## Re: RE:Pressure and Flow

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• Hi, JG. Just think of bricks. If you are the bottomest brick, you have all the bricks above you pressing down on you, so the pressure on you is greatest. But
Message 1 of 3 , Jun 30 8:00 PM
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Hi, JG.

Just think of bricks. If you are the bottomest brick, you have all
the bricks above you pressing down on you, so the pressure on you is
greatest. But you can't move sideways, because your neighbour bricks
have the same pressure on them, and the brick above you has a brick
above it, and on up to the top. Now if you were a molecule of water,
you'd have all the molecules stacked above you and your pressure would
be greater. You can still move a little (which is why ink will spread
around in water even if nobody stirs it; there is molecular motion
which would be like brick motion, but LOTS smaller :) Anyway, no
particular molecule has any pressure to move around very much, just
kind of randomly wandering.

And anywhere you meauser a pressure in a static vessel, the pressure
will be proportional to the depth of the fluid at the depth where
you are measuring. When the stuff starts to move, then the pressure
differentials start to show up, even at equal depths.

Does that help?
Donald.

--- In hobbicast@y..., "geoff" <cyrano@a...> wrote:
> Hi, the recent thread has me wandering. If you measure the pressure
in a
> glass of water, the pressure increases as the tip of the manometer
goes
> further in the water, but there is no motion of the water [ i don't
think
> convection currents are the answer ] and yet the pressure
differential
> remains. i don't understand the pressure's not causing currents in
the
> water. Also i don't understand most of the recent posts. Just
geoff
• In a message dated 6/29/01 7:06:40 PM Pacific Daylight Time, ... That is what us inginears call static head . It is just the weight(or mass if you are ISO
Message 2 of 3 , Jul 1, 2001
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In a message dated 6/29/01 7:06:40 PM Pacific Daylight Time,
cyrano@... writes:

> . If you measure the pressure in a
> glass of water, the pressure increases as the tip of the manometer goes
>

That is what us inginears call "static head". It is just the weight(or mass
if you are ISO inclined) of the fluid above the manometer end. When the
fluids (water or gas) start moving is when the fun begins. Look at what may
be called a "cellar drainer" or a gadget boaters use to bail out water it is
a venturi that uses water flow to create a low pressure to "suck" water out
of a low place. For home shop users I think it would be enough to remember
that as fluids flow through an expanding section pressure rises, velocity
drops. A constriction is the other way around, velocity rises, pressure
drops. Just keep you change of sections gradual.

John Meacham
High Desert of California, Palmdale, Littlerock.

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