Results 1 to 4 of 4

Thread: Fan Anemometers

  1. #1
    Join Date
    Nov 2007
    Location
    NW Indiana
    Posts
    3,078

    Fan Anemometers

    This discussion on the validity of the fan type anemometer was buried at the end of another current thread. I am starting this thread to have a topic just about it.

    In 2016, I posted a thread about the results I got doing some testing with a Fan Type Anemometer called "Measuring Dust Collector Performance - Fan Type Anemometer" . What I found is using one can get wildly ranging results depending on how close you held it to the opening of your duct or hose. This does not even take into account the angle that you hold it. Lastly, the worst place to measure air flow is at the end of a pipe as you have air flowing in all directions. The presence of a large device like the fan anemometer also changes the flows.

    After I posted the above mentioned thread, I spent more time reading on the Australian Forum - woodworkforums dot com. I would provide a link but not allowed here. They have a Dust Extraction Forum that is very good and an expert BobL who is a frequent contributor. I am going to put a couple of quotes from here for people to read. If you are interested, I would encourage you to read this forum to gain a better understanding.

    This has to do with the capability of the 2 hp units

    Yep I have measured the standard 2HP DCs from carbatech, timbecon, H&F and an older Ron Mac.
    They are all the same.
    Using either one of the two 4" inlet ports the pull about 425 CFM.
    Removing the plastic 2 x 4" adapter reveals a 4.5" port that draws about 570 CFM
    Pleated filters add a few %, less to the straight 4" port and a bit more to the 4.5" port

    Modified to full 6" flow according to the generic 2HP DC thread produces about 870CFM and about 10% more with pleated filters.


    These have to do with the fan type anemometers

    Propellor type anemometers are not appropriate for measurement of air speeds in or around wood dust extraction ducts.
    In general they significantly over estimate the air speeds and as their measurements are highly position sensitive cannot be used reliably even as a rough indicator..
    Most changes that are made to DC systems produce small (a few % changes) so basing changes on propeller type measurements can end up making things worse rather than better.


    I would advise against the use of any anemometers at the entrance of any DC duct to measure flow. It's dead easy to measure an air speed at a specific point but the air flow patterns at entrances are so complex and turbulent that it is near impossible to convert that into a flow. This is why a test duct is used.



    you cannot place an anemometer at the front of an inlet where the air flow is highly sensitive to orientation and position and what is in line further down the air path. You can have the exact same position and orientation but something you did further along the air path alters the preferred air flow path back at the propellor and this will give you you a wrong reading.

  2. Also, the simplest issue with fan type anemometers is that they block a significant percentage of the duct opening. This increases the air velocity through the fan (smaller area = higher velocity) and thus over-estimates the ariflow if the full cross sectional area of the duct is used in conjuction with this velocity to calculate the estimated volumetric flow. For a 4" duct a fan type anemometer can be a huge % of the cross sectional area. Cheers, Dom

  3. #3
    Join Date
    May 2005
    Location
    Highland MI
    Posts
    4,511
    Blog Entries
    11
    Again looking for data, not opinions. Best I can readily find is Larry's (OP on the thread resurrected by himself) in his 7-28-2016 post: "My tests with the same hose and conditions using the digital thermal anemometer gave about 490 cfm and 5621 fpm. Based upon the graph, it appears that holding the fan anemometer about a half an inch from the end of the 4” hose would be about right." To me this validates that a fan anemometer can provide fairly accurate and useful results if held fairly close and in the center of the orifice. My example and pic showed that I was about flush rather than 1/2" out, but I bet if I tried again the difference between flush and 1/2" would be within 10%, which in a hobby shop means squat. You measured 490 "actual" cfm at the end of what appears to be a 4" slip connector vs my 570 cfm, a difference of 16%, easily attributable to about a dozen factors in system design. Thanks for posting that. The one thing I noticed from your 2016 post is that you did not get a data point flush with the orifice (at zero), and the curve you superimposed over the data points implied infinite velocity at 0" from the orifice. Been a long time since I did any statistical curve fitting, but one more point at zero and a different polynomial (or even an old french curve) would be really interesting. Also did you notice a decrease in actual flow as you moved the anemometer closer to the orifice as many claim would happen? Good stuff.
    NOW you tell me...

  4. #4
    Join Date
    Sep 2006
    Location
    Deep South
    Posts
    3,970
    The thing I like most about those little fan anemometers is you can keep positioning them and reinterpreting the results until you validate whatever conclusions you are looking for. At one point in my career, I had to validate the performance of high velocity environmental chambers for use in automotive electronics testing. I would never have allowed an equipment vendor use one of those things because different brands give different results and are highly subject to positioning in the air flow. I only allowed hot wire anemometers because they are consistent and don't alter the air flow of what you are trying to measure.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •