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I think the venier motion detectors use a "time of flight" ultrasonic
system, not doppler, and so measure the range to the relecting surface,
and track changes in the range to calculate speed/acc etc, and as such
probably would see a stream of sand as a stationary boundary.
A simple and "obvious to the public" ( can we ever assume this?)..
Method might be to locate an undershot paddle wheel in the sand stream,
and either link that to a tachometer to gauge the speed ( if the volumes
of sand and the speeds are sufficient, the paddle wheel could be linked
to a dc generator and directly output a signal to feed into an
ammeter/volt meter, or other display/logging system).
I don't know what the system is trying to show or do. But if you let the
sand run off the end of the slope, you can calibrate (calculate) its
speed from the curve the sand takes in flight, as the path is directly
related to its speed, no moving parts, just a graphic beside the sand
"waterfall" and nice opportunity to disucss a basic bit of physics.
Hope this helps.
Stuart Kohlhagen
Manager Exhibition services
Questacon the national science and technology centre
Canberra
-----Original Message-----
From: Informal Science Education Network
[mailto:[log in to unmask]] On Behalf Of Allan Ayres
Sent: Friday, 13 April 2007 8:39 AM
To: [log in to unmask]
Subject: Re: Radar guns
ISEN-ASTC-L is a service of the Association of Science-Technology
Centers Incorporated, a worldwide network of science museums and related
institutions.
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Phil wrote:
>We're trying to find a way to measure the speed of something like sand
>as it flows down an inclined plane. We were thinking of maybe trying a
>radar gun at the bottom of the plane pointed up at the sand, but we're
>kind of concerned about it...well, not working. Does anyone have any
>experience with this type of thing, or have any suggestions?
We're also looking for speed-sensors right now. In our case, we'd like
to gauge the speeds of steel balls rolling in tracks that are right next
to each other, so something that seems to be a feature the vendors like
to tout (wide-angle sensitivity) is actually a bug for us. :-)
Plus, the cheap and easily available ones seem to be calibrated for
2-100 mph or so (the speed of golf balls, baseballs, go-carts, cars,
etc.), and our speeds would be significantly less than that.
Does anyone have any experience with Vernier's motion detectors?
http://www.vernier.com/probes/motion.html
They seem like total overkill for just getting a single number -- speed
-- as opposed to graphing position and calculating acceleration and
everything else you can do with them, and they don't look particularly
promising in terms of angular discrimination (i.e.
returning the speed of just one ball). But, compared to some of the
industrial sensors, they're actually fairly inexpensive.
Hmmmmm.
Thanks for any insight,
--Allan
--
~~
Allan Ayres
Exhibit Developer
Lawrence Hall of Science
University of California, Berkeley
Berkeley, CA 94720-5200
[log in to unmask]
510-642-1254
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