Hmm, yes, or pair metal tubes intruding into the flow, upstream has a
DS18B20, downstream a DS18B20 and a heating resistor in some thermal
potting compound.

On Sun, Aug 28, 2016 at 06:19:31PM +1200, Brent Brown wrote:
> Similar to what is done with a thermistor. Thermistor most of the time me=
asures air temperature for reference, then for a short time constant power =
fed into the thermistor and measure what it's temperature rises to. With no=
/low air flow the thermistor temperature rises a lot, conversely with high =
air flow the thermistor stays relatively cool. With calibration very accura=
te air flow can be deduced. A variation on the theme is 2x thermistors, one=
 heated and one not.
>=20
> -------- Original message --------
> From: Justin Richards <justin.richards@gmail.com>=20
> Date: 8/28/16  5:50 PM  (GMT+12:00)=20
> To: "Microcontroller discussion list - Public." <piclist@mit.edu>=20
> Subject: Re: [EE] air flow sensing=20
>=20
> I recall the use of light globe filaments used to detect air flow.
>=20
> Current is passed thru the filament and the resistance is measured.
>=20
> >From memory the results are repeatable and the can be calibrated to give
> reasonable measurements.
>=20
> Justin
>=20
> On 28 August 2016 at 12:52, James Cameron <quozl@laptop.org> wrote:
>=20
> > Thanks for the replies on liquid level sensing.
> >
> > Another aspect of the problem is detecting fan failure, which can be
> > done by measuring current or a rotation sensor, but doesn't actually
> > tell if the fan is moving air.=A0 Blocked intake makes fan spin faster;
> > it is not moving as much mass, but the same could happen if the
> > chimney is working well from the heat of the day.
> >
> > I'm interested in comments on anemometers, air flow sensors, mass flow
> > sensors, or pressure sensors relative to ambient.
> >
> > The space to work within is a few yards of 3" diameter plastic pipe.
> >
> > I'm fine with drilling it, because if I change my mind I can plug it
> > with kapton tape or silicone caulk.
> >
> > The environment can be excessively dirty or wet.=A0 Occasionally an
> > insect gets into the system, lays eggs, and a few days later a swarm
> > will try to leave.=A0 Occasionally the dew point is just right for
> > condensation in the pipe.
> >
> > The fan (at the moment) is a 90mm 12V DC rated at 59 CFM, 3.8 watts,
> > 0.32 amps.=A0 The corners get cut off and foam padding added to fit it
> > into 100mm nominal (4") plumbing pipe.=A0 Wish for circular fans.=A0 ;-=
)
> >
> > --
> > James Cameron
> > http://quozl.netrek.org/
> > --
> > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> > View/change your membership options at
> > http://mailman.mit.edu/mailman/listinfo/piclist
> >
> --=20
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> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
> --=20
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> View/change your membership options at
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--=20
James Cameron
http://quozl.netrek.org/
--=20
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