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From: bernie@metapro.DIALix.oz.au (Bernd Felsche)
Subject: Re: ELECTRONIC WINDSENSOR
Message-ID: <Cw9nvG.81p@metapro.DIALix.oz.au>
Organization: MetaPro Systems, Perth, Western Australia
References: <AMF.94Sep15113140@kongle.idt.unit.no> <sarge-1509940947210001@ohno.ppp.verdix.com> <35a75m$m5s@handler.Eng.Sun.COM>
Date: Sat, 17 Sep 1994 09:04:28 GMT
Lines: 40

In <35a75m$m5s@handler.Eng.Sun.COM> cmcmanis@Sun.COM (Chuck McManis) writes:

>Matthew C. Sargent (sarge@Rational.Com) wrote:
>: hot wire cooling method

>I'm not sure what you mean by this however it occured to me that a rather
>interesting flow analyzer might be made of Nitinol aka Muscle Wire. This
>wire contracts when heated and relaxes when 'cool' so if you connected
>a length of wire to a pressure sensor you could heat it with a current
>and measure the pull on the pressure sensor. Now if you knew the current,
>the pressure, the ambient temperature, the time to cool the wire in still
>air. (And you'd need to know the thermal convection capability of the gas

Isn't this a hot-wire anemometer?  A good thermodynamics reference book
allows you to work out the air mass-flow rate.

>you were measuring) You could calculate how much air was moving past the
>wire by how quickly it cooled (and released pressure on the pressure
>switch). You'd probably also need to know the barometric pressure since
>the density of the air would effect how efficiently it cooled the wire.

The *mass* flow of air determines the cooling of a hot wire.  It's one
of the reasons why most fuel injection systems in motor cars use either
hot-wire or hot-film flow sensing. Other systems use manifold pressure
or a swinging flap.

Go to the nearest BMW, pop the bonnet and detach the hose leading from
the air cleaner box to the throttle valve...  you'll probably find a
short piece of rigid tube on the hose with an electrical connector at
the top....  carefully remove and have a peek down the tube. If there's
a wire, you'll hardly be able to see it.

The trick is to try to keep the wire at a constant temperature (simple
enough if you use an NTC sensor or similar for feedback) and measure
the current required to keep it there. It's also necessary to measure
ambient air temperature.
-- 
Bernd Felsche, MetaPro Systems Pty Ltd
328 Albany Highway, Victoria Park, Western Australia
Phone: +61 9 362 9355  Fax: +61 9 472 3337
