Vented Loops

macd said:
Indeed.
It's easier in metric: around 10m, obviously depending on atmospheric pressure.
KellysEye's loop would be fine if he could find a partial vacuum to sail around in, although his sails wouldn't work very well. Couldn't make a decent cuppa, either ;)
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I think lake Titicaca is somewhere on the way. At least, you could reduce the length of the pipe a bit. I don't know about the cuppa. The highest tea I have drunk was at something over 2000m in Ethiopia, which was actually rather delicious.
 
john_morris_uk said:
????

Sorry but that doesn't make any sense to me.
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Sorry, only just noticed it was you. In order to break a syphon you have to break the body of water with either a gas, air, or a vacuum. A mercury barometer will sustain a column of about 760mm of mercury, befor a vacuum is created above it, and mercury being around 13 times as dense as water, this means that an equivalent column of water would be around 3/4 x 13 metres, roughly the 30ft I proposed. I think this is why most of us prefer valves.
 
john_morris_uk said:
????

Sorry but that doesn't make any sense to me.
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Look up 'Torricellian vacuum'.

A countinuous loop of water will allow water to siphon through it - bad news on a boat. As johnalison wrote, to prevent this, you need a siphon break. Usually this is a hole (which may be valved to make it one-way), which allows air to enter the top of the loop. However, if the loop is sufficiently tall, the weight of the liquid in it will create a vacuum at the top of the loop, which will also work as a siphon break. For mercury, this height is about 760mm. For freshwater, it's about 10m.

The effect varies with atmospheric pressure. Mr Torricelli was bright enough to deduce that a device could use this effect to measure atmospheric pressure (which was actually his main area of interest). We call this a barometer (although other mechanisms have since been developed). A Torricellian barometer uses a column of mercury in precisely this way - which is why we talk about 'the mercury rising/falling'. What we actually mean by that is that the vacuum gets smaller or greater in volume in response to changing atmospheric pressure.

Torricellian barometers are easy to recognise, since they obviously have to be greater than 760mm tall.
 
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Thanks for the explanation and it all makes perfect sense.

Of course, one has to have conditions under which the siphon actually starts (and is sustained).

My gut reaction is that the sustainability of the siphon action is a function of pipe size and the fluid properties. Presumably offset levels and heights of the siphon also affect things.

It's forty years since my days in the physics deoartment of UC Swansea and I was useless at fluid dynamics then!
 
>That's not enough to prevent syphoning; it would have to be something like 30ft I think.

That's wrong we never got syphoning sailing 10,000nms and Jane and I would use the loo three times a day each on average.
 
KellysEye said:
>That's not enough to prevent syphoning; it would have to be something like 30ft I think.

That's wrong we never got syphoning sailing 10,000nms and Jane and I would use the loo three times a day each on average.
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What they are saying is that syphoning can take place up to 30ft............not that it will. Thats the way I read it anyway.
 
Sandyman said:
What they are saying is that syphoning can take place up to 30ft............not that it will. Thats the way I read it anyway.
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Quite.
I suspect that what may happen is that a certain amount of air remains in the outlet pipe, and may even be replenished due to turbulence as the loo is pumped out. Although unintended, this would have the effect of a siphon-break. (This is supposition on on my part, but seems realistic). However, it would not be prudent to rely on it.

What is clear is that if a hose without a break is completely full of water, and the interior exit is below sea level, it will siphon unless there is a physical impediment to its doing so. No ifs or buts.

I might add that when we bought our current boat, it had a loo out-loop much as KellysEye described. On two occasions the bowl spontaineously filled to sea-level, which was luckily about four inches below the top of the bowl. There's now a holding tank in the circuit, so no worries with siphoning back.
 
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