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  1. #14
    Join Date
    Feb 2016
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    Louisburg Kansas
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    I hope you are correct.
    One story about the digital hoods I'll share with you. Keep in mind this was about 10 years ago.

    I got a call from a company that wanted me to take readings on some fans that cooled critical safety controls. When the guy told me where he was located I told him it will take me longer to get there than to run the tests. There is a balance company within a couple of miles of you that can do it a lot cheaper than I can. He said I know that but want a separate opinion. I knew the company close to him never used an analog hood. I suspected he had some bogus readings that are almost always high.
    When I got there I took 10 readings with the digital hood and played them all back for him and ask him what the flow was. He had no idea nor did I. I then took readings with the analog hood and because of the slight wind it varied a little but was easily averaged mentally. That convinced him I should use the analog hood. He told me the balance company down the road had taken the readings with a digital hood. His problem was his control equipment was over heating. They did the design thinking they had enough airflow to cool the equipment but didn't. Their product was literally life safety and had to work at all times.
    You won't run into that on houses but I thought you would like at least one example of why I only used the digital hood on VAV systems.
    No man can be both ignorant and free.
    Thomas Jefferson

  2. #15
    Join Date
    Jan 2015
    Location
    San Diego, CA
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    Thread Starter
    Interesting. Without knowing the details of the electronics in the design it's not clear to me why the digital hood would be more inaccurate assuming that the sensing elements such as a pitot tube are the same. The issue could be with inaccuracies in the transducers when the physical quantities such as pressure is converted to an analog signal or errors in the ADC during the digitization process. Once the signal is digitized it is unlikely that the processor would create any additional errors such as when averaging.

    Another possibility is that fluctuations in the air velocity is averaged out by the inertia of the analog gauge which would act as a low pass filter. In the digital domain such fluctuations could be captured in the data and then the residual error could be present event after digital averaging.

    It would be interesting to understand the reasons for the discrepancies. Maybe the manufacturers have been able to resolve these in the latest versions of the tools.

  3. #16
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    Feb 2016
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    Louisburg Kansas
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    Your theory on inertia is correct along with persistence of vision. Airflow fluctuates in air systems but is a lot more exaggerated in low pressure systems. The digital hoods I have read out instantaneous values which are correct at the instant read. Consider a sine wave type airflow where one reading can be at the top of the wave, another at the bottom and more readings at random locations on the wave. You can store the readings and average what you read but all you have is the average of your readings. You don't get a true average of the actual airflow.
    No man can be both ignorant and free.
    Thomas Jefferson

  4. #17
    Join Date
    Sep 2008
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    VA
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    I’ll preface this post by stating, I have nowhere near the air balancing experience as Wayne, but I have done my fair share of testing, taking notes, and comparing different methods for “residential applications.”

    I trust the Alnor Lo-Flo as my #1 for residential supply air diffuser readings so far. It seems to read slightly low, but it’s linear and repeatable. I’ve compared it to high volume flow hoods, high end large vanes, small vanes, hot wires, pitot with auto zeroing digital dual ports, analog magnehelic and red oil tubes, Retrotec powered duct leakage tester configured for air measurements, and probably a few others I’ve forgotten about. I even set up a lab test with a small sealed return and supply ducting system.

    So, when comparing several diffusers, you can weight them. When balancing a residential system, your trying to get a certain % of the total volume to certain spaces. The actual cfm doesn’t really matter much if the meter reading is repeatable and linear, because you’re simply comparing it to the other vents. Basically “robbing Peter to pay Paul”. As long as your static isn’t affected that much.

    The total volume is more accurately read by the fewer return vents. Of course there’s leakage to be considered. Unfortunately, the Lo-Flo is not capable of reading return air volumes above 500cfm. I have found flow hoods to be extremely accurate with return air readings. Larger cfm flow hoods are less accurate for residential supply air readings.

    I found:

    the hot-wire to be accurate at higher velocity without a grille (less turbulence).

    The vanes to be ok most of the time with a grille but less so without a grille. I find the expensive large vane to be a waste of money for small residential supply air vents. It works well with the larger return air vents, but the small vane is probably a better buy for small supplies. Just don’t get a cheap off brand one from e-Bay.

    Pitot only good for in duct, Magnehelic most accurate other than very expensive auto zeroing manometer. Cheaper digital manometers under $400 are not worth connecting to a Pitot. The Magnehelic works very well and is relatively inexpensive.

    High flow hood very good at large return air measurements. Probably good with high volume large supplies, I don’t know.
    Not good with small supply measurements.

    Lo-Flo Alnor very good with small return air, and small supply air.
    Again, it reads a little low on supplies I think, but it’s very reliable IMO. I don’t even bother reaching for anything else when taking residential supply air readings. It has the small 16”x16” short hood.

    For more accurate readings I made a cardboard box with a hyperbola shaped insert. This made both the high flow and low flow hoods read very close to one another.
    "The only true wisdom is in knowing you know nothing" Socrates

  5. #18
    Join Date
    Sep 2008
    Location
    VA
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    The other thing I like about the Lo-Flo is the field correction factors that can be programmed to use on different types of grilles. Most residential homes use similar grilles, so the correction factor can be set for: factory, correction A, or correction B, in order to yield very accurate and repeatable results.
    "The only true wisdom is in knowing you know nothing" Socrates

  6. #19
    Join Date
    Feb 2016
    Location
    Louisburg Kansas
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    I like your posts mgenius33 because we have both dug into this stuff pretty deep. I did commercial and you have covered residential and flow quantities do make a difference. I remember when you were running some experiments and read all your posts and know you didn't take anything for granted. I used the Alnor Lo-Flo hood but for a fairly short period. It was a two month trial before buying. I had 5 flow hoods and was thinking about buying a low skirt hood for office spaces where cubicle walls were tall enough to make reading airflow difficult.

    Post #14 was based on the use of the big bucks digital hoods and big bucks analog.
    No man can be both ignorant and free.
    Thomas Jefferson

  7. #20
    Join Date
    Aug 2019
    Location
    Central Florida
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    105
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    The lo flow hood from Evergreen is bad ass . Its not cheap though .

  8. #21
    Join Date
    Jan 2015
    Location
    San Diego, CA
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    397
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    Thread Starter
    Quote Originally Posted by mgenius33 View Post
    I’ll preface this post by stating, I have nowhere near the air balancing experience as Wayne, but I have done my fair share of testing, taking notes, and comparing different methods for “residential applications.”

    I trust the Alnor Lo-Flo as my #1 for residential supply air diffuser readings so far. It seems to read slightly low, but it’s linear and repeatable. I’ve compared it to high volume flow hoods, high end large vanes, small vanes, hot wires, pitot with auto zeroing digital dual ports, analog magnehelic and red oil tubes, Retrotec powered duct leakage tester configured for air measurements, and probably a few others I’ve forgotten about. I even set up a lab test with a small sealed return and supply ducting system.

    So, when comparing several diffusers, you can weight them. When balancing a residential system, your trying to get a certain % of the total volume to certain spaces. The actual cfm doesn’t really matter much if the meter reading is repeatable and linear, because you’re simply comparing it to the other vents. Basically “robbing Peter to pay Paul”. As long as your static isn’t affected that much.

    The total volume is more accurately read by the fewer return vents. Of course there’s leakage to be considered. Unfortunately, the Lo-Flo is not capable of reading return air volumes above 500cfm. I have found flow hoods to be extremely accurate with return air readings. Larger cfm flow hoods are less accurate for residential supply air readings.

    I found:

    the hot-wire to be accurate at higher velocity without a grille (less turbulence).

    The vanes to be ok most of the time with a grille but less so without a grille. I find the expensive large vane to be a waste of money for small residential supply air vents. It works well with the larger return air vents, but the small vane is probably a better buy for small supplies. Just don’t get a cheap off brand one from e-Bay.

    Pitot only good for in duct, Magnehelic most accurate other than very expensive auto zeroing manometer. Cheaper digital manometers under $400 are not worth connecting to a Pitot. The Magnehelic works very well and is relatively inexpensive.

    High flow hood very good at large return air measurements. Probably good with high volume large supplies, I don’t know.
    Not good with small supply measurements.

    Lo-Flo Alnor very good with small return air, and small supply air.
    Again, it reads a little low on supplies I think, but it’s very reliable IMO. I don’t even bother reaching for anything else when taking residential supply air readings. It has the small 16”x16” short hood.

    For more accurate readings I made a cardboard box with a hyperbola shaped insert. This made both the high flow and low flow hoods read very close to one another.
    Thanks for your feedback on this. Yes I realize that the LoFlo cannot used for returns. I guess I could use my rotating vane assuming it is accurate. The correct technique is to traverse the return grill about 1" away, average the velocity readings and then multiply by the Ak of the grill correct?

    I might try creating a ductboard test fixture with a pitot traverse to see how far off the other tools are - based on a previous post that a pitot traverse wound be the most accurate out of all the techniques.

  9. #22
    Join Date
    Sep 2008
    Location
    VA
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    Quote Originally Posted by szw21 View Post
    Thanks for your feedback on this. Yes I realize that the LoFlo cannot used for returns. I guess I could use my rotating vane assuming it is accurate. The correct technique is to traverse the return grill about 1" away, average the velocity readings and then multiply by the Ak of the grill correct? yes

    I might try creating a ductboard test fixture with a pitot traverse to see how far off the other tools are - based on a previous post that a pitot traverse wound be the most accurate out of all the techniques. Be sure to radius the inlet side of the box. Straight edges on the inlet side will not create laminar flow in a short section of straight duct. This is why flow hoods do so well on return air readings. (Or my theory anyway)
    ..

    I’ll likely be doing more testing this year. I’ve added a Testo 416 mini-vane to my arsenal.
    "The only true wisdom is in knowing you know nothing" Socrates

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