Cheap flow detectors

Only when the flow is fast enough to be turbulent - a Reynolds number in excess of about 2300 in a lot of situations

And sound propagation. And if the flow is turbulent, you can detect vortex shedding behind a bluff body in the channel.

This article doesn't mention vortex shedding flow meters, but they are commercially available.

-- Bill Sloman, Nijmegen

It's called a turbine meter, and you've been able to buy them since the 1970's - probably earlier.

That's a magnetic flow meter, and they've been around since the 1960's at least.

-- Bill Sloman, Nijmegen

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Put a "balloon" in series with the pipe to "detect" water pressure; a "Y" for flow.

...another way is to use a ball in a vertical pipe,,size of ball a little smaller than pipe ID; increase of flow rate will raise ball more.

On Thu, 15 Apr 2010 16:43:07 -0700, whit3rd wibbled:

Wow - I like that - very elegant.

What about a thermistor in self-heating mode? A pair (one in self- heat mode, one not) should get dowt to a reasonable flow sensitivity.

The shuttle (piston) type runs around $20 apiece one-off. Cheap enough?

Best regards, Spehro Pefhany

I didn't even know Slowman.

Who would even want to?

The feeling is perfectly complementary.

Mike Terrell and krw are perfect examples of the fact that evolution - as an essentially random process - throws up a lot more maladaption than useful variations.

-- Bill Sloman, Nijmegen

Yeah, but that would be an "invasive" approach; "cut the pipes". I was hoping for something that could sit "outside" them. E.g., John's sound and temperature ideas are on the right track. I don't know how to even approach the sound technique as I don't know how I can "filter" out the effects of neighbors, etc. Maybe I could put something *in* the supply line to accomplish that (?) -- but, its a big unknown.

OTOH, temperature might work *if* I do everything relatively; with a slow learning algorithm (so, as soil temperatures change and interior temperatures change, I change my expectations of what constitutes a 'difference' in the pipe)

Hmmm... but, would that be "sensitive enough" (whatever that means)? Or, would it just detect "big flows"?

speed of the flow.

As I said in response to Charlie's suggestion: how sensitive would this be? I.e., I would assume it reacts to the *speed* of the water/fluid (and not the mass)?

Yes, I've used some "fan sensors" like this.

Ouch! I guess a lot would depend on the thermal mass to which the sensor (and heater) is attached? I.e., using this on a 1" galvanized pipe outdoors might prove "expensive"?

How would you sense a *leak*? (presumably, that is a much finer resolution)

Welcome to engineering. You can either work to optimize the galvanized pipe outdoors (insulate well, use a pulsed strategy, budget for a separate heater so you can more reliably detect small changes with the thermistor, or even the two thermistors someone else suggested) or you can change a section of pipe or the location of sensing - it depends on the constraints. If the location must be outdoors and not cutting the pipe is a prime constraint, then you have to work with those constraints.

At some point cutting the pipe (or calling a plumber to cut the pipe) and putting a water meter in-line becomes the more cost effective approach.

Normally you put two contacts near the floor, and when there's a puddle, you have a connection and sound an alarm. They have issues with contamination causing false alarms when things like salt films building up across the sensor body and connecting the contacts without liquid water. Floor paint will reduce problems as compared to bare concrete floors. If you can contain the leak (ie, use a pan under the water heater or clothes washer), a float switch is less prone to false alarms. Won't do a thing for your sprinklers, of course.

Or, find another sensing technology that doesn't have these liabilities. : At some point cutting the pipe (or calling a plumber to cut the pipe)