Simple flow sensor wanted

flow switches are available (try a commercial plumbing outlet)

otherwise the can be made... here are two ideas.

stick two stainles steel wire electrodes along opposite sides (of the inside) of the tubing and put a magnetic field through the gap. (not across) measure the voltage with a high impedance voltmeter.

may not work well if mounted too close to the pump or if the pump develops an electrical fault. running the water through a metre or so of metal pipe (copper/stainless/galv) pipe before the sensor may ba all that's needed,

This one may collect rust particles if they are present in the water and the flow is too slow.

some boats use a setup like this for their speedometers.

you can use copper wire electrodes to test-drive this one, but stainless lasts much longer in service.

another option is this, (cross section)

if garbled view in a fixed-pitch font (like courier) or paste into notepad

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Reply to
Jasen Betts
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Just put a pressure switch after the pump. when the tank is empty the pressure will drop.

Bye. Jasen

Reply to
Jasen Betts

the threads beneath those compression nuts should match the threaded adaptors used for 19mm plastic pipe too...

19mm is just the metric name for 3/4" pipe. it's still 3/4".

Bye. Jasen

Reply to
Jasen Betts

how many watts are you going to need to push through tat thermistor to get a result in under minute?

that setup works great with air but water based solutions take much much more energy to warm up.

Bye. Jasen

Reply to
Jasen Betts

I've got problem with that, but WTF is 22mm -- which is the size these come in.

The 1/2" equivalent appears to be called 15mm, and the 3/4" equivalent is called 22mm!

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John H
Reply to
John_H

What sort of pressure drop do you have in mind?

The frictional loss in 19mm tubing flowing 20 litre/min is something less 3 metre of head per 30 metre. (A google on Hazen-Williams should find the various tables for frictional losses)

I'm pumping through 6 metre of hose, which will give around 600mm head at the pump. This works out to be around 0.1 bar (1.5 psi) pressure.

I'd also suspect that the problem would be compounded by the fact that I'm feeding to the bottom of the receiving tank (to minimise the head pressure, and hence the pump load). This will have the effect of significantly varying the static head at the pump outlet, depending on the level of the main tank and the slope the machine is working on (which can be relatively steep on occasions -- 30° or so).

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John H
Reply to
John_H

Look at placing a plastic encapsulated bar magnet in a flaring high pressure plastic pipe with a magnetic reed switch above the magnet and against the pipe. Flow causes the magnet to rise, actuating the reed switch. Put the reed switch in series with a relay actuation coil. No issues with corrosion and very simple. As with any pumping application, size a foot strainer to ensure particles will pass by the magnet in the flaring tube. The flaring tube can be an adaptor from 3/4" to 1", the magnet diameter is sized to have sufficient restriction in 3/4" tube to have the liquid flow cause the magnet to rise into the bigger cross section tube above with flow.

Reply to
Einstein

PS, if you don't want to use an electrical circuit, use a magnet on the outside of the pipe, located on a pivoting arm, so that the outside magnet follows the movement of the inner magnet. The position of the outside magnet will indicate whether you have fluid flow or not.

Reply to
Einstein

I was looking through our Omega catalog for other bits and pieces and this caught my eye after having read your post.

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If you insert a small tube into the flow, and it doesn't have to be very far... like the dripper take-off in small irrigation systems... you will develop a negative pressure in the tube (venturi effect). Cut the end of the tube to a 45 - 60 degree angle and by placing it facing towards the flow you get a positive pressure, facing away you get a negative pressure. A sensitive low-pressure switch can then be used, similar to the one above.

Reply to
David (one of the many)

Sounds neat... same principle as the flow meters used on gas cylinders I presume, but is it commercially available as a switch?

Whilst it's not too difficult to fabricate something for a special purpose, ease of service and readily available replacements is a major consideration in this particular application, where downtime can be very costly.

Thus whatever I decide on needs to be serviceable, or readily replaceable, by the machinery operator -- which does tend to eliminate some of the cleverer solutions. Any potential problems that might be associated with the device also need to easily diagnosed by the operator -- such as a blockage or a failure to work as it should.

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John H
Reply to
John_H

It's the size of the threaded part. pipe is sized by the diameter of the hole (as that's the most important part) but the couplers are sized by the thread (as that's where the action is)

Yeah that sounds about right I haven't dealt with that stuff for over a year now.

Bye. Jasen

Reply to
Jasen Betts

If you want a DIY solution, can I suggest a plastic ball or slug with an embedded magnet operating a reed switch on the outside of a short piece of tubing next size larger than your delivery tubing. The ball or slug will need to be slightly larger than the ID of the delivery tube, and have some kind of restraint to limit its travel. If you cannot arrange the flow vertically up, you would also need a light ss spring ahead of the ball/slug. Food for thought.

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Regards,

Chas.
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Reply to
Chas

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