Thread: Anemometer on special

Rainy day in Sydney so up to the shed to do some more measurements
First Set was with both the vane and Hotwire mounted at entrance to the 4” pipe, same distance in and, as expected lots of turbulence and wild readings.
0CF1EF09-5B6A-41CD-998D-3F0A303D3CA7.jpeg 6BA3455D-5F6B-4AE6-A447-65B6F793FFEF.jpg
These measurements were with the duct fully open. Partially closed gave vane 20.2 and Hotwire 10.8 and almost fully closed gave vane 5.5 and Hotwire 3.54 for variations of 79%, 87% and 55%. Definately no constant there.

So I tried the inlet to my saw table. It is a flap that extends the width of the dust box, is hinged, and open when the extractor is turned on.
664A497D-AFB1-497A-A366-C1296CFBFCF1.jpg 510CE46B-4435-477A-A22E-FD4CA1C54E8A.jpg
Once again turbelence etc with a vane reading of 5.4m/s and Hotwire 3.64.

What I wanted was a fair comparison with minimal turbulence so I decided to measure in the chamber box itself. At the front is the flap and the rear RHS a 150mm outlet. It’s a large chamber.
Results were good though they took a bit of creative photo shooting for the vane display

C099994A-AA41-4210-9BEF-5CBEE8D77507.jpg

You will have to expand the vane picture but it reads 1.4 m/s. It only reads to onedecimal place.
2428F781-67F4-4423-913C-6A163166F2A6.jpeg


The Hotwire reads 1.43

7CF7325A-1917-4CC0-98A5-9F4C16410814.jpg

Not on a huge reading re speed but it’s not blocking an airway so it’s not affecting the reading.
So I would say it’s a reasonably accurate instrument IF used in the right conditions.

Also tried it in front of my floor fan on Hight - vane 1.9m/s, Hotwire 1.93m/s.

Now to find a way to use it in a measuring tube so people can get a reasonably accurate airflow measurement for their systems with a cheap instrument.

BobL must be on holiday no comment from him

I think BobL has said all there needs to be said. The anemometer is not a good way to measure air flow in a duct. Also, measuring airflow at the end of a duct with any instrument is problematic due to turbulence and other factors.

For reasonably accurate readings you need to do it in the middle of a long tube or duct so that turbulence is minimized and you need to do readings across the duct at certain locations. BobL has described this a number of times and responded to the anemometer question several times.

We would all like a cheap and easy way to measure performance but unfortunately there is no easy way. I have done measurements with a hot wire anemometer doing traverses but I know I still have significant error compared to the rigorous method that BobL has shown. If you want to see changes in performance, try measuring pressure and see what changes there are.

Using a Hotwire anemometer, if you traverse the correct distances and allow the correct distance from openings, elbows etc that cause turbulence, then your feadings should be as accurate as any that BobL has taken, so I can’t understand your statement “significant error”.

I’ve used pressure readings with pitot tubes, static pressure readings with and without blocked inlets and used Hotwires. I get accurate, repeatable readings. I know when things have changed or if one modification makes the system better or worse. I like to fiddle and experiment (See the thread on fitting a wok). People will say “this won’t work, that wont work” but never show pictures or figures or numbers showing why it won’t work - that’s what I’m doing.

The purpose of these series of tests is to see if a cheap vane anemometer can provide a method of checking whether a modification has made the system better or worse and maybe, with modification/whatever, provide a reasonably accurate figure (even if it does involve a calibration figure for each pipe diameter) for flow like the USA mines depot achieved using a vane anemometer.

I wonder if testing the windvane anemometer in a tube / pipe the same diameter as the vane housing would diminish the effect of turbulence on readings? If so, can you then extrapolate from say, 50mm pipe readings to say 100mm pipe or even 150mm pipe?
I don't know if any o the above is possible but tying to think of a work-around use for the windvane.
Geoff

Sometimes i wonder if the old fashion airspeed indicators used on aircraft that relied on pressure of the air on a spring/paddle would be just as effective as anything else. Just needs the scale changed to cfm.

https://sites.google.com/site/pieten...-plan-packages Down the page a bit.

https://www.alamy.com/stock-photo-pr...-23882797.html

Maybe upgrade it with a tiny pressure transducer and readout for those that like complication.

Pete

TinyTool
Biggest problem with that is that the cheap vane units have a maximum speed spec. of 30m/s although it does display higher figures.
If you look at the reading on the edge of the 100mm pipe, it’s reading 29 m/s.

I would have to find a steady, low air flow source in a small diameter pipe.

QCInspector
Not sure on those. Appear to be designed to work outside a structure, such as the vane unit.

I reckon a pitot tube and plastic pipe manometer would be a simple method except the calibrated pitot tubes cost a bundle. I made a unit out of a diesel injection pipe but it needs two entry points; one pipe for total pressure and one for static pressure instead of the one for a proper pitot tube. I calibrated it against the Hotwire so some may question it, but it worked very well. Might see if it’s easy to cobble up a cheap unit that anyone can make.
More testing to do.

I got to thinking about my tongue in cheek comment about tiny pressure transducers in my last post. Would something like these placed at the end of a thin diameter tube or rod work? They are only about 8mm to 10mm in diameter so would disturb the air a lot less than any fan and not much more than a hot wire or pitot tube.


https://www.aliexpress.com/item/Micr...AbTest=ae803_3

https://www.aliexpress.com/item/FSR4...AbTest=ae803_3


Or perhaps something like this for a pitot tube.

https://www.aliexpress.com/item/DC-5...AbTest=ae803_3

I don't have the brains or education to implement but I bet Bob could if it was feasible. As if he doesn't already have enough on his plate.

Pete

The second one you linked to we already use to measure pulsations in vehicle inlet systems and also exhaust systems. It’s hooked to a picoscope, is very sensitive and works extremely well. It could certainly measure the pressure from a tube like a pitot but you would need to be able to convert the signal to an accurate pressure vs voltage scale reading. We only look at the pattern to determine cylinder pressure problems.

The first one still needs a pressure source from eg. a tube, and some circuitry to convert the signal to a measureable reading, so a pitot tube and a water manometer would be a cheaper option IMO.

The third option operates at too high a pressure for what we want to measure if those you listed are the only ones available. Once again, if one was available, there would be a need to develop an accurate pressure vs voltage scale.

New tool just arrived. Be good to get some comparisons.
4C33D6A5-D80E-46CD-AA79-2A2200077422.jpeg