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help with measuring airflow

zacmobile Member Posts: 211
I am having trouble measuring duct airflow, I took a ventilation course and it seemed so easy but in the field I have never been able to replicate the procedure done in the classroom.

For example, I am trying to read the airflow being exhausted by a kitchen range hood through a 4" duct, I have a Dwyer 0-1" water magnahelic & a pitot tube (series 160, the coaxial kind with high & low connections on the same unit) but the gauge barely flickers when the probe is inserted in the airstream. I ran the calculation based on what I think it says on the gauge (.02" water) and get a ridiculously low CFM number.

Any suggestions?


  • BillW
    BillW Member Posts: 198

    What number are you getting?  Are the tubes clean, and the instrument functioning properly?  How many cfm is the fan designed for?
  • zacmobile
    zacmobile Member Posts: 211

    I'm getting such a small amount of movement on the needle that I am estimating the amount at .02" water.The tubes are clear, I can blow through them easily. It's a brand new gauge and seems to respond when I blow or suck on the tubes. I don't know what the fan is listed at, (part of the reason I'm trying to measure it) It is a mid-range below microwave one I would say it should be in the 200-300 CFM range. It's going through approx 14' of 4" ducting with 6 elbows and seems to be blowing pretty good out of the hood outside.
  • zacmobile
    zacmobile Member Posts: 211

  • BillW
    BillW Member Posts: 198

    I'm stumped as to why your gauges aren't working.  I assume that they are Dwyer, and they probably have a website or tech support line.  Try contacting them, or if someoone at the supply house you bought them from may be helpful.  Don't feel bad, they are a bit tricky to use.  Sorry I don't have a better answer.
  • zacmobile
    zacmobile Member Posts: 211

    I tried the support line, it's really hard to get through and all they said was the same thing you did in your first reply: are the tubes clear etc. I thought maybe my math is wrong, can you double check?

    This is the formula i've been using:

    velocity pressure in inches of water squared X 4005 X area of duct in feet = ?

    (.02 X .02) 4005 X 3.14 (.25X.25) = .314 CFM

    I have one of those Dwyer air velocity slide rules but I can't figure out how to read it for the life of me.
  • BillW
    BillW Member Posts: 198

    I don't see anything wrong with the formula or the math, but I am not the world's best math guy.  I don't have my Dwyer slide rule anymore, since I'm retired.  The air flow rate should be around 2-300 cfm for a typical kitchen exhaust fan.  Maybe post this question on the main wall, and  everyone can help.  These are good people, and nobody is going to laugh or think anything less of you.  Nobody knows everything!
  • RJ_4
    RJ_4 Member Posts: 484
    air flow

    Go with a smaller range magnahelic, like a 0-.25 or 0-.5  remember if you think your looking for under 500 cfm your 2 inch mag is to big.  also are you measuring free area, if there is a grate, register or diffuser in the air stream you need to account for that, make a number of cross section readings and get an average reading.   In some cases registers etc can account for a 50% reduction in air flow readings.  There are also air flow meters you can attach to your volt/amp meter,  field peice makes a good one. the Trane air conditioning manual has a excellent chapter on air flow
  • hws
    hws Member Posts: 43
    edited February 2013

    a vane anemometer would be better for this particular situation?

    I use the magnehelic for balancing procedures, and the anemometer for measuring at terminals such as diffusers, grilles etc. I don't use the pitot tube very much
  • Eugene_Silberstein
    Eugene_Silberstein Member Posts: 349
    Just Stumbled onto This Post

    Sorry, but I just stumbled onto this post.

    The equation being used is incorrect, which is why your numbers are causing you grief.

    The formula for airflow is:

    CFM = Air Velocity (ft/min) x Cross sectional area of the duct

    The velocity of the air is given by 4005 multiplied by the square root of the velocity pressure, not the square of the velocity pressure.

    So, your numbers will look something like this:

    CFM = 4005 x Square root of 0.2 x 3.14 x 2"/144

    where the 2" is the radius of the duct and the 144 is the conversion between square inches and square feet.

    CFM = 4005 x 0.14 x 3.14 x 2/144

    CFM = 49.2

    Hope this helps
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