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Old 03-07-2008, 20:36   #16
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Sorry that doesn't compute. 40 ft of water column = 17.34 PSI. Will never push water through a salt water RO filter
Wrong. A 5 gallon container with a surface area of 1 sq' would exert 17.34 psi X the surface area of the membrane = psi on membrain.
So you would need a 1 sq' surface area of container @ 40' with a membrane that has 46.13 sq' to get 800 psi on membrane. Basic hydraulics.
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Old 03-07-2008, 21:06   #17
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You have a very basic flaw in your understanding of hydraulics. It doesn't matter how large a tank is in diameter, the head pressure is developed by the height of the column of water. The formula for calculating this is P=S.G. x h. If you were to develop that pressure by an external force as is done in a closed system then you have what we all use.
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Old 03-07-2008, 21:11   #18
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Originally Posted by easterly View Post
Wrong. A 5 gallon container with a surface area of 1 sq' would exert 17.34 psi X the surface area of the membrane = psi on membrain.
So you would need a 1 sq' surface area of container @ 40' with a membrane that has 46.13 sq' to get 800 psi on membrane. Basic hydraulics.
SOOOOoooooo. My 800 sq inch car tire has 35# x 800" = 28,000# of pressure...... that is interesting math....

No, you have 17.34PSI, or Pounds per square inch, on each inch of the surface area of the membrane. This will not work.... would be nice if it would though.
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Old 03-07-2008, 22:15   #19
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First off two different membranes. A Home Depot RO membrane is NOT a SW membrane. The pressure required is far less for a fresh water membrane. Try to install a real Salt Water membrane and you could hoist your five gallon jug up a 200' mast and all you'd have at the end of the day is five gallons 200' in the air. If you want to spend $100 on an emergency WM you can get a pur survival 06 all day long on Ebay. Of course Lancerbye is correct, diameter does not determine head pressure. To the previous poster who is running their WM at 1000 psi, you are at the max pressure any membrane of that type can handle, in short it will not last as long as it should. Membranes are not of the Tim Allen school of more power is better. If you need to run it at 1000 psi to get it's rated gph output you probably have a minor issue with your WM.
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Old 03-07-2008, 22:25   #20
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Yeah yeah, if the pressure was wrong, why did you tell people to do it? Obviously you've never seen it work, nor tried to go sailing with an extra 40+ pounds of dead weight up at the masthead. Filling a system with 'dirty water' isn't at all the saline as desalinating seawater.

Posting a link to the Seapack is pretty useless too. That's a one-shot emergency desalinator, it won't work for long. And it DOES REQUIRE ENERGY INPUT, it requires $100US of energy for ONE SHOT. Whether it takes gas, electricity, or money, it still requires "energy" up front. I can buy 100 gallons of water, in bottles, for the same money. That's way more effective.
Plus you can get a recycling fee for the empty bottles.
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Old 03-07-2008, 22:31   #21
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but i guess no one wants to think that anything less than a 2000 dollar system that can produce 10 gallons a min will work at all. i guess i will have to scrounge the parts up to make one


First, Glad to have you aboard Scotty Second, If you could come up with a 10 gallon per minute watermaker for 2k send me a private message and we'll be rich by Saturday. <G>
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Old 04-07-2008, 02:04   #22
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This will not work.... would be nice if it would though.
No it wouldn't. You car tire example is the reason why :-)

10Gall per minute?? Not even a real and expensive desalinator could produce that much per minute could it?? That's a lot of water.
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Old 04-07-2008, 02:37   #23
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... if you dilute salt water 50 % ( which is safe but nasty tasting ) ...
The generally recommended drinking water standard is a maximum of is 500 ppm (500 mg/L) of Total Disolved Solids (TDS).
Seawater, at 35,000 ppm, would have to be diluted at least 70 times (70 parts fresh to 1 part salt water) to reach a potable level. Even so, these salt levels may upset your blood chemistry & renal function.
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Old 04-07-2008, 05:47   #24
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To the previous poster who is running their WM at 1000 psi, you are at the max pressure any membrane of that type can handle, in short it will not last as long as it should. Membranes are not of the Tim Allen school of more power is better. If you need to run it at 1000 psi to get it's rated gph output you probably have a minor issue with your WM.
Your right actually it runs at 800 to 850 psi with max pressure being 1000psi. It produces 16 gal per hour I think the rejection point is at 100 to 120ppm(adjustable) I tried running it as low as 50ppm but it would kick in and out too often.
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Old 04-07-2008, 06:35   #25
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You have a very basic flaw in your understanding of hydraulics. It doesn't matter how large a tank is in diameter, the head pressure is developed by the height of the column of water. The formula for calculating this is P=S.G. x h. If you were to develop that pressure by an external force as is done in a closed system then you have what we all use.


1sq" x 1 psi acting on a 15sq" surface will yield 15 psi on that surface.
F2=F1(A2/A1) Pascals law
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Old 04-07-2008, 06:45   #26
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Here is a link to explain head pressure.Relation of Pressure to Elevation
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Old 04-07-2008, 06:57   #27
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OK if you have 17psi and it acts on a surface area of 43sq' you will have 800psi. You are correct about head pressure but if you use the pressure on a larger area the force is multiplied. Pascals law.
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Old 04-07-2008, 07:01   #28
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SOOOOoooooo. My 800 sq inch car tire has 35# x 800" = 28,000# of pressure...... that is interesting math....
Only works for incompressable liquids.
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Old 04-07-2008, 07:19   #29
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Easterly,

Work though the units--the "square inches" cancel out.

(17 lb/sq in) x 43 sq in = 731 lbs of force on the 43 sq in surface

The pressure is still only 17 lb/sq in

pressure drives the reverse osmosis, not force.
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Old 04-07-2008, 07:24   #30
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Pounds per Square Inch, or PSI, are a unit of pressure or of stress (force per unit area), resulting from a force of one pound applied to an area of one square inch.

A 1000 pound weight, that is spread evenly over a surface area of 1000 square inches, will have a pressure of 1 psi.
On the other hand, if you take that same 1000 pound weight, and put it on a surface area of 1 square inch, it will have a pressure of 1000 psi.

17 psi is a force of 17 pounds acting upon 1 square inch, or any other arithmetic combination that equals a product of 17 (8.5# on 2 sq. in, etc).
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