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whats the best method for measuring air flow

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newagedawn
newagedawn Member Posts: 586
in A-C, Heat Pumps & Refrigeration
curious as to what you guys think about measuring airflow.
is static pressure or velocity better? when it comes to checking manufactures spec's
"The bitter taste of a poor install lasts far longer than the JOY of the lowest price"

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  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,524
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    best is to check with a variety of equipment. Air reading are sometimes difficult to measure accurately.

    I prefer a pinwheel to measure airflow across a coil. Or use a pitot tube with a magnahlic gage or a manometer
  • Jamie Hall
    Jamie Hall Member Posts: 23,286
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    EBEBRATT-Ed said:

    ...Air reading are sometimes difficult to measure accurately....

    That is putting it mildly! Some 60 years ago I was involved in accurately measuring the airflow into an early jet engine. There was some interesting gadgetry involved...

    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • newagedawn
    newagedawn Member Posts: 586
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    i like the static pressure measurement the best as total static gives you an exact place to reference in manufactures specs, then if you want to check actual CFM use a vane or hot wire
    "The bitter taste of a poor install lasts far longer than the JOY of the lowest price"
  • Harvey Ramer
    Harvey Ramer Member Posts: 2,239
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    You can also calculate the airflow accurately by measuring the change in the physcometrics after it passes through an appliance with a known or calculated input value.
  • Leonard
    Leonard Member Posts: 903
    edited May 2017
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    Would guess either is fine , if your instruments are accurate enough.

    I'ld measure it in center of duct. Boundary effects will slow down air flow as you near the walls.

    Overall flow will be less than center velocity because of boundary effects. Amount will depend on if flow is laminar or turbulent..... real can of worms to get right

  • wasup23
    wasup23 Member Posts: 3
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    newagedawn said:

    curious as to what you guys think about measuring airflow.
    is static pressure or velocity better? when it comes to checking manufactures spec's

    Duct traverse - the only way to check airflow in ducts if the measurement is critical
    Hot wire anemometer is the most accurate in low flow, but both hot wire anemometers and pitot tubes with manometers to convert to FPM then CFM are both accurate at higher flows
  • jumper
    jumper Member Posts: 2,247
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    Easiest but not very exact is measuring pressure drop somewhere that a chart applies too.
  • RayWohlfarth
    RayWohlfarth Member Posts: 1,481
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    I use an air capture hood and it seems to be the most accurate. I have a hot wire and rotating vane anemometer. The duct traverse is time consuming and I found varied results but its better than guessing or taping a tissue to the diffuser.
    Ray Wohlfarth
    Boiler Lessons
  • Leonard
    Leonard Member Posts: 903
    edited August 2018
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    Can measure air velocity at center of duct, and ASSUME it's constant for whole cross section of duct, as an estimate.

    But if you want to get accurate, an error source that can show up is air velocity is not constant as you start getting close to walls of duct (boundary layer, velocity is zero at duct wall ). Guessing it's effect shows up more so in small dia ducts than in larger ones.

    Guessing that boundary might be 1/2 -1 inch thick, can be calculated but been a while since I ran compressible flow numbers.

    How fast velocity changes in boundary layer (velocity profile) also depends speed of flow ( if flow is laminar or turbulent)
  • Jamie Hall
    Jamie Hall Member Posts: 23,286
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    The only really accurate way is to take a traverse of the duct (both side to side and top to bottom, if rectangular) at close intervals. Then... DON'T just average the numbers! Rather, compute the air flow in each layer measured from its velocity and the cross section area of the layer.

    Make sure that you are several diameters downstream and upstream from any transitions, corners, takeoffs, what have you.

    Now... tedious, you say? Indeed. There are usable approximations. Depends on how accurate you want to be.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Harvey Ramer
    Harvey Ramer Member Posts: 2,239
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    You can also use psychometrics to calculate air flow for an AC unit. Provided of course that you are taking the refrigerant readings as well.

    For a furnace or electric strip heater, temp rise works well.

    For individual branches, a flow hood.
  • GBart
    GBart Member Posts: 746
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    I've found a flow hood to be the most accurate but not always practical. if you can out it up against other methods someday, you'll go hmmm.
  • Leonard
    Leonard Member Posts: 903
    edited August 2018
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    Jamie Hall ...... sounds right.
    At least 3 diameters down/up stream, think preferably ~ 10 .
    Total air flow is the sum of individual air flow of the layers.

    If you know electronics, Self heated thermistor might make a good Hot wire anemometer. Can be nice and small , size of a pin head for fast response. Arranged in a bridge with a non self heated one as temperature compensation. Buy them with wires already soldered on, about impossible to not cut THIN leads while scraping off paint coating. (~44 AWG)

    Calibrate in your car on a windless day, probe extending "far" in front of car to catch "clean air" not effected by car's presence. Likely non-linear voltage output with air speed, so calibrate with car at different speeds. IF your car tires are 2 diameter sizes off stock your speed maybe off by 3-4 MPH at 60. (235-75-15 instead of factory 215-75-15)

    Seem to remember an exact solution for a round duct, just need centerline velocity. Involves integrating the flow equations......, been years. Likely it's in a chart somewhere.

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