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) *********************************************************************** 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. The ISEN-ASTC-L email list is powered by LISTSERVR software from L-Soft. To learn more, visit http://www.lsoft.com/LISTSERV-powered.html. To remove your e-mail address from the ISEN-ASTC-L list, send the message SIGNOFF ISEN-ASTC-L in the BODY of a message to [log in to unmask]