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Thank you everyone for your help and quick reply.

I would like to keep the interactive 'all natural' as possible but using 
pumps is being considered.

We have tried and will be trying many of the ideas mentioned here, such 
as somehow floating the drain 'opening' at a fixed height below the 
surface and a device that work on 'water level-float-draining opening 
restricting device.

I did make a simple inline 'auto' valve restricting device that changed 
based on flow pressure (or rate). It did auto adjust as the water 
level/flow/pressure changed. I have yet to get it to keep the flow rate 
constant (did reduce the difference) though and am intrigued enough to 
want to played around with the design a bit for get better results... 
but time is fast running out and thus the request for help.

For the rest of the system... the tank is open at the top and will be 
filled from above at the same rate as the tank empties. There will be 
multiple drain and filling valves  that can be opened to change. For the 
'filling' valves I was going to use the 'overflowing-constant head' tank 
concept with a valve to match the main tank's (a) drain flow rate.

I figure this may be a bit confusing sorry - somewhat in a rush today.

I'll let you know what happens, what solution we go with/find.

Thanks again.
-Peter

Ian Russell wrote:
> ISEN-ASTC-L is a service of the Association of Science-Technology Centers
> Incorporated, a worldwide network of science museums and related institutions.
> *****************************************************************************
>
> A fascinating challenge! Mariotte's Bottle is new to me. I found this
> simpler version of it for constant flow:
> http://en.wikipedia.org/wiki/Mariotte%27s_bottle
>
> However, you can't have more water flowing into the main tank during
> operation. So I've a feeling that a closed system might not be what you
> want.
>
> If not, here's another solution. The classical way to achieve gravity-fed
> constant flow is a constant-head tank, with extra water continuously running
> to waste through an overflow pipe. I'm guessing that you aren't doing this
> because you don't want to waste water.
>
> You say that you must drain water at a constant rate from the bottom of your
> tank, whose water level varies. I suggest that you drain this water into a
> separate constant-head tank, allowing it to overflow from there, not to
> waste but into a third, lower tank. This third tank has a simple
> float-switch actuated electric pump returning the overflow water to your
> original tank.
>
> The second, constant-head tank will then deliver a gravity-fed flow which is
> perfectly constant. I hope this description is clear enough.
>
> Promoting public engagement with science
> through a contagious delight in phenomena
> *
> [log in to unmask] * http://www.interactives.co.uk
> *
> Give people facts and you feed their minds for an hour.
> Awaken curiosity and they feed their own minds for a lifetime.
> *
> Ian Russell (Twitter: ianrusselluk)
>
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