"Martin Brown" wrote in message
...
On 05/08/2019 15:18, Tim+ wrote:
Harry Bloomfield, Esq. wrote:
Chris Green formulated the question :
Is there a size of pipe above which a syphon won't work because the
water empties out of the down pipe without 'sucking' the water above
down? It works well with small pipe because, I assume, surface
tension helps to prevent the water from 'dropping out'.

Correct!

Air has to be prevented from being sucked up the discharge pipe, by
having the discharge under water, or the syphon will be immediately
lost with a larger pipe.

Nonsense! Have you actually tried it? Ive syphoned water plenty of times
and never had to keep the outlet underwater.

He is basically right at least when the pipe size becomes large enough
that the water in free fall near the middle allows bubbles of air to get
back up the top side where friction slows the water to a crawl.

Talking large diameter pipes here not thin hose pipes. It doesn't take
much to prevent air going back up the outlet but it does require a bit of
care or bubbles of air will rise up the top side of the pipe and
accumulate at the top of the syphon.

I presume if the outlet pipe is kept vertical, there is less chance of air
bubbles getting in at the bottom and rising to the top of the loop, breaking
the siphon.

I imagine few of us have ever used a very fat pipe for siphoning - normally
it is something about the diameter of a hosepipe.

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?