Newsgroups: sci.electronics
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: hot wire anemometers
Message-ID: <1989Dec15.172519.22348@utzoo.uucp>
Organization: U of Toronto Zoology
References: <5653@orca.wv.tek.com>
Date: Fri, 15 Dec 89 17:25:19 GMT

In article <5653@orca.wv.tek.com> philb@ptolemy.TEKTRONIX.COM (Phil Biehl;685-2122;60-850;684-2867;orca) writes:
>	Some one in sci.aeronautics mentioned using a device called a 
>	"hotwire anemometer" to measure windspeed across a aircraft
>	wing. I've heard of these things before (I think the Voyager
>	Mars landers used one) but have never been able to find out 
>	many details of how they work and from what/how they are made.

The idea is exceedingly simple:  you run a current through a wire, enough
to heat it up, and measure its temperature (e.g. by using a wire whose
resistance changes in a known way with temperature).  The stronger the
air currents over the wire, the better the cooling and the lower the
temperature will be.  Of course, you have to compensate for variations in
air temperature.  In principle it's a trivial device, although I'd imagine
that calibration could be quite a bit of work.

>	Anybody know anything about these things? Can one fairly accurately
>	measure wind direction as well as speed? ...

You could measure direction by measuring X and Y components of velocity,
if you can devise some sort of ducting system to get only one component
flowing past each of two wires.  Actually I guess it would have to be a
bit more complicated than that, since the hot-wire anemometer won't tell
you *which way* the air is moving through the duct.  It should be practical,
although again it would have to be calibrated.
-- 
1755 EST, Dec 14, 1972:  human |     Henry Spencer at U of Toronto Zoology
exploration of space terminates| uunet!attcan!utzoo!henry henry@zoo.toronto.edu
