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I'm designing a rain gauge which uses accurate electronic scales to measure weight of water (and therefore volume) that has entered the sump. Ideally the system will not require any manual intervention.

When the water level exceeds some suitable value I switch on a pump (I'm using a low voltage submersible centrifugal pump) for a fixed time. That time is long enough for the water level to drop so that the pump is now "empty". I then reset the scales to get my new zero weight level.

So far so good - but the outflow tube now has air in it. Some of the water in the tube may flow back down into the sump. This will create an error in the weight measurement.

A solution would be to stop pumping when the weight reaches some lower value, but which is enough to stop air entering the pump. Now, the outflow tube should not have any air in it so the water in it will stay there. But in a dry spell the water in the sump could evaporate, so we are back to the original problem.

I suspect (but don't know) that some kind of lift pump might get over the problems. Is such a pump available at low voltage and low cost? Or some other kind of pump perhaps?

keble
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  • reset the scales after any water has flowed back - simple. – Solar Mike Mar 25 '21 at 21:33
  • look for a pump described as "self priming" ... – Pete W Mar 26 '21 at 01:35
  • why do you need a pump? Just use a gravity drain. Or don't ty to pump the water uphill. Or don't have an outflow tube at all. – Tiger Guy Mar 26 '21 at 07:29
  • Re "self priming" - Priming isn't the problem, at least to my understanding. It always starts to pump! Re "why do you need a pump" - it's a rain gauge, so I have to collect the water in a tank/sump in order to weight it, and hence measure the change in rainfall over a period of time. – keble Mar 26 '21 at 07:30
  • @Tiger Guy I need a pump because the water is being weighed periodically. That takes time. I need to empty the water before it overflows! – keble Mar 26 '21 at 11:15
  • But a gravity drain could be useful - that needs a bit of work to the sump in order to make a leak-proof electrical valve, so a bit more complex. I should add that this is intended as a school project! – keble Mar 26 '21 at 11:17
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    @keble whether you're weighing water in the collector or outside of it you don't need a pump to move water. A solenoid operated ball valve would be easier to install than a pump. Valves generally do not leak. – Tiger Guy Mar 26 '21 at 15:38
  • can you provide a sketch of your setup? I have a hard time warapping my head around your concept: Specifically: Do you really weigh the pump along with the water? What pump specifically do you have? flow rate, head? – mart Mar 26 '21 at 16:30
  • @mart Here are some photos 1 shows the sump with pump inside it. Water level is low, so pump is not activated. – keble Mar 27 '21 at 09:05
  • @mart next photo 2 shows water left in the outflow tube after the pump has activated.It is this water that can flow back into the sump and some time after the pump has switched off, and it causes the weight measurement to be in error. – keble Mar 27 '21 at 09:08
  • @Tiger Guy OK that idea sounds good. I found a small one here. I'll see if I can find a 5 volt one – keble Mar 27 '21 at 09:13

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