# A Question About Water Pressure

6 replies to this topic

### #1 Guest001

Guest001

Curious

• Members
• 2 posts

Posted 26 April 2012 - 02:58 AM

We have an underground pool for irrigation. The pool's wide 6m, long 20m, deep 2.5m. A pipe goes out of the floor of that pool. The pipe's diameter is 7.5cm.
Could someone tell me how much is the pressure of the water in the pipe in atmospheres, pascals or bars?

### #2 Turtle

Turtle

Member

• Members
• 14883 posts

Posted 26 April 2012 - 03:22 AM

the pressure in the pipe will vary with the depth of the water. in any case, if you need the water delivered at a level higher than that of the water in the tank you will have to pump it. but i guess you knew that. here's a page that gives equations for making your calculations.

Static pressure and pressure head @ engineeringtoolbox.com

Edited by Turtle, 26 April 2012 - 03:24 AM.

### #3 Guest001

Guest001

Curious

• Members
• 2 posts

Posted 26 April 2012 - 03:30 AM

no, it's not going to go upwards, there is a fall. I just didn't want to make any mistakes with the equations. The pipes I wanted to buy hold up to 4 atmospheres. I also wanted to know how big the fall should be so that I can use the maximum of the pipes and not lose any of the pressure. Hope this makes sense.

### #4 CraigD

CraigD

Creating

• 7314 posts

Posted 26 April 2012 - 12:37 PM

We have an underground pool for irrigation. The pool's wide 6m, long 20m, deep 2.5m. A pipe goes out of the floor of that pool. The pipe's diameter is 7.5cm.
Could someone tell me how much is the pressure of the water in the pipe in atmospheres, pascals or bars?

The page Turtle linked to give the details, but the calculation’s pretty simple:

$p = d g h$
where [imath]p[/imath] is relative, or gauge, pressure, [imath]d[/imath] is the density (of water, in this example, 1000 kg/m3), [imath]g[/imath] is the acceleration of gravity (about 9.8 m/s/s), and [imath]h[/imath] is the height (which you can also call depth) of the column (2.5 m in this example).

You pressure, then, would be 24500 kg/m/s/s, in standard SI pressure units, 24500 Pa. A standard atmosphere is 101325 Pa, so this converts to about 0.24 atm.

As a rule of thumb, for every 10 m of height, you get 1 atm of pressure.

What’s amazing (or at least, legend has it, was to Archimedes back around 250 BC) about this equation is that the other dimensions of the container, even if it’s a very complicated shape, don’t matter – only the water height does. You can use this practical science knowledge to amaze you friends unclogging drains with garden hoses on rooftops, and other stuff sure to make your reputation as a science-y person.

The pipes I wanted to buy hold up to 4 atmospheres. I also wanted to know how big the fall should be so that I can use the maximum of the pipes and not lose any of the pressure. Hope this makes sense.

You can work the pressure equation the other way to calculate that your 4 atm max pipe could handle up to 40 m of water column height – though you’d have to be careful that that maximum rating is a service, not a failure, rating, and other practical engineering and plumbing considerations.

### #5 Ludwik

Ludwik

• Members
• 97 posts

Posted 14 May 2012 - 10:51 PM

The page Turtle linked to give the details, but the calculation’s pretty simple:

$p = d g h$
where [imath]p[/imath] is relative, or gauge, pressure, [imath]d[/imath] is the density (of water, in this example, 1000 kg/m3), [imath]g[/imath] is the acceleration of gravity (about 9.8 m/s/s), and [imath]h[/imath] is the height (which you can also call depth) of the column (2.5 m in this example).

You pressure, then, would be 24500 kg/m/s/s, in standard SI pressure units, 24500 Pa. A standard atmosphere is 101325 Pa, so this converts to about 0.24 atm.

As a rule of thumb, for every 10 m of height, you get 1 atm of pressure.

What’s amazing (or at least, legend has it, was to Archimedes back around 250 BC) about this equation is that the other dimensions of the container, even if it’s a very complicated shape, don’t matter – only the water height does. You can use this practical science knowledge to amaze you friends unclogging drains with garden hoses on rooftops, and other stuff sure to make your reputation as a science-y person.

You can work the pressure equation the other way to calculate that your 4 atm max pipe could handle up to 40 m of water column height – though you’d have to be careful that that maximum rating is a service, not a failure, rating, and other practical engineering and plumbing considerations.

My recollection (from using the quoted equation for water) is that an increase of 10 meters in h (depth in this case) adds one atmosphere to pressure.

Ludwik Kowalski
/

### #6 Mountain

Mountain

Thinking

• Members
• 18 posts

Posted 09 September 2013 - 12:58 AM

In case you were wondering 4 atmospheres is 56 psi, most water pipe is rated at 60psi minimum, you would have a hard time getting that much head pressure unless you have one whopper of a drop.

Edited by Mountain, 09 September 2013 - 12:59 AM.

### #7 cujet

cujet

Thinking

• Members
• 11 posts

Posted 22 September 2013 - 11:49 PM

For those in the United States:

Each foot of water "head" is roughly 0.43PSI.

But a very "quick and dirty" way to think about it is like this: Cut the head in half for PSI. Of course, it's off a good bit, but when estimating the pressure in a rain downspout that's clogged, it's close enough.