. If you can't understand this, too bad..

• posted

Im not 100% certain what you were getting at but anyhow I found the answer which is that they are both the very same...they simply omitted minor forces from the equation (meaning they considered the weight of the liquid as minimumal for the press equation).

Manometer model at the very bottom of: