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need to measure very low air velocity

Discussion in 'Electronic Basics' started by Jim Basinger, Sep 16, 2003.

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  1. Jim Basinger

    Jim Basinger Guest

    I need to measure the velocity of very low airflow.........less than 1/2
    foot per second or so. When I say measure, I mean an analog signal or that
    I can store in a data logger and calibrate against some known. This is not
    a switch application.

    I have an Extech "hot wire" anemometer but aside from the fact that it does
    not measure airflow at the velocities I need, the sensor "head" is
    directional.

    I have seen some ideas for using self-heated transistors, but don't know if
    this type of idea is sensitive enough.

    I think I have seen a design using a piezo sensor of some sort that could
    sense the wafting of a hand over the sensor, but I don't remember if it
    output an analog (relative) signal or not.

    The application for this device is to measure the air movement at a forest
    floor to about 6 inches from the ground. The sensor and data logger would
    be left in place to collect data, since a persons presence might have a
    large effect.

    Any ideas on how I might build this?

    Thanks
    Jim
     
  2. Baphomet

    Baphomet Guest

    The application for this device is to measure the air movement at a forest
    floor to about 6 inches from the ground. The sensor and data logger would
    be left in place to collect data, since a persons presence might have a
    large effect.

    Any ideas on how I might build this?

    Thanks
    Jim



    Here is a link http://www.specialinstruments.com/pdf_e/28-29.pdf that might
    prove helpful. I'm not sure that these devices will measure (at least with
    any accuracy) 0.5 ft. / sec. air flow rates. You might try experimenting
    with a ribbon mic http://www.wesdooley.com/pdf/r84_short_manual_ver1.pdf.
    They are extremely sensitive to minute air flow. It has a figure eight polar
    pattern so wind direction shouldn't be a consideration; keeping it dry
    probably will. You will also require about 60 db. voltage amplification to
    get the minute voltage generated by the ribbon up to a useful level.
     
  3. Baphomet

    Baphomet Guest

    Of course, if you have a steady breeze (no fluctuations) all bets are off
    :)
     
  4. Baphomet

    Baphomet Guest

    I think you might wind up having to construct your own transducer. You could
    use a pressure sensitive transistor
    http://www.omega.com/Pressure/pdf/PX70.pdf mechanically coupled to a
    diaphragm of some sort. This might provide the required sensitivity that you
    need but wouldn't address the directionality problem. Integrating the output
    from several such units might do the trick. I don't know of any simple
    answer. Any transducer engineers out there?
     
  5. Baphomet

    Baphomet Guest

    So I'm thinking of my relies above and the more I think about them, the less
    satisfactory they become and the problem becomes increasingly less trivial
    which is making me increasingly manic.

    We're starting out with a lousy 0.5 ft/sec wind velocity which needs to be
    "captured" by a sail...or diaphragm. We can't put any unnecessary "drag" or
    "damping" on the diaphragm as the pressure sensitive transistor would
    provide, so why not silver the diaphragm and let it deflect an off axis IR
    beam. This essentially frictionless beam displacement could then be detected
    with a photo diode array, and then scaled to useable output levels.

    Again, this method suffers from directional sensitivity so several units off
    axis to each other would have to be used and the outputs integrated.
     
  6. Start with a funnel. At the "focus" of the funnel, the air velocity will be
    much higher. Now, in a narrow tube that is driven by this higher velocity air,
    you place a thin tissue paper strand and use an optical sensor to see it move.
    Alternatively, place your hot wire sensor at the narrow end of the funnel in
    a tube.

    Cheers!

    Chip Shults
    My robotics, space and CGI web page - http://home.cfl.rr.com/aichip
     
  7. Hmmm. I wonder if a small, frictionless anemometer would work.
    Build a conventional anemometer spinner, but make it a couple of
    cm across, sense it's rotation with LED and phototransistor, then
    suspend it with passive maglev like this:

    Maglev seismic sensor
    http://www.jclahr.com/science/physics/diamag/seismo/index.html

    Maglev toy
    http://www.teachersource.com/catalog/images/lev100.gif
    http://www.teachersource.com/catalog/page/Electricity_Magnetism_Engines/Magnet_Products/

    Maglev seismic sensor
    http://www.jclahr.com/science/physics/diamag/seismo/index.html

    Or perhaps you could make two separate 1-axis sensors as
    above, using levitated pencil leads with small vanes
    dangling below like a pendulum, one for X and one for Y.

    High-diamagnet pyrolytic graphite samples are available from
    scitoys for a few bucks:
    https://4ua.com/scitoys/cgi-bin/shop.exe?page=magnet_desc.html



    Or less exotic: dangle a few inches of lightweight fishing line
    downwards, attach an opaque horizontal foil square to the bottom,
    then sense X and Y deflection with photosensors and diffused LED
    bars. You could even use EM damping to stop any resonant swinging
    if the foil was copper, and a supermagnet disk was held just below.
    If fishing line doesn't offer enough of a sail, glue on some foil
    bits.
     
  8. onestone

    onestone Guest

    Try a search for ultrasonic anemometer. These have the sort of precision
    you're looking for I believe.

    Al
     
  9. Baphomet

    Baphomet Guest

    Thanks for the info; I didn't know such a beast existed. Sounds like the
    best solution to the problem by far. I would always opt for an off the shelf
    solution rather than screwing around with custom instrumentation.
     
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