measuring air movement
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  1. #1
    Join Date
    Sep 2001
    Location
    NY
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    234

    measuring air movement

    Is there some sort of formula to figure out how much air is moving by way of my fans?
    I got a 80mm sunon exhaust fan, and then the stock HSF that came with my processor. Also what does cfm stand for?

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  2. #2
    Join Date
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    CFM stands for Cubic Feet per Minute, i.e. how many cubic feet of air a fan can shift.

    I don't know of any way of measuring the CFM other than adding the rated CFM's of the fans in your system and totalling them.

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  3. #3
    Join Date
    Jun 1999
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    Austin, Texas, USA
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    The CFM rating of a particular fan is generally rated in free air. That is no pressure or restriction that exists in a computer case. The CFM for an axial drops off pretty quickly when a pressure of even .2 inches of water exists within the chassis. If you were to total the CFM of all of the fans in your system you would get a number, but it wouldn't be a very accurate one.

    The only true way of arriving at a CFM number would be to hook your system up to a flow bench and run airflow testing on it. You can see the following link for a description of a flow bench:

    http://www.me.pdx.edu/~gerry/class/M...enchManual.pdf

    In the absence of a flow bench you may be able to guesstimate the total system impedance and take a look at the Air Flow/Air Pressure perfomance curve. If you just look at the vertical axis for the system impedance then you can trace your way down to the horizontal axis and that will give you the CFM at that pressure.

    I'm quite sure this is not the answer that you wanted to hear though.

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    Mike
    Mechanical Engineer
    Dell Computer Corporation
    Storage Systems Group
    Mike
    Mechanical Engineer
    Dell Computer Corporation
    Server Development

  4. #4
    Join Date
    Dec 1998
    Location
    Grand Haven, Michigan, USA
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    11,332
    Tape a trash bag over the fans and heatsink and let the fan inflate the bag. The resistance is almost nothing until the bag is almost completely full. The bags are rated for a cirtain amount of storage (like 30 gallon) and you can measure the time it takes in seconds. Convert it to cubit feet per second and your good to go.

    I'm guessing that most heatsinks add anough turbulance and back pressure to cut the airflow on most fans by 50-75% or more. That is one reason why fan addaptors do not work too well. the airflow is restricted even more and these fans are not made to generate much pressure. Their perfromance drops off quickly. That is why less restrictive fan grills are usually more important then the actual fan.

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