If you have an anorak, put it on now!
No, you're not sad. It's good to come across somebody who's interested in what's going on. I'm not a plumber either, i'm an electronics engineer. Although I do all of my own plumbing. And a few other select plumbing jobs as specials.
Being in IT, whilst you probably know what the problem is with a one eyed god I just threaten it with a through investigation of its inards, which usually does the trick.
Hmmn. I have a feel for what you're saying there about the pipe not being in full flow until the overflow inlet is submerged by a certain amount.
My guess as to why would be -
Taking your current configuration of overflow as an example. Obviously the overflow configuration is restrictive enough on water flowing down it to give rise to this situation. Given a big enough overflow then the following situation would not exist. But therein lies my concerns about many of the overflows you see in use.
With a rising tank water level and a gentle fall on the pipe, the water enters the overflow and flows down it at a sedate (good word, glad I thought of it
) rate. Both water and air enters the top of the pipe, trickles down it and out of the end.
The tank level rises a bit more to where the overflow inlet is nearly covered (perhaps just over to the top of it?). The flow down the pipe is still at a rate that's less than it's fully capable of. It now starts to make a noise as it sucks air with it. I'll guess that what's happening at this point is that the falling water going down the pipe, whilst sometimes making a seal at the top, due to the suction, occasionally breaks that seal in the easiest way it can. The pipe is a mixture of air and water and complicated by the fact that the air wants to rise and water wants to fall.
The tank level rises even more to the point where there is enough water pressure at the inlet for the water flow to flush the remaining air out of the overflow pipe. At some point, and it's easily seen in small bore pipes with air bubbles in them, flow wins. I have no idea what the equation would be (where are the fluid dynamics people when you need them?). Now the pipe is fully loaded with water. Here positive feedback kicks in, the fully water loaded pipe then increases in flow up a rate determined by pipe resistance, head of water at the inlet and probably a couple of others. The higher flow allows the pipe to suck the tank to a lower level than the full flow could ever start at. The cycle repeats.
Pity they don’t make ‘transparent’ pipes and connectors then we could watch what is going on inside.
They don't specifically but you can make them. One of my first posts to this group was about the problems and cure of a central heating system. Many people had had a go at trying to establish what was wrong with a friend's system. There were all kinds of effects going on: Pump cavitation, Hydrogen being produced continuously in one particular radiator, periodic noises, corrosion, magnetite buildup in select pipes, poor heat flow, motorised valve ball had rotted through, kettling in the boiler. In the end I made some clear plastic sight glasses (basically transparent pipes) to go inline with some of the 22mm copper pipes and built some visual pressure gauges (lengths of vertical clear pipe tapped into the system in and around the pump so I could see in real time the effects of flow and the pump) from polythene pipe. The result was a revelation and allowed me to see changes in pressure and also the fact that it was virtually blocked in one particular place and that there was a constant ingress of fresh water into the system from other problematic plumbing. It let me properly see what was going on rather than blindly replacing components/piping and not learning much from it.
I've been fortunate enough at times to have had the time to investigate some of this and learn from it, although my friend nearly throttled me on about day 4. They did get a fully fixed system. In the unknown there's usually something to be learned.
I often find myself wanting to joint different systems together, and between what the plumbers merchants have or know about and what the Pneumatics/Hydraulics people have and/or know about. Thankfully they both cross in British standard pipe thread (BSP)! Although trying to describe what you want when you're not sure of the name for it without going into why you want it (as that usually causes far too many questions) can be hard work.
The link is most amusing. I never realised that the OED had that wrong. Mind you it's not the first place I'd look for that kind of info. I never attributed the workings of a siphon to air pressure.
