"Harry Bloomfield"; "Esq." wrote in
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NY explained on 06/08/2019 :
I imagine few of us have ever used a very fat pipe for siphoning -
normally it is something about the diameter of a hosepipe.

Even an hosepipe size, suffers the syphon action being disturbed.

Odd that I've never noticed it. But it's something I'll look out for. I
presume in a clear pipe you can see air collecting at the top of the pipe
and maybe even bubbling up the outlet pipe.

Is it purely the diameter that causes air to bubble back up, or is it the
ratio of the diameter to the length? In other words, is a short fat pipe
more likely to do it than a longer pipe of the same diameter?

The water's skin (name?) is the main thing which allows it to work well in
the smaller pipes. Add in some soap, disrupt the skin and then you have
more issues syphoning with even a small diameter pipe.

That sounds like the makings of a "try this at home" experiment: siphon
first from a "reservoir" of clean tapwater and then from one with a lot of
soap in it, using the same pipe laid out in the same way (*). I wonder if
the flow rate is noticeably worse with soapy water? Repeat with the outlet
of the pipe under water.


(*) Eg at 45 degrees, discharging into a vessel which a) always keeps the
output out of water, and then into a vessel which keeps the output under
water.