On Thu, 05 Jul 2007 20:17:01 GMT, "JoeSpareBedroom"
wrote:

What about the suction being added by the pump which sends the water to the
filter?

I believe that even if there were a total vacuum below the water
drain, the maximum pressure would be the weight of the water and air
above the drain. That's the principle of a mercury barometer (except
that there is a vaccum in the tube above the mercury instead of below
it), and if water barometers were made 600 feet tall, that would be
the principle there too.

I don't think suction really exists. All there is is unresisted
pressure, and the pressure is caused by the weight of the atmostphere
(and the water, when something is under water.)

Sort of like centrifugal force doesn't really exist (that is equal and
opossite to the centripetal force, as elementary school science books
sort of said). All there is is inertia that tends to make the
something go straight ahead, perpendciular to its axis of rotation,
not away from teh center of rotation.