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Posts: 550
Reply with quote  #1 
I'm thinking about getting an anemometer and was wondering what your experiences have been with them and what features/specs I should look for.  I'm not currently doing side-by-side comparisons.  I would just be using it to get an idea of the cfm I'm getting in each collector.  On previous collectors I just used the output temp to size the fan.

- Besides wind speed are there any other features I would need for this applicaton?
- I assume the anemometer would need to be able to start at low wind speeds and have good accuracy at low wind speeds.  One designed for high speeds (like 100mph) is probably not as good as one designed for lower speeds.
- What wind speed measuring units should I look for?  Should I make sure it has ft/min?
- What are your experiences with measurements?  Some of the discussion in the following thread makes me question using an anemometer, since the readings vary so much across the duct opening.  One article says to take 25 measurements at different points.
- Does having a long straight section of duct smooth out the air flow at the measurement point so it doesn't vary as much?  (Seems to be what Gary at BIS does in his measurements.)
- Kestrel seems to be a popular brand, but the cheapest model is $75.  What am I getting compared to a cheaper model?

The other option is using the garbage bag method, but the difficulty is knowing how many gallons the bag really holds when filled.  I don't think you can trust what the bag is labeled and collapsing the open end of the bag around a duct reduces the volume of air in the bag.

Kevin H


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Posts: 2,206
Reply with quote  #2 

I think for the solar heater or RC enthusiast, an anemometer is very handy to have. However, as an experience DIYer, I have never had a use for mine outside of my solar collectors. I have the Kestral 2000, which, in addition to wind speed, also measures air temps. Most can be set for MPH, FPS, MPS and more. I managed to get a great deal on an Amazon lightning deal. You could certainly get by without the thermometer though. Kestral anemometers are what the olympics use to determine, so you can assume they are accurate. But I think other brands could work as well. A couple of features you may want to look for is ball bearings for your fan. It is much smoother than plastic bushings. A replaceable fan is nice too. Another very nice feature is the ability to take an average of your wind speed. As you try and measure wind speed the numbers will bounce around like crazy, and averaging will give you a more accurate overall reading. Below is a model that looks pretty good for a lot less than the Kestral brand. It reads winds as low as .4MPH and has wind speed averaging.


 As far as taking the actual measurements, a straight piece of duct at least 2' long is desired as well as a flow straightener which is nothing more than a cross made from flashing to smooth out the random flow. The straight duct is needed as measurements at an elbow the inside of the corner air flow is much slower than the outer part of the corner. Accurate wind speed is probably the most difficult measurement to take accurately. Taking a dozen or more readings is rather impractical on a 6"-8" duct. However, you should strive to take a few measurements in the exact same location(s) within the duct. That could be done by using a jig of some sort, or a mark on the edge of the duct lining up with a certain spot on your anemometer. When I was testing last winter, I had a day that for some reason the wind speed measurements were all over the place. I thought I might have power fluctuations or something worse, my fan or Anemometer were going bad. It turned out that it was really windy which pushed on the glazing, resulting in massive flow fluctuations.


Posts: 4
Reply with quote  #3 
Not sure and didn't find on Google [as yet] as I may be looking for wrong place. But is there not a rule in engineering for duct work that says some like the best  efficiency with fan is to have 6 times the diameter in length before and after the fan to remove turbulence. 

and to get best results  add all the curves together and deduct 5% for each amount of 90 Degrees bends. 

Like I said it maybe buried in places I can not verify, does anyone know for sure???????


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Posts: 2,206
Reply with quote  #4 

I'm sure there is some rule of thumb regarding best fan placement within a duct run. What you may be referring to is the best way to accurately measure air moving through a given duct. You certainly do not want to take air speed readings immediately after a 90˚ bend. But if that is the only point of measurement, a temporary straight section should be added. I like to add a + or X made out of sheet metal and placed within a duct before taking readings. They will help settle the flow of air within a duct for more accurate readings. Keep in mind that taking accurate airflow readings is far more difficult than temperature readings.

As our collectors are DIY, we often using recycled fans, and less than ideal ducting runs. Below is a link to a page with a PDF that lists a few common EL, or equivalent length ducts, and various duct pieces, such as elbows and transition pieces. A 1' piece of straight duct has an EL of 1, while a 90˚ elbow, can be the equivalent to 20' of straight duct. So while we strive to have optimal duct and fan setups, for DIYers, it's more important to learn and understand so improvements can be made in the future, while still making a functioning solar device. 


Greg in MN
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