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Pressure switches (flushing loo with rain water project)

Postby NigelG » Sat Nov 08, 2008 6:28 pm

Sorry I mucked up, I posted the following as a response to myself instead of as a new topic. I'll get the hang of it soon?

I am now very close to having our system working i.e. I now have the submersible pump (whale 881) in the butt and on its own it is filling the cistern no problem. Next I have piped in a T to take the pressure switch. I have been given 2 to try; both are adjustable: 0.11 - 0.4psi and 1.0 - 4.0psi respectively and both have Common, NC (‘normally closed’) & NO (‘normally open’) terminals. In both cases I am getting the same result: if I use the Common/NC option the switch clicks on and off repeatidly making the pump works but it hunts/pulses and is very slow no matter how I adjust the pressure range but, it does stop when the cistern fills and starts again on demand. If I use the Common/NO option the switch is silent and the pump runs very smoothly and fast but once the cistern fills and the valve closes, the pump carries on pumping no matter what setting I try on the pressure adjustment. My guess is that the Common/NO option is the correct wiring option but neither switch is the right one (PSI range) to suite the whale pump I have.

By the way I did consider using a level switch to control the on/off function but, unless there is a wireless version, to wire in the switch (based in the cistern) back to the pump would be difficult to put in place so hence the pressure switch idea.

Does anyone know how the pressure switch works i.e. from the point of which of the NO or NC options is the right one to use and why?

Thanks again
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Simply Build It

Postby dennis graham » Sat Nov 08, 2008 9:54 pm

hi, i tried to patent the very same thing some 8 years ago! i did make one that is still working and supplying 3 loos!
dennis graham
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Postby plumbbob » Sat Nov 08, 2008 9:59 pm

All the pressure switches I have ever seen are just straightforward on/off two way switches ie, as the pressure reaches the preset level, the intrernal connection swaps between Common/N/C contacts and common/N/O.

Thinking about it now, I can see why the pressure switch option may not work reliably.

I should have thought the N/O option would be correct because using the N/C would reverse the operation, ie, switch the pump off when the pressure was low instead of on, so basically it wouldn't work.

If we assume the switch is working and wired correctly, maybe the problem is with loss of pressure in the pipework. Are you sure the pump is not allowing a backflow causing it to pulse? What about the cistern valve? Often these valves don't simply cut off the flow but continue to dribble for a while. Pull the float up with a finger. Does this stop the pump?
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