Posted by *Morris Dovey* on February 14, 2011, 3:53 pm

A while back Curbie provided this link to manometers

http://www.efunda.com/formulae/fluids/manometer.cfm

which answered my question of the moment, and I filed the link away as

being helpful.

Now I'm dealing with a similar problem (attempting to use a standpipe

arrangement to maintain a minimum pressure in a heat engine) and find

myself with a severe case of mental indigestion - and what feels like a

dumb question...

What if a U-tube manometer has a different diameter on each side?

I've posted a sketches of two manometers at

http://www.iedu.com/DeSoto/Misc/Manometers.png

to illustrate. According to the handy-dandy equations describing these

things, applying a given pressure to one side will produce a difference

in height of the fluid that depends only on the pressure, the density of

the fluid, and the gravitational acceleration constant.

I'm comfortable with that for the case where both sides of the U are of

equal diameter, but when they aren't the result looks absurd because it

seems to require a magical change in the volume of the fluid.

The density variable would seem to be telling me that I should be

looking at mass, rather than column height - but none of the articles

I've read seem to want to address mass or tube dimensions.

What am I missing here?

--

Morris Dovey

http://www.iedu.com/DeSoto/

PGP Key ID EBB1E70E

Posted by *Jim Wilkins* on February 14, 2011, 5:58 pm

*> A while back Curbie provided this link to manometers*

*> http://www.efunda.com/formulae/fluids/manometer.cfm *

*> which answered my question of the moment, and I filed the link away as*

*> being helpful.*

*> Now I'm dealing with a similar problem (attempting to use a standpipe*

*> arrangement to maintain a minimum pressure in a heat engine) and find*

*> myself with a severe case of mental indigestion - and what feels like a*

*> dumb question...*

*> What if a U-tube manometer has a different diameter on each side?*

*> I've posted a sketches of two manometers at*

*> http://www.iedu.com/DeSoto/Misc/Manometers.png *

*> to illustrate. According to the handy-dandy equations describing these*

*> things, applying a given pressure to one side will produce a difference*

*> in height of the fluid that depends only on the pressure, the density of*

*> the fluid, and the gravitational acceleration constant.*

*> I'm comfortable with that for the case where both sides of the U are of*

*> equal diameter, but when they aren't the result looks absurd because it*

*> seems to require a magical change in the volume of the fluid.*

*> The density variable would seem to be telling me that I should be*

*> looking at mass, rather than column height - but none of the articles*

*> I've read seem to want to address mass or tube dimensions.*

*> What am I missing here?*

*> --*

*> Morris Doveyhttp://www.iedu.com/DeSoto/ *

*> PGP Key ID EBB1E70E*

http://www.grc.nasa.gov/WWW/k-12/WindTunnel/Activities/Pascals_principle.ht=

ml

The fluid in the larger column certainly weighs more, but that weight

is spread across a proportionally larger area so the unit area

pressure at the bottom is the same.

Morph the U tube into a 1" open pipe dipped into a 55 gallon barrel.

Would you expect a different liquid level in the 1" pipe?

jsw

Posted by *Morris Dovey* on February 14, 2011, 7:02 pm

On 2/14/2011 11:58 AM, Jim Wilkins wrote:

*> http://www.grc.nasa.gov/WWW/k-12/WindTunnel/Activities/Pascals_principle.html *

*> *

*> The fluid in the larger column certainly weighs more, but that weight*

*> is spread across a proportionally larger area so the unit area*

*> pressure at the bottom is the same.*

*> *

*> Morph the U tube into a 1" open pipe dipped into a 55 gallon barrel.*

*> Would you expect a different liquid level in the 1" pipe?*

Thanks! That was exactly what I needed. As usual, I was trying to make a

hard problem out of what should have been an easy one. :-/

--

Morris Dovey

http://www.iedu.com/DeSoto/

PGP Key ID EBB1E70E

Posted by *Jim Wilkins* on February 15, 2011, 12:42 pm

*> On 2/14/2011 11:58 AM, Jim Wilkins wrote:*

*> ...>*

*> Thanks! That was exactly what I needed. As usual, I was trying to make a*

*> hard problem out of what should have been an easy one. :-/*

*> Morris Dovey*

The great minds from Aristotle's to Newton's time did the same.

jsw

Posted by *Morris Dovey* on February 16, 2011, 12:53 pm

On 2/15/2011 6:42 AM, Jim Wilkins wrote:

*>> On 2/14/2011 11:58 AM, Jim Wilkins wrote:*

*>> ...>*

*>> Thanks! That was exactly what I needed. As usual, I was trying to make a*

*>> hard problem out of what should have been an easy one. :-/*

*>>*

*>> Morris Dovey*

*> *

*> The great minds from Aristotle's to Newton's time did the same.*

Regrettably, so have all the more ordinary minds - and I think /I/ might

be even better at it than either/both those guys! ;-)

I just can't seem to get quite enough traction to get ahead of the

learning curve - and the hurrier I go, the behinder I get.

The best news is that I seem to be surrounded by folk who've already

acquired knowledge I lack and are willing to help me learn.

--

Morris Dovey

http://www.iedu.com/DeSoto/

PGP Key ID EBB1E70E

> A while back Curbie provided this link to manometers> http://www.efunda.com/formulae/fluids/manometer.cfm> which answered my question of the moment, and I filed the link away as> being helpful.> Now I'm dealing with a similar problem (attempting to use a standpipe> arrangement to maintain a minimum pressure in a heat engine) and find> myself with a severe case of mental indigestion - and what feels like a> dumb question...> What if a U-tube manometer has a different diameter on each side?> I've posted a sketches of two manometers at> http://www.iedu.com/DeSoto/Misc/Manometers.png> to illustrate. According to the handy-dandy equations describing these> things, applying a given pressure to one side will produce a difference> in height of the fluid that depends only on the pressure, the density of> the fluid, and the gravitational acceleration constant.> I'm comfortable with that for the case where both sides of the U are of> equal diameter, but when they aren't the result looks absurd because it> seems to require a magical change in the volume of the fluid.> The density variable would seem to be telling me that I should be> looking at mass, rather than column height - but none of the articles> I've read seem to want to address mass or tube dimensions.> What am I missing here?> --> Morris Doveyhttp://www.iedu.com/DeSoto/> PGP Key ID EBB1E70E