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Siphons and fluid dynamics

Working on a science fair project with my 3rd grader we got an unexpected result I am at a loss to explain.  We made a mock up toilet with two different drain pipes / siphons.  When we use the 3/4″ pipe we get a slow drainage that stops when the water in the bowl goes down to about the level of the crest of the siphon.  When we use the 1 1/2″ drain pipe we get a very strong flow and the water level drops all the way down until air can enter the siphon, so the water level drops way below the top of the siphon, down to the intake opening.  I understand that more water travels though the larger pipe, hence the faster flow rate, but I can’t explain the difference in the ending water level in the bowl.  What forces are at work here?  How can we further test or prove these principals?

 

Asked Dan

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