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Thread: Dust collector air flow measurements

  1. #31
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    Quote Originally Posted by David L Morse View Post
    Wrong context Andrew. HVAC operates at much lower velocities than DC, hundreds of CFM not thousands.
    I think you're mixing up your units this time. In this case it would be FPM, and generally speaking it's not that far different, about 2-4x, with HVAC running about 2000-2,500 vs 4000-5000 fpm for a dust collector. The CFM are about the same, ~1000 CFM, with the ducts being somewhat larger in HVAC, though personally I run 6" residential HVAC ducts in my system.

  2. #32
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    Oh, and the other reason the pitot tubes are a bad idea is that the velocity inside your ducts isn't really important either. What really matters is what you're able to deliver to the blade and the dust, which is where the anemometer comes handy.

  3. #33
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    Quote Originally Posted by Eric Arnsdorff View Post
    Andrew - David explained it. I only reported the flow rates.
    Nothing was changed for the comparison.
    I used the anemometer at the end of an open hose (a practice I've seen done on youtube, etc.) with all other flow paths closed off. The inner diameter at the measurement point was used to determine the flow area and calculate the flow rate.
    The pitot tube measurement requires proper straight upstream and downstream distances to straighten the flow along with fully developed turbulent flow. I measured this in a 6" duct section that came close to proper straight upstream and downstream lengths. I traversed the duct and took multiple readings to calculate a velocity measurement. The inner duct diameter was used to determine the flow area and calculate the flow rate. One could even argue that the pitot measurement had extra flow from the leakage of the closed dust gates and such.


    Bill I fundamentally agree with what you said and I can tell a significant difference functionally in my old and new setups. The improved function is really what matters. The thing is that my personal bias comes into play without an accurate measurement. Doing all the work would make me have an emotional response to think it was better. Having an accurate measurement gave a definite quantitative assessment rather than my biased qualitative judgment. It was extra work and I still haven't spent the time to assemble the information which may be of use to others interested in doing something similar. It also helps me understand some of the choices in this space and may help others as they put together their systems.

    Additionally, I'm not sure how one can ever trust an inaccurate measurement. If you compare 2 measurements which have the same bias (such as measuring with the same tape measure) then the comparison is valid and this fact is often used appropriately. But in this case the anemometer doesn't create a consistent bias even if it is positioned identically. There is some level of error cancellation in the comparison but still it just puts question on an already highly questionable measurement. It not only leaves doubt but has certainty that it is incorrect.

    Additionally, it may be of use to others and having a reasonably accurate measurement may help others understand how to do something similar.
    All the above has already been done and the information published on the Australian Woodworking forum, search for "Ubeaut Forums". The person who did it is a physics professor who specialised in clean rooms development and the surrounding air flow needs of clean rooms from what I understand. Ducting systems for WW's have been studied and discussed there for many years with all the information gained being available on the above forum. It is most probably the single best repository of information in the world but I am not allowed to put any links here.
    Chris

    Everything I like is either illegal, immoral or fattening

  4. #34
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    In this quest for accuracy - do you really know whether your tool is accurate ?

    How?

    Did some org. like NIST certify it so ? If they did , how long is the calibration and the certification good for ? Meaning , is it accurate next year when you change out some piping or fitting and go to measure again ?



    Andrew - I think your simply mistaken here. HVAC units don’t move nearly the amount of air as most dust collectors we use. Not even close. Even if you have a high velocity type.

  5. #35
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    Quote Originally Posted by Andrew More View Post
    Oh, and the other reason the pitot tubes are a bad idea is that the velocity inside your ducts isn't really important either. What really matters is what you're able to deliver to the blade and the dust, which is where the anemometer comes handy.
    For more information on measurements of air flow in a duct and why some approaches are both inaccurate and what can make them either better or worse please read the information which is available in the Australian Ubeaut WW forum dust extraction sub forum. The anemometer approach has been proven not to be useful and if you really want to get down to it the absolute none instrument recipe for dust extraction ducted system is a 15" impeller driven by a 5hp motor @60hz and 6" ducting. No measurements are needed as 100's of systems have been installed, simply work and have never been measured for air flow.

    Why I am not allowed to link to something that is primary to woodworker's health I have no idea.
    Chris

    Everything I like is either illegal, immoral or fattening

  6. #36
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    I'm not sure what to make of these comments.

    I'm more than familiar with NIST certification and measurement uncertainty. My professional applications of these have been relied on by the Nuclear industry along with several other industries. The idea that the ducting is NIST certified is really an odd statement. I won't bother with this topic other than to say the pitot measurements have reasonable accuracy levels and more than adequate for the purpose of dust collector velocity/flow measurements when used properly. Anemometers (the type typically employed for these measurements) can be a bit qualitatively useful even in small ducts such as these but they can't provide any quantitative values for these measurements.

    If you'd like to get into measurement uncertainty analysis (including transient analysis), I'd be glad to help.

    Otherwise, I will at some point put together some pics and description of what I did because I do think it can be valuable to others as it did prove to be a significant improvement. Interestingly, it was done at a much lower cost than I was about to spend on another system.

    Maybe the biggest bonus is the footprint of the system as well as several other aspects are really a huge improvement and worth what I did spend on the extra. I'm a one man hobbyist woodworking shop in my basement. I think this is more typical of who visit these forums. Dust collection is the first thing I ignored until I finally realized it should've been one of the key items in my shop.

  7. #37
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    Quote Originally Posted by Eric Arnsdorff View Post
    I used the anemometer at the end of an open hose (a practice I've seen done on youtube, etc.) with all other flow paths closed off.
    That is your first mistake, 99.99% of DE information on YT is wrong, misleading and downright dangerous to health if you choose to believe but in the end it is your choice and no one else's.
    Chris

    Everything I like is either illegal, immoral or fattening

  8. #38
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    This thread is confusing and often times just wrong. The fan anemometer is just the WRONG tool to use.

    The Aussie forum - Dust Extraction has the best information on measurement that I have found.

  9. #39
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    Quote Originally Posted by Larry Frank View Post
    This thread is confusing and often times just wrong. The fan anemometer is just the WRONG tool to use.

    The Aussie forum - Dust Extraction has the best information on measurement that I have found.
    Larry, a voice in the wilderness but I don't think some here like the idea that better information can be had outside the US.
    Chris

    Everything I like is either illegal, immoral or fattening

  10. #40
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    On the Aussie forum, there are a couple very good threads by BobL. He has the credentials to do very good measurements and provides great info but can be a bit cantankerous at times. BobL and Pentz provide excellent information.

    HOWEVER........Neither of these are easy reading .

    Making reasonable measurements is tedious and requires more of a setup than most people are willing to do. It requires setting up a proper test duct and doing multiple measurements across the duct. I think only a couple of people on this forum have taken the time to do this.

    My advice is to skip trying to do measurements and just follow best practices that you can do such as duct size, duct layout, less flex, etc.

  11. #41
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    Yall are right. Everyone should leave this forum and go to the Aussie forum!!!
    Really??!!??!!??

  12. #42
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    Quote Originally Posted by Eric Arnsdorff View Post
    Yall are right. Everyone should leave this forum and go to the Aussie forum!!!
    Really??!!??!!??
    I never suggested this at all. I am just suggesting additional resources.

  13. #43
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    So I should throw out my anemometer? How accurate do we need to be when choosing between 4" and 6" duct? Like using a measuring tape vs a micrometer when dealing with wood dimensions. Is the fan anemometer the wrong tool to use in any measurement of duct velocity? If so why does the HVAC industry rely on them?
    NOW you tell me...

  14. #44
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    If you want to use the fan anemometer, then use it. In large ducts like HVAC it is fine. But, with small ducts there are issues with turbulence that cause problems.

    Personally, I have done tests using a test duct and and did measurements across it away from the ends. I have also tried using a fan anemometer and got numbers that were way off.

    If you are happy with the fan anemometer, then use it.

  15. #45
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    Quote Originally Posted by Eric Arnsdorff View Post
    Yall are right. Everyone should leave this forum and go to the Aussie forum!!!
    Really??!!??!!??
    Perhaps not having a closed mind and investigating other recommended sources might be a good idea. As I mentioned above it seems that those on this forum in particular have some weird idea that information from other sources can't possibly be any good. As Larry said, carry on and your health is your concern and no one else's.
    Chris

    Everything I like is either illegal, immoral or fattening

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