The pressure sensor method is usually performed as follows:

Assume you have a pipe with two sections, one which is 1" in diameter,
the other is 3/4" in diameter.  If the liquid is not moving, then the
pressure is the same in both sections of pipe.

If the liquid is moving at one gallon per minute, then the flow
through the larger pipe is slower than the flow through the narrower
pipe.  This translates to a pressure difference in the two pipes,
which is related to the speed of the flow.  A single sensor can be
used for this - connect one side to one pipe, the other side of the
sensor to the other pipe, and the sensor only outputs the difference.

Not great for a wide range of flows - a setup that can handle and
measure hundreds of gallons per minute won't be able to easily detect
a 1 gallon per minute flow.  Conversely, a setup that can measure
differences of mL per second isn't going to be able to handle gallons
per second.  So as long as you have a reasonably narrow range where
you need accurate results you should be able to design a cheap sensor
of this type.

-Adam

On 10/31/06, cdb <colin@btech-online.co.uk> wrote:
> Well I don't need the accuracy to be to industrial standards this is
> for home construction after all.
>
> So boring one or two holes for a thermistor or two (perhaps
> thermistor/thermocouple combination) would be easy from a construction
> point of view.
>
> The air pressure one, I'm sure would work, but obviously some sort of
> membrane would have to be used to stop water getting into the pressure
> sensor.
>
> For some reason doing something 'weird' with piezo stuff appeals to
> me. Is there a formula that might correlate force/compression into
> flow? I'm thinking along the lines of the faster or slower a liquid
> flows, pressure is expanded against the sides of the vessel (in this
> case a pipe) which a thin film piezo device might be able to pick up,
> two would be needed one above a constriction and the other below for
> measuring time difference - or have I just confused myself?
>
> Still the thermistor or possibly ultrasonic methods might be the way
> to go. I looked for some capacitive sensors that might do the trick,
> but drew a blank.
>
> Colin
>
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