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Actually another factor might be where the water is going to...is it going into open air, or another sealed tank, or...
Sorry, this was just a fun interesting mental distraction for me...

On Aug 19, 2010, at 10:59 AM, William Katzman wrote:

> ISEN-ASTC-L is a service of the Association of Science-Technology Centers
> Incorporated, a worldwide network of science museums and related institutions.
> *****************************************************************************
> 
> I agree - use a constant volume water pump. 
> 
> I could be wrong, but I don't believe a valve will work at all since the cross-area would be the same at the different heights, but the pressure would still vary, so the volume coming out would change.  Of course if the valve changed its opening aperture relative to the pressure, that would work (which I believe is essentially what you tried).
> 
> In theory you could rig up a situation where the water always comes out from 4" below the water surface, but in practice this would not be simple thing to do.  You can always set up a siphon situation with a floater and a sinker holding the hose 4" below the water's surface, but since the height the water going up to get out of the tank varies, the back pressure on the siphon would vary and the water volume would vary slightly (am I wrong here?).  So perhaps a more critical question is how close of a volume do you really need?
> 
> Ah...now a logic problem comes to mind...If you fed a hose hanging from 4" below the surface of the water through the bottom of the tank you would have a constant pressure at the hose's entrance which should lead to constant flow rate into the hose - right?  But you can also consider the situation at the bottom of the hose (at the bottom of the tank) - where all the water in the hose is causing more pressure causing the rate of flow to increase.  So what happens - does the water come out at a fast speed as if it were exiting the bottom of the chamber or at a slow speed as if it were exiting 4" below the surface of the water....take a moment to think about it and discuss it...  ;)
> 
> -William
> 
> PS.  You could in theory put a channel in the side of the tank and have a floating plate that has a hole in it move up and down (staying 4" below the surface) in order to keep a constant size hole at a constant depth - but let's be honest - that would be really hard to make happen!  It might be fun to do, but a constant volume water pump is easier.
> 
> 
> On Aug 19, 2010, at 10:16 AM, Michael Ross wrote:
> 
>> ISEN-ASTC-L is a service of the Association of Science-Technology Centers
>> Incorporated, a worldwide network of science museums and related institutions.
>> *****************************************************************************
>> 
>> Does it need to be drained by gravity? Can you just connect a little peristaltic or small inline circulator pump to force it to drain at the desired rate? 
>> If it needs to be gravity drained, then you will have to figure out how fast the water naturally drains at the lowest level. Then find a valve that will keep it draining at that rate at higher water levels. With gravity drains, you can always find something to decrease the flow of water, but not something to increase it. So if you can use a pump, it might be the easiest option. Regards.
>> 
>> 
>> Michael Ross
>> Program Educator
>> Connecticut Science Center
>> 250 Columbus Boulevard
>> Hartford, CT 06103
>> Direct: 860.520.2104
>> 
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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 11:08 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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> 
> William Katzman
> Program Leader
> LIGO Science Education Center
> "Inspiring Science"
> [log in to unmask]
> (225) 686-3134
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> ***********************************************************************
> For information about the Association of Science-Technology Centers and the Informal Science Education Network please visit www.astc.org.
> 
> Check out the latest case studies and reviews on ExhibitFiles at www.exhibitfiles.org.
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William Katzman
Program Leader
LIGO Science Education Center
"Inspiring Science"
[log in to unmask]
(225) 686-3134

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