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Author Topic: Real life math problem
TheMoMan
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Icon 1 posted June 20, 2007 04:02      Profile for TheMoMan         Edit/Delete Post   Reply With Quote 
golfgimp______________________________The thirteen Psi sounds correct, depth or height of the water is the sole pressure creater, ask a diver. Flow versus pipe size will reduce the pressure.

http://www.onr.navy.mil/Focus/ocean/water/pressure1.htm

Now if you could contain the water near the eaves you would have more pressure, but that might be ugly and or dangerous.

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Benjamin Franklin,

Posts: 5837 | From: Just South of the Huron National Forest, in the water shed of the Rifle River | Registered: Sep 2002  |  IP: Logged
The Famous Druid

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Icon 1 posted June 20, 2007 06:13      Profile for The Famous Druid     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Originally posted by golfgimp:
Well, yes I am collecting rain water for the garden. I am going to trench the pipe in and if I can't get much water pressure I'll go drip.

(keeping well out of the water-pressure debate, my memory of those physics classes is very hazy)

Drip's definitely the way to go.

18 55 gallon drums might sound like a lot of water, but if you want to last through, say, a month without rain, it won't go far. Drip watering systems are far more water-efficient than sprinklers.

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Posts: 10670 | From: Melbourne, Australia | Registered: Oct 2002  |  IP: Logged
TheMoMan
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Icon 1 posted June 20, 2007 06:39      Profile for TheMoMan         Edit/Delete Post   Reply With Quote 
Golfgimp & TFD_______________________Or soaker hoses, any water put into the air will suffer from evaporation.

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Benjamin Franklin,

Posts: 5837 | From: Just South of the Huron National Forest, in the water shed of the Rifle River | Registered: Sep 2002  |  IP: Logged
GameMaster
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Icon 1 posted June 20, 2007 19:39      Profile for GameMaster   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
Spungo is right, the thickness in this case doesn't matter, as the water is "stacked" by it's height. Changing the pipe width will do nothing. To increase pressure you need to "stack" more water... put the tanks higher and get longer pipe.

Increasing the diameter of the pipe will only increase the area over which the pressure will be applied but assuming that opening in the tank is as big as the pipe (read: the valve fits the pipe) no pressure is lost.

The Venturi Effect and Bernoulli's Principle applies to horizontal lengths, where the height of water is the diameter of the pipe. If at the bottom you had an elbow, and after that went from a large diameter pipe to a smaller diameter of pipe, it would decrease the pressure in the smaller pipe (the amount stacked horizontally) and increase flow/velocity in the smaller pipe. Because, the same amount of water has to pass through the a point in the large pipe and a point in the smaller pipe over the same time -- so the velocity increase as the pressure decreases as teh diameter of the pipe decreases in horizontal lengths, which would mean the force coming out of the horizontal pipe would be more.

If in doubt, do it. Gallileo dropped fruit (a grape and an orange -- i believe) at the cardinals house to prove that different weighted objects fall at the same rate (neglecting friction).

So, here's what I propose. Get two straws, a think one and a thin one. Cut a peice that is a centimeter long of each, and a peice that is 2 cenimeters long of each. Get a soda bottle with 2 caps. Cut holes matching the size of the straws. Then in turn put them both over a bathroom scale (you may have to tilt the scale so that the water runs off and doesn't tilt the experiment, and they should be as close the the same height above the scale as you can get -- measure if possible). Read the scale and record the results. Then to get an idea of the correct pressure, simply divide the weight by the area of the opening of the straw. That should get you the force of the weight of the water in a 1x1 column or in other words a good approximation of the pressure at that point.

That said, pressure is in force per unit area, increasing the area (diameter of the pipe) will increase the effected area and the overall exerted force. The bending and narrowing of the pipe at the end may or may not be a good idea depending on the amount of force lost in the direction change, I would play with it and use a scale to measure the resulting forces until the desired effect is obtained. Please report your findings and the lab sheet is due on Tuesday. [Razz] [crazy]

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GameMaster
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Icon 1 posted June 20, 2007 19:48      Profile for GameMaster   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
Almost forgot after a 30 foot drop, the pressure is:

At the bend:
~2 Atmospheres
or
~56 inches of mercury

And I also agree with TFD, drip is a better idea.

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GrumpySteen

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Icon 1 posted June 20, 2007 20:43      Profile for GrumpySteen     Send New Private Message       Edit/Delete Post   Reply With Quote 
Just one small detail that should be considered during the design of all this:

Will the roof of your house support over 8,000 lbs of water for an extended period of time? Not that I really think you're going to get -that- much rain, but life has a way of becoming more interesting than you wanted it to be sometimes.

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The Famous Druid

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Icon 1 posted June 20, 2007 21:57      Profile for The Famous Druid     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Originally posted by Steen:
Will the roof of your house support over 8,000 lbs of water for an extended period of time? Not that I really think you're going to get -that- much rain, but life has a way of becoming more interesting than you wanted it to be sometimes.

You might be surprised.

Assuming, say, a 200 m^2 roof, 20mm of rain will deliver 4m^2 (or, if you prefer 4 tons) of water.
Very heavy rain is about a mm a minute, so you could fill those drums with a good 20 minute downpour.

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golfgimp
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Icon 1 posted June 20, 2007 22:38      Profile for golfgimp   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
The water barrels are going under our raised front walk way so rain barrels falling into the bedroom at night isn't a worry. Having read all this I will definitely go drip and thank you everyone for the clarification on pressure, force and flow. Some of it even makes sense!
Posts: 3 | From: Salt Spring Island, BC | Registered: Jun 2007  |  IP: Logged


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