# Formula For Water Pressure

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• posted on January 25, 2004, 2:05 pm
My neighbor wants to estimate the pressure on his 2nd floor based upon a reading of pressure on the 1st of his house. He says there's a formula. Anyone know?
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• posted on January 25, 2004, 2:18 pm
John Gregory wrote:

In very round numbers, the (static) pressure goes down 1/2 PSI per foot of elevation. That's easy to remember.
The more exact number is .433 PSI per foot.
Jim
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• posted on January 25, 2004, 6:18 pm

For some reason I find 2.31 ft/psi easier to remember and either multiply or divide by that. Either way, from waist high on 1st floor to waist high on 2nd floor is only about 3 or 4 psi static pressure difference (unless the neighbor has very tall ceilings).
If 2nd floor flowing pressure seems much lower, it could be a restriction like debris in a faucet aerator or mineral deposits (especially in galvanized piping).
--
David Efflandt - All spam ignored http://www.de-srv.com /

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• posted on January 25, 2004, 6:41 pm
Speedy Jim writes:

Or a scuba diver would say, 34 ft/atm.
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• posted on January 25, 2004, 7:39 pm
Ah, gee, I misinformed the poor fellow. Sorry out there!
--

Christopher A. Young
Jesus: The Reason for the Season
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• posted on January 26, 2004, 1:57 am
Speedy Jim wrote:

I believe that only works when the system is open to atmospheric pressure.
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• posted on January 26, 2004, 2:17 am
George E. Cawthon wrote:

Nope. Works just fine either way... Jim
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• posted on January 26, 2004, 5:14 pm

To clarify that: The RELATIVE pressure goes up by about 1/2 psi per foot, reguardless of whether the system is opened or closed. Since the OP wanted to calculated pressure on one floor, by reading a guage on another, this is sufficient. What depends on an open system is figuring the ABSOLUTE pressure, which in an open system near sea-level will be 1/2 psi / foot, plus atmospheric pressure at the top of the water column.
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• posted on January 27, 2004, 6:35 am
.433 pound per foot
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• posted on January 30, 2004, 9:04 pm

As a general rule (if you don't need to be exact) you can assume a 5psi loss per floor above ground level, or .5 psi per foot. This is the way fire engineers and fire protection system installers generally figure pressure loss due to elevation.
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• posted on January 31, 2004, 1:17 am
This has been rattling around for quite a while. The static loss (no water flowing) is easily found. A cubic foot of water weighs 62.4 lbs. A square foot is 144 sq. inches. The pressure from 1 foot of water is 62.4 lbs divided by 144 sq. inches or 0.433 lbs/sq inch. Ten feet of elevation looses you 4.33 lbs/sq in of pressure.
The real issue though is how much is lost due to "friction" or drag as water moves through pipes and around corners and bends in the plumbing. One needs to know the diameter of the pipe, how many bends and the extent of those bends (30 degree, 45 degreee, or 90 degree,) the length of the straight bends, and the flow rate. Once all of that is known you can calculate the pressure at the outlet end.
RB
TexasFireGuy wrote:

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• posted on January 31, 2004, 6:01 am
But aside from "The White House" could diameter of pipe, bends, length of straight runs possibly be a significant factor in a single family residence when merely attempting to calculate the drop in pressure at the 2nd floor of a two story house?

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• posted on January 31, 2004, 6:57 am
Yes. It made enough of a difference when my home was being designed that both my architect and I felt that 1" copper was needed to avoid demand related pressure drops.
RB
John Gregory wrote:

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• posted on February 5, 2004, 10:09 pm

he wanted to know was how much prassure was lost due to elevation of the second floor.
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• posted on February 1, 2004, 9:48 am

62.4 lbs?
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• posted on February 1, 2004, 12:00 pm
wrote:

-- Doug Miller (alphageek at milmac dot com)
How come we choose from just two people to run for president and 50 for Miss America?
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• posted on February 1, 2004, 5:40 pm
Weight of water. 62.4 lbs/cu ft. Also 1 gram per cu cm.
RB
Brandon wrote:

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• posted on January 25, 2004, 2:45 pm
There is but more info is needed. The basic formula is Q=AV where Q is the flow rate, A= the area of the pipe, V= the velocity. Velocity can be derived from the formula Delta P=V^2/2g or V^2=2g(delta P). Then: Q=A* sq root (2g*delta P). Then there is the constants to assure all the proper units. Then there is the much easier way and that is to go to any hydraulic text or Hydraulics Handbook (like Crane's) that gives you all the formulas without doing any work. Go into Google with *pressure drop* http://www.xs4all.nl/~kostermw/dP / http://www.processassociates.com/process/fluid/dp_1.htm http://www.pipeflow.co.uk/theory.html MLD

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• posted on January 25, 2004, 3:31 pm
--
Joseph E. Meehan

26 + 6 = 1 It's Irish Math
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• posted on January 25, 2004, 8:21 pm
Well, gentlemen.... I now know everything I ever wanted to know about the difference in static water pressure as a function of height in a domestic residence.
I COULD measure it a mic, mark it with a chalk... but we're just gettin' set ta' cut it with an ax so... 1/2 #psi/foot of elevation will surely suffice.
By the way. I'm curious that Irish Math, Joe; "26 + 6 = 1 It's Irish Math". I'll bite. What's the riddle?