Thread: Inexpensive hot wire anemometer..Any recommendations..?

If you are hoping to insert a hot wire anemometer into a DC ducting line and get a sensible reading then you could well be wasting your time as most DC systems have too much turbulence in space and time.

A 4" duct typically can carry a max of 425CFM which translates to an

Accurate measurements should be performed using a long straight test ducts that are larger in diameter than the diameter of the actual ducts you are trying to measure.
The larger diameter duct sets up a much slower air speed that is less turbulent and easier to measure
eg I use a 240mm diameter test duct to measure flow in a 150mm section of DC ducting.
Testduct2.jpg

It would be worth your while to read

DRAFT: FAQ - Dust Extraction (Practical Aspects)

As Bob said, a hot wire anemometer really isn't the best choice for measuring the kind of velocities you're looking at. A pitot-static tube combined with a digital manometer is better suited for that. You really should have a digital manometer anyway.

Pitot-static tube for example. The AFP-6B would be my choice.

Digital manometer for example. I have one of these and it works. I also have a Testo 510i, which I prefer. It's operated from a phone and records the data for you.

Dave I wouldn't argue that pitot static is the best. Gregg is likely asking about a hot wire anemometer because on another forum I told him the fan type were inaccurate and read higher than actual. I suggested the hot wire as a better instrument. So in the grand scheme of things it is junk, better, best. Should he want to set up a test duct both the hot wire and pitot static will work with the nod going to the pitot static. The hot wire consistently placed in a hole in the duct is going to be far better than putting a fan type in front of an open duct.

Greg going back to your question, for that use you would want a hot wire anemometer that reads at least 30 meters/second (6,000 ft/min) but I'll happily sit corrected if I'm wrong. To keep dust in suspension the recommended flow in horizontal ducts is about 4,000 FPM (20 metres/second) and I think 4,500 FPM (23 metres/second) in vertical ducts.

Pete

Ok Pete, I understand. I agree that 30 m/s is high enough for most measurements. That's the highest you're likely to find in a low cost unit, at least from what I've seen. The HWA is clearly a better alternative to the "fanemometer".

If you only measure flow though, you're only getting half the story. You still need a way to measure pressure. So add a pitot-static to the manometer and you have both for lower overall cost than HWA plus manometer.

Of course, if you really want to go low cost you can, with care, fabricate your own instruments. A total pressure tube in a duct section along with sidewall static ports and a simple water gauge can make some decent measurements for very little cost.

Thanks Bob, Dave, Pete...
Lets take it as innocent enthusiasm rather than naivete. I'm commonly thinking about things flying through the air...never in my life sofar thinking about air flowing through a thing...

Bob, I was enjoying already reading some of your "Dust Extraction (Practical Aspects)". And reading some test results from Dave mapping results accross a duct, amenometer vs pitot-static.

It's very interesting wondering what is the most realistic notional model of the flow in these ducts....what the form of the turbulence actually looks like...

I'll read some more Bob et Dave and look for something on unsteady flows in ducts.

Cheers,
Gregg.

Originally Posted by GreggMacPherson

Thanks Bob, Dave, Pete...
Lets take it as innocent enthusiasm rather than naivete. I'm commonly thinking about things flying through the air...never in my life sofar thinking about air flowing through a thing...

Bob, I was enjoying already reading some of your "Dust Extraction (Practical Aspects)". And reading some test results from Dave mapping results accross a duct, amenometer vs pitot-static.

Thanks

It's very interesting wondering what is the most realistic notional model of the flow in these ducts....what the form of the turbulence actually looks like...

Think of a streamer fluttering in a stiff varying breeze ie fluttering all over the place. The closer to any junction or machine restriction the worse it is going to be. Averaging is needed to get a reliable measurement. A wider test duct reduces the amount of turbulence which also helps

Greg if it helps any watch how water behaves. Air will move in much the same way. You can even introduce ink or dye by way of a needle like smoke in a wind tunnel.

Pete

Originally Posted by GreggMacPherson

...It's very interesting wondering what is the most realistic notional model of the flow in these ducts....what the form of the turbulence actually looks like...


Cheers,
Gregg.

Google "CFD videos" for lots of that. "CFD" is "Computational Fluid Dynamics". It's a popular research area and a hot software market.

Edit: Ok, I wasted some time exploring some of those videos. Perhaps a better search is "CFD turbulent flow in ducts video".
I also found this one. While not a CFD simulation video, it's well researched and presented. It's a fun watch.

Hey Pete. I'll have a sniff around and find something easy to read by a physicist. Also there will be lots of duct flow model results from FLUENT accessible and discussed.

A couple of days ago I bought a good swag of used components for DE really cheap, so I can play if I want. Maybe I'll do a junk gloat on another thread...

Cheers.

Originally Posted by dmorse

Google.... "Computational Fluid Dynamics"....

Hey Dave, I don't need encouragement to go down that rabbit hole. I have to hold myself back. Twenty years ago I was hard at it using "3D panel" software (CMARC 12, cousin of VSAERO) for research in developing a sailplane. Full CFD codes like FLUENT were/are ten times more demanding to use, but are good at modeling separated flow. Should be good for cyclones and unsteady flows in ducts.....It always was horrifically expensive, but now at least the computers are cheaper. There are open source codes, and FLUENT etc can be a free student licence. Enough mesh detail for a round duct, my guess, but maybe not a cyclone.

Edit...Just started looking at "this one" (that one)...that guy is enormous fun..I saw him for the first time a couple of days ago, re a really perplexing inertial quirk....
The Bizarre Behavior of Rotating Bodies, Explained
The Bizarre Behavior of Rotating Bodies, Explained - YouTube

Cheers.

I just purchased one of these from Trade Tools in Qld.
0-30 m/s range. Max or average mode.

about $40

Originally Posted by Enfield Guy

I just purchased one of these from Trade Tools in Qld.
0-30 m/s range. Max or average mode.

about $40

Maybe have a read of this to see why they can't be used to accurately measure DC related air flows.
DRAFT: FAQ - Dust Extraction (Practical Aspects)

Originally Posted by BobL

Maybe have a read of this to see why they can't be used to accurately measure DC related air flows.
DRAFT: FAQ - Dust Extraction (Practical Aspects)

I might be better off asking this in the sticky, but whats the reason the propeller style meters need to be in ducting 20x their size?



I actually bought this guy quite a while ago and thought i may have been able to measure the duct work air flow. I think the guy i bought it off was an air con tech but was nice enough to leave his little calculation sheet.

Thought i may have been able to mount it inside the duct work easy enough, but doesnt seem like that will be the case as its 100mm across and im not converting to 2m duct work. Didnt buy it with that intention in mind.

havabeer69 take a look at this publication. It applies to the outlet end of the duct. I don't know how measuring at the inlet affects the readings but suspect it might be worse. https://www.cdc.gov/niosh/mining/use...dfs/ri9061.pdf

Pete