Dust collector air flow measurements

[Eric Arnsdorff]

I’m changing up my dust collection and figured I’d check my flows while I’m at it to see if my changes make measurable differences. I went to buy one of the vane anemometers on Amazon and the geek in me convinced me to look for something that may be semi-accurate.
I see measurements with these posted a lot but I also know it’s very inaccurate.
I know there’s not much that can be trusted on Amazon with instrumentation. But some things are both cheap and reasonably accurate.
The pitot tube devices seem like they should work and one could map a duct to get a decent average (given a straight run long enough for fully developed flow). I’ve never tried any of the Amazon digital manometers. One could check it with a homemade water nanometer. Has anyone used these?
I see you can get some hot wire anemometers. Those seem like they’d have some chance of being reasonable. But would likely need checking against the above mentioned pitot setup or such.
I’m hoping to get some decent advice to learn if it is worth getting something cheap from Amazon for my shop use without going through the cost and effort of trial and error.
In the end the vane anemometer probably would give a reasonable comparison of the effect of the changes. But I’d like to have a somewhat trustworthy flow measurement just because of that geek side of me.

[John Lanciani]

See my post from a while back here; https://sawmillcreek.org/showthread....fer&highlight=

LMK if you're intrrested.

[David L Morse]

Dwyer makes well respected equipment but I use this lower cost pitot-static tube.

I have a manometer that's an older version of this one and it's ok but I prefer this one, in part for it's Bluetooth data logging ability.

I also have a hot wire anemometer but it has some difficulties in dust collection size ducts. It's good at HVAC velocities though.

[Thomas Wilson]

Dwyer makes well respected equipment but I use this lower cost pitot-static tube.

I have a manometer that's an older version of this one and it's ok but I prefer this one, in part for it's Bluetooth data logging ability.

I also have a hot wire anemometer but it has some difficulties in dust collection size ducts. It's good at HVAC velocities though.

I have the Testo 405i hot wire anemometer. It is limited to 30 m/s which is 5905 fpm. Both of my dust collectors exceed this in 4 inch duct. I find it useful for checking air velocity in gaps around the tool cabinet. I also have the Testo 510i manometer. It could measure flow with a Pitot tube. But I only have a static pressure probe for measuring differential pressure to ambient mainly for measuring filter clogging. I think the vane type anemometer also works well for this kind of testing.

[Eric Arnsdorff]

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.

[Andrew More]

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.

[David L Morse]

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.

[Bill Dufour]

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

[Eric Arnsdorff]

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

[Larry Frank]

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.

[Ole Anderson]

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?

[Larry Frank]

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.

[Thomas Wilson]

The problem with flow measurements is that the flow velocity varies in magnitude across a channel. The good news is that a velocity across a round duct for fully developed, turbulent flow is fairly uniform. One just has to have a straight round duct about 10 pipe diameters long to have fully developed flow. Most people do not have a suitable, accessible length of duct that they are willing to drill holes in in an existing system that can serve as a test section. So what can be measured?

Velocity varies a huge amount in both magnitude and direction across an open duct outlet which seems to be the way YouTubers measure velocity. The pitot tube sensor and the hot wire anemometer measure at a particular point and direction. That point and direction can be very different from the average flow in the duct. Spatial averaging, ie taking a series of flow measurements across a duct can compensate for point versus average problem if the averaging is done correctly.

Also the sensor response, ie the sensor reading versus the true flow velocity at that point, can be different. The reality is that the point response of hot wire and pitot tube is pretty good. This analysis changes for the vane anemometer. It averages over an area in a complicated way. Plus, its sensor response under uniform velocity field is not so linear. So it has to be used within its calibrated range of velocity. Ideally the velocity profile across vanes should be flat to improve the accuracy of its response. The limitation of the hot wire anemometer is range. My Testo 405i maxes out at 5900 fpm. For duct velocity in 6” duct, 5900 fpm is not enough for a true high performance dust collection.

The question for the shop owner is mostly how good is my own system and could I make better at a reasonable price. The good news is that all the measurement methods generally track velocity proportionately if not accurately in magnitude. Since we don’t know exactly what velocity is needed, this knowledge can be enough. I take readings on velocity readings around the air inlets and gaps of cabinet tools. I find 200-300 fpm enough to entrain fine dust particles. Your mileage and collection efficiency may vary.

[Chris Parks]

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.

[Eric Arnsdorff]

Thanks David! Those were exactly what I was looking at on Amazon. I’m hoping the cheap digital manometer gives accurate enough readings. It’s easy enough to check with a homemade inclines u-tube manometer it seems. Hopefully, the pitot tube is made well enough for some light use. I’ve put those on order.

John it looks like you have some equipment you’ll loan. I’ll get the pitot tube measurements and see how that goes. If I find I’m unable to get reasonable measurements with the pitot I may take you up on the offer. I can certainly report back some of my readings along with improvements.