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Thread: Dust collector air flow measurements

  1. #16
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    Ok - yeah 1000 cfm is more reasonable and is achievable but does require more than some equipment can produce.
    Iím just happy the engineer wife (who self studied the patent bar law exam - meaning thereís not much that gets by her) agreed with going with the higher cost pitot ($120) rather than the low cost knockoff!

  2. #17
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    Quote Originally Posted by Eric Arnsdorff View Post
    Thanks Chris! I looked briefly and it does look like there are several threads out there addressing my hunt for a low cost and fairly accurate method for measuring flows and pressures. Pitots and manometers seem popular. I did see a link to a Wood magazine article. I wasnít crazy about all their methods but it appeared better than most (https://www.woodmagazine.com/video/dust-collector-test).

    I understand the science and have practical experience in the measurements and analysis. This is why seeing those prop anemometers used makes me cringe!

    Itís been a while now (around 20 years ago now) but I spent about 5+ years doing measurement and analysis for nuclear plants to determine their capability of removing heat load during accident scenarios. It involved many measurements of their systems including air flows for heat exchangers and such. I had to also determine all measurement uncertainties and project those to accident conditions. The plants must have enough capacity including the extrapolated uncertainties to remove accident scenario heat loads to prevent meltdown. Iíve even authored a text and taught a class for the nuclear industry sponsored by EPRI on the measurement and analysis so the plants could maintain a program to measure and verify they met the NRC requirements.

    While I have knowledge and experience in that area, I havenít measured a small system such as my dust collector. So these forums and info are very helpful. Heck This weekend is the first time I did the math and I was quite surprised with the results. You know what assuming does!

    Obviously, when I did such measurements professionally we had some of the best instrumentation and all very well calibrated and maintained. I have been pleasantly surprised with some cheap instrumentation from Amazon and such sources. But I also know most arenít remotely what the claims say.

    I will add those vane/prop anemometers stuck on an open duct with who knows what flow sections prior to it are useful in proving oneís theory. I mean you could pretty much get that to read anything you wanted it to ;-)
    On the Australian site that Chris mentioned I posted some data comparing the Testo 405i that Thomas and I have to my pitot/manometer combo. Go to their advanced search usine keyword testo and user dmorse. I think you'll see why I prefer the pitot methid.
    Beranek's Law:

    It has been remarked that if one selects his own components, builds his own enclosure, and is convinced he has made a wise choice of design, then his own loudspeaker sounds better to him than does anyone else's loudspeaker. In this case, the frequency response of the loudspeaker seems to play only a minor part in forming a person's opinion.
    L.L. Beranek, Acoustics (McGraw-Hill, New York, 1954), p.208.

  3. #18
    I was trying to measure the flow through a fairly wide opening in an enclosure and had issues with the vane style manometer- the very sensitive voltage sensor that measured the spinning vanes was getting swamped with noise from the induction motor on the fan itself! (It was a vent fan, not a dust collector, but similar technology). A hot wire probe measured it just fine.

  4. #19
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    Quote Originally Posted by Eric Arnsdorff View Post
    Andrew where did you see 1000 fps for fine dust?
    Sorry, wrong units. I believe the 1000 CFM is correct, my bad for trying to do things off the top of my head.

  5. #20
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    Getting back to the initial post, measuring improvements in your dust collection system will be difficult. It is difficult to get accurate measurements and takes a lot of time and effort. After reading many of BobL's posts, I measured my system using a hot wire anemometer and doing traverses. I would be happy if I was +/- 10% of the real value. However, this suggests that I would need a huge difference in two dust collector setups to see a difference.

  6. #21
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    Nice study David! The pitot tube simplicity is hard to beat. I copied your chart below.

    Bert I don't know how those prop/vane or whatever spinny thing happens to be on those are useful at all - I just don't see how it could be remotely accurate. I'm sure they spin and give a number. I have used that style (granted a very high quality and well calibrated version) for mapping very large intakes where the device was very small compared to the inlet duct (some would be around 8-10' tall/wide).

    David's study shows the complexity of using a hot wire - there's just a lot going on to get to work in a duct. The elements can be small and possibly some designs may work but this is why I asked the question originally.

    I have the Dwyer 167-12 on order and hope to get some reasonable/repeatable measurements.

    Larry I hope my measurements can be within 10% and hopefully within 5% and be repeatable. I will be making some pretty big changes (4" ducts to 6", Thien to SDD XL, blower mods, eliminating a mess of hose and fittings, etc.). I'm turning it into a bit of a science experiment with it as well which will take more time and a little more cost as well. But it's kind of fun doing some engineering on my hobby. So I'm ok with all that. But yes it wouldn't be worth it for a smaller change.

    Thanks for the feedback! Not only comments here but yall have helped me find information elsewhere that helps a lot.
    Pitot vs HWA.jpg

  7. #22
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    I just use the suck your tape measure up method.
    And my particle counter.
    The Plane Anarchist

  8. #23
    Quote Originally Posted by Eric Arnsdorff View Post
    Bert I don't know how those prop/vane or whatever spinny thing happens to be on those are useful at all - I just don't see how it could be remotely accurate. I'm sure they spin and give a number. I have used that style (granted a very high quality and well calibrated version) for mapping very large intakes where the device was very small compared to the inlet duct (some would be around 8-10' tall/wide).

    David's study shows the complexity of using a hot wire - there's just a lot going on to get to work in a duct. The elements can be small and possibly some designs may work but this is why I asked the question originally.
    The blade style anemometers are good for really large flow areas, like getting windspeed outdoors or something. They're also OK for other areas where, like you said, they're very small compared to the overall area. My use case wasn't for dust collection, it was for a laser cutter enclosure vent with a large open area. Thus it was closer to HVAC levels of operation, not dust collector ducts. I also have a hot wire anemometer that I got for that same application. Whether or not the hot wire one will work in a given system will definitely be affected by the thermocouple geometry that David mentioned in his other post. I have an old Dwyer 471, which has the temperature sensor right next to the hot wire sensor. It should be fine for use in ducts that aren't terribly small, but I agree overall that a pitot tube used correctly will be the easiest way to do things. It worked great for my usage of a giant rectangular vent hole.

  9. #24
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    I've made some changes to my dust collector including going from a Thein baffle seperator design to a cyclone along with several other changes. The functional results are very noticeable and it all proved very worthwhile. At the same time the footprint is much smaller and the system is much more convenient to use due to adding a filter instead of exhausting outside (I always forgot to open or close that dang window). I also made some measurements to see the difference. I haven't taken the time to assemble the information and pics. I'm actually excited about installing my Inkleind vise and bench dogs currently (it's coming out very nicely). I'll get more details on my dust collector changes and the effects on here shortly.

    My measurements for flow are based on pitot tube measurements and this is the simplest way I could come up with to obtain pretty accurate flow/velocity readings.
    As a reference, I borrowed an anemometer from work just to see how bad they are in a measurement of small ducts used in dust collector systems.

    I checked the flow with the anemometer similar to what I've seen others post on the internet/youtube/SMC/etc.
    Anemometer flow rate result on 4" hose end (really the hose ends up closer to 3.5"): 470 cfm

    The same flow was measured using a pitot tube in a straight section of 6" duct as close as I had to recommended upstream/downstream straights.
    Pitot tube flow result in 6" duct: 255 cfm

    That's a 83% difference!!! And I didn't even account for the pitot tube flow geometry. Full corrections would have been a little less flow value for the pitot but not greatly different.

    The difference/error of the anemometer was even more than I expected it to be just from a SWAG.

    The point is those typical anemometer readings that are often posted and used in any any duct size we use for dust collection are very inaccurate. It's fine if you just want to see some relative impact of a change (assuming you do the exact same measurement). But don't bother using it as a flow measurement.

    The most important factor comes back to the functional improvements of noticeably better dust collection and a reduced footprint/easier dust collector.

  10. #25
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    You do realize that comparing a 4" hose to a 6" duct is going to result in differences, right? The speed is going to decrease, regardless of what you use to measure.

  11. #26
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    Quote Originally Posted by Andrew More View Post
    You do realize that comparing a 4" hose to a 6" duct is going to result in differences, right? The speed is going to decrease, regardless of what you use to measure.
    Of course the velocity is different but Eric is talking about flow rate, CFM, which is the same in either section. Read the first part of your link, the part titled "The basic continuity". It doesn't matter where you measure, the CFM is th same. Obviously it matters how you measure.
    Beranek's Law:

    It has been remarked that if one selects his own components, builds his own enclosure, and is convinced he has made a wise choice of design, then his own loudspeaker sounds better to him than does anyone else's loudspeaker. In this case, the frequency response of the loudspeaker seems to play only a minor part in forming a person's opinion.
    L.L. Beranek, Acoustics (McGraw-Hill, New York, 1954), p.208.

  12. #27
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    I will jump in here without having read all the previous posts. I think for most people you do not really care too much about the actual flow rates and pressures. You are simply trying to get max flow and pressures at the collection point. Yes you want certain minimum air speeds and flow rates but you will have to take what your fan can produce.
    The best way to determine if they are adequate is to use the machine and measure dust in the air.
    That said repeatability is far more important then accuracy. Same idea as use the same tape for all measurements and it does not matter if it is acccurate or not. I used to have one that was 10" short since the tip broke off and had to be moved. It was very good and useable, worked fine as long as I only used that one tape.
    Bill D

  13. #28
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    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.

  14. #29
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    Fair enough. This doesn't match up with my experience in using both approaches, or with descriptions I've read online from HVAC engineers indicating that the pitot tube is less accurate than a anemometer. Maybe you've got a bad meter or setup.

    Quote Originally Posted by Eric Arnsdorff View Post
    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.
    Because on some level it doesn't matter. ALL measurements are by their nature inaccurate until you get to the subatomic level and the laws of nature say pick speed or position. The question is how close do you need to be for the job you're doing. For the purposes of what we're attempting to do 10-20% doesn't seem significant to me.

    The truth is that almost all of the shrouds on dust collectors are poor, and how they are setup matters a heck of a lot more than what your dust collector can do, at least in the range of most hobby level dust collectors. Maybe the 10 HP commercial setups it starts to matter a bit. The saw blades on table and miter saws shoot out dust at about 120 mph, most dust collectors move the air at about 60. Going 61 or 65 doesn't matter much. You'd need to double your performance to start seeing an improvement over clever shroud placement. Not sure how fast a router bit moves, but it's probably got similar issues. Bandsaw dust collection has issues the stuff above the table. Below it and a couple of 4" ports usually work pretty well. Jointer it doesn't matter, since everything falls, and there's little fine dust anyway. Similarly with thickness planers. With a drill press is all thick chips, so you've better off just using a brush and a bucket. Most hand held sanders work best with a shop vac of some sort due to their compact size.

    So maybe performance starts to matter with wide belt sanders?

    So given the issues you're working with, your time is often best spent focusing on the shroud, rather than the HP out of the collector, unless it's something relatively easy to do, like buttering the ducts, or making sure the runs are relatively straight.

  15. #30
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    Quote Originally Posted by Andrew More View Post
    ... descriptions I've read online from HVAC engineers indicating that the pitot tube is less accurate than a anemometer. ...
    Wrong context Andrew. HVAC operates at much lower velocities than DC, hundreds of CFM not thousands. At those velocities the output of a Pitot-static tube is very low, it varies with the square root of velocity. It's difficult to measure pressures that low. For example, wnile at 4000 fpm the pressure output is 1" wg, at 400 fpm it's only 0.01" wg. It that situation it's easier to get some accuracy with an anemometer. But, a Pitot-static tube is a primary standard and needs no calibration while an anemometer not only needs calibration but multi point correction factors. You might almost say that an anemometer is only as accurate as the Pitot tube used to calibrate it.
    Beranek's Law:

    It has been remarked that if one selects his own components, builds his own enclosure, and is convinced he has made a wise choice of design, then his own loudspeaker sounds better to him than does anyone else's loudspeaker. In this case, the frequency response of the loudspeaker seems to play only a minor part in forming a person's opinion.
    L.L. Beranek, Acoustics (McGraw-Hill, New York, 1954), p.208.

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