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  1. #1
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    Proper method for using manometer to check static pressure

    I recently purchased a digital differential manometer (fieldpiece). I've used it once to verify airflow on a small (3-5 ton) package unit that had been iced up. I drilled holes into the plenums and screwed the threaded brass fitting in. I got a reading (.68 inWc). My question- is this an acceptable method or do I need to use the air pressure probes that go into the ducts? any other advice on taking air pressure measurements to verify air flow is appreciated.

  2. #2
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    I would use the pitot tubes to take the measurements with. If you insert a tube into the duct, you can get different readings when the tube is in different positions due to the velocity pressure of the airflow. The pitot tubes are designed to eliminate that and give you a true static pressure.
    Also try to get in a straight run of duct away from elbows, transitions, or outlets. That way you have a better chance of getting a true reading that isn't skewed.

  3. #3
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    Quote Originally Posted by lunchbox View Post
    If you insert a tube into the duct, you can get different readings when the tube is in different positions due to the velocity pressure of the airflow. The pitot tubes are designed to eliminate that and give you a true static pressure.
    Every thing I have seen, and read on pitot tubes, the pitot tube needs to be positioned directly into the airflow for an accurate reading.

    Just like a static probe, the holes on the side of a pitot tube need to be perpendicular to the airflow, or the reading is inaccurate.

    I will admit that pitot tubes are easier to use in commercial applications, because it is easier to find long straight sections of duct.

    I you are meticulous in all your reading and calculations, a pitot tube is probably more accurate than a static probe, but you have to drill sooo many holes.

    Although static probes aren't 100% accurate, they are easier to use.
    Instead of learning the tricks of the trade, learn the trade.

  4. #4
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    Quote Originally Posted by Paulerockalot View Post
    I've used it once to verify airflow on a small (3-5 ton) package unit.

    I drilled holes into the plenums and screwed the threaded brass fitting in.

    My question- is this an acceptable method or do I need to use the air pressure probes that go into the ducts?
    I'm assuming you are using static probes for your testing?

    Static probes should be placed as close to the unit as possible, in the supply and return plenums.

    Be sure to read the notes on the manufacturers blower performance chart to find out the specifics for the unit you are working on.

    These charts will tell you if the coil needs to be DRY or WET, if the FILTER is included, and if any ELECTRIC HEAT STRIPS need to be allowed for.
    Instead of learning the tricks of the trade, learn the trade.

  5. #5
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    Quote Originally Posted by rundawg View Post
    I'm assuming you are using static probes for your testing?

    Static probes should be placed as close to the unit as possible, in the supply and return plenums.

    Be sure to read the notes on the manufacturers blower performance chart to find out the specifics for the unit you are working on.

    These charts will tell you if the coil needs to be DRY or WET, if the FILTER is included, and if any ELECTRIC HEAT STRIPS need to be allowed for.
    I didn't use probes of any kind, just screwed the open brass fittings into the side of the plenum. They have a diagram of this in the instructions, but they recommend the L- shaped spike probes for accurate measurement. I don't know if these are the static probes or the pitot tubes everyone is mentioning. I have seen those permanently mounted in large boxcar air handles attached to magnahelixes. Also, the unit I used it on was very old and had no info in or on it other than a superheat charging chart. A few knowledgeable people have told me .68inWC differential is acceptable.

  6. #6
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    Quote Originally Posted by Paulerockalot View Post
    they recommend the L- shaped spike probes for accurate measurement. I don't know if these are the static probes or the pitot tubes everyone is mentioning.

    If they are permanently mounted, my guess is they are talking about Static Pressure Probes.

    See attachment for the difference in a Pitot Tube and Static Probes.
    Attached Images Attached Images
    Instead of learning the tricks of the trade, learn the trade.

  7. #7
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    Quote Originally Posted by rundawg View Post
    Every thing I have seen, and read on pitot tubes, the pitot tube needs to be positioned directly into the airflow for an accurate reading.

    Just like a static probe, the holes on the side of a pitot tube need to be perpendicular to the airflow, or the reading is inaccurate.

    I will admit that pitot tubes are easier to use in commercial applications, because it is easier to find long straight sections of duct.

    I you are meticulous in all your reading and calculations, a pitot tube is probably more accurate than a static probe, but you have to drill sooo many holes.

    Although static probes aren't 100% accurate, they are easier to use.
    You are correct. I wasn't thinking about the difference in static tubes and pitot tubes. I have both and use the static tip if I only need static pressure.

  8. #8
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    Based on that handy picture, they are the static probes. Thanks

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