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How to Measure Airflow Rate?

Discussion in 'Hobby Electronics' started by David L. Jones, Jul 28, 2005.

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  1. Maybe a little off-topic...
    I want to measure the airflow rate (from a small axial fan) through a
    ducted system. I am thinking of using a smoke test and time how long it
    takes to get through the system (which comprises 100m ducting and a
    similar overall area channelled box).
    Any traps?
    What type of "smoke" works best?
    What errors are typically in this sort of measurement?
    How is this sort of thing is usually done?
    Are there any better ways to measure air flow rate? Maybe measure
    pressure somehow and use the pressure vs airflow graph from a
    Any other traps?

    Any help appreciated.

    Dave :)
  2. Geoff C

    Geoff C Guest

    I would use a hot-wire anemometer cos I have one, or a vane type. Losses in
    100m of duct is large. Timing smoke I would imagine is tricky because it
    will smear due to differences in flow rate between walls and centre as the
    flow will likely be laminar after some distance. You could try a smoke
    detector at the oultet I suppose.
  3. Sorry, typo, I meant 100mm ducting. My total length is maybe 10-20m.

    I'll look into an anemometer, thanks.
    Presumably you would simply use it on the output and work out the flow
    rate based on the diameter of the ducting?
    That's what I was thinking...
    Also thought of that one, but I thought there might be some lag in the
    smoke detector response time?

    Dave :)
  4. dmm

    dmm Guest

    Apart from heated anemometers as Geoff mentioned, there's rotating vane, ultrasonic,
    laser-doppler-anemometers (LDA), and pitot tube anemometers.

    I'm sure Tech Rentals would have a selection.

  5. Terry Given

    Terry Given Guest

    doesnt dick (head) smith sell flow rate measuring devices...

  6. budgie

    budgie Guest

    You can use differential pressure and a decent graph (if one is available).
    Otherwise I'd look to use a vane anemometer. Note that any anemometer gives a
    "local" flow rate (really a velocity sensor) and you need to have a decent
    technique for moving the anemometer aperture across the duct aperture to obtain
    a sensible average reading due to non-uniform flow across the duct.
  7. Phil Allison

    Phil Allison Guest

    "David L. Jones"

    ** Sure is.

    Try " alt.ducted.air.con.tragics " .

    .......... Phil
  8. Geoff C

    Geoff C Guest

    Yeah, 100mm is a bit different... The hot wire anemometer is good cos it
    allows spot air velocity measurements. However the airspeed profile
    across the outlet duct is usually very irregular, most flow is on an
    outer annulus. most of the time I prefer a vane anemometer (cheaper too)
    and try to get a consistent placement at the outlet so I get repeatable
    results when trying different experiments. Total flows are very hard to
    measure without affecting the actual conditions. Presuming the objective
    is for cooling, I usually regard temperature measurements as the premium
    information. If you are still very interested in flow metrics, have a
    look at data books or websites for companies like Papst who state their
    measurement conditions in the front of their apps books and this is the
    best sort of practical info I have seen about duct system design. Its a
    few years since I have looked though.
  9. That's what I want, repeatability across various experiements. Plus I
    also want an absolute airflow figure to allow calculation of total
    system efficiency etc. So Budgies idea of taking an average across the
    duct sounds good too.
    Will take a look, thanks.

    Dave :)
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