water feature problem

[metal fabric]

Hi guys, I came across an interesting problem recently while building a water feature out of an old kettle and a terracotta tub. I mounted the kettle at a 45 degree angle on a piece of pipe and had this feeding into the tub as per my rough drawing at: http://erickendall.gallery.netspace.net.au/water-feature

The return pipe to the resevior gave me the water level in the tub, but to make the whole unit look better and to stop leaves and debris going down the return, I made up a brass cap so the water would have to be drawn from below the water surface to get to the return. I thought this would work quite well, but after about 2 minutes of running the unit, it made a gurgling noise and then the water level dropped to nearly the bottom of the brass cap. I couldn’t believe it!! This happened a couple of times, so then I thought it may need some bleed holes near the top of the brass cap. I drilled three 3mm holes around the circumference near the top. Turning the unit on again I found it took a little longer, but it still did the above action. I then drilled another 3 in between the original holes. This fixed the problem, but I still have no idea what the physics are behind it. By the way, the kettle spout is about 12 mm diameter with the water coming out under gravity. Any ideas?

[MrMongoose]

When the entire pipe/cup section is enclosed, the hydrostatic pressure is atmospheric at the bottom and decreases linearly to the top of the pipe, so the water is having to flow against an adverse pressure gradient. By drilling holes, you relieve the negative pressure at the top, so the water is flowing from an area of atmopsheric preussre to an area of atmospheric pressure under gravity.