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Peristaltic pump or floating drain line are the only two ways I can think
of.  Any gravity flow is under the effect of the hydrostatic head and will
vary over time without a way to maintain constant head.  This model delta
http://serc.carleton.edu/NAGTWorkshops/sedimentary/models/desktop_delta.htmlhas
a recirculating/overflow system to maintain constant head/constant
flow
input that could be modified to maintain constant head/constant flow output,
but it's a lot of trouble when a simple pump might do the job.

Dave Smith



On Thu, Aug 19, 2010 at 11:07 AM, Peter Ford <[log in to unmask]> wrote:

> 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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-- 
David L. Smith
Da Vinci Science Center
Allentown, PA
http://www.davinci-center.org

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