ISEN-ASTC-L is a service of the Association of Science-Technology Centers
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If you can use a siphon, keep the loose end of the hose at a constant distance below the water level in the tank (maybe a float in the tank can help with this). The pressure differential will be constant, and so will the flow rate.

Brian McWilliams
Public Programs Manager
Sci-Port: Louisiana's Science Center
820 Clyde Fant Parkway
Shreveport, LA 71101
Main Line: (318) 424-8688
Toll free: 1-877-SCIPORT 
(1-877-724-7678)
Fax: (318) 222-5592
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-----Original Message-----
From: Informal Science Education Network [mailto:[log in to unmask]] On Behalf Of Peter Ford
Sent: Thursday, August 19, 2010 10:08 AM
To: [log in to unmask]
Subject: How to drain a tank of water at a constant rate?

ISEN-ASTC-L is a service of the Association of Science-Technology Centers
Incorporated, a worldwide network of science museums and related institutions.
*****************************************************************************

Hi all,

Here's the technical dilemma that I'm trying to solve.

I want to drain a tank of water, from the bottom, at the same rate 
regardless of how much water is in the tank. So whether there is 5" or 
20" of water in the tank it will drain at ½ gal a minute (values are 
somewhat arbitrary for this example).

So how can I do this? I've tested Pressure-Compensating Orifice from 
McMaster but that didn't work (I think the pressure is to low for them 
to work). I have seen devices that automatically regulate but they are 
for larger industrial systems (and cost a lot), or they are for higher 
pressure systems.

For more specificity it's a 11" diameter tank and I'm draining through a 
1" valve. The water height varies between 4" to 30"

Any solutions, or ideas, would be appreciated.

Thank you in advance.
-Peter

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