### Author Topic: Anemometer basics  (Read 2091 times)

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#### SLOweather

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##### Anemometer basics
« on: January 07, 2016, 03:41:50 PM »
An anemometer measures the speed of the wind. There are a few types. For this, we're concerned with cup type anemometers.

After a little thought about how one works, it seems to me that in a perfect world, the speed of circle the cups trace out, the "circumference" would match the speed of the wind.

However, factors like bearing friction, aerodynamic friction of the cups moving through the air, rotational inertia, any measurement induce loss, etc will slow the cups down.

For instance, a Davis 7911 anemometer cup set has a radius of 3" from the shaft center to  the outside of a cup. The circumference is then 3.1416 * 3 * 2 (pi x the diameter) or 18.85" inches, or 1.57 feet.

A mile is 5,280 feet. That would be 3,361 rotations. So, in the perfect world, a one MPH wind would cause 3,361 rotations of the cups.

However, in their data sheet for this device, Davis says that 1 MPH = 1,600 rotations in one hour. That's a correction of 2.1, or 47.6%.

I'm certain that the correction factor will be different for different designs of cup assemblies.

#### SLOweather

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##### Re: Anemometer basics
« Reply #1 on: January 07, 2016, 04:01:56 PM »
One other thing I noticed about the Davis cups. The inside edge of the cup is about one cup diameter out from the shaft.

BTW, there are several papers on the web about anemometer, cup design etc. I'll post links when I have a chance.

#### Jáchym

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##### Re: Anemometer basics
« Reply #2 on: January 07, 2016, 04:32:49 PM »
My dad bought the NetAtmo couple of months ago so for Christmas I bought him the brand new anemometer for NetAtmo station. The interesting thing is that it uses ultrasonic technology, so I am curious how well it will perform.

#### Old Tele man

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##### Re: Anemometer basics
« Reply #3 on: January 12, 2016, 04:23:15 PM »

However, in their data sheet for this device, Davis says that 1 MPH = 1,600 rotations in one hour. That's a correction of 2.1, or 47.6%.

Could it be that Davis is using TWO contacts per revolution...to provide better 'slow' wind registration?
• SYS: Davis VP2 Vue/WL-IP & Envoy8X/WL-USB;
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#### SLOweather

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##### Re: Anemometer basics
« Reply #4 on: January 12, 2016, 06:00:39 PM »
at Davis is using TWO contacts per revolution...to provide better 'slow' wind registration?

The spec sheet (http://davisnet.com/product_documents/weather/spec_sheets/6410_SS.pdf) says:

Wind Speed Translation Formula . . . . . . . . . . . . . . . . . . 1600 rev/hr = 1 mph
V = P(2.25/T)
(V = speed in mph, P = no. of pulses per sample period
T = sample period in seconds)

If you reduce 1600 res/hour down, it comes out to 1 mph = 1 rev in 2.25 seconds. The first time I saw that, I figured that was the basis for the ISS's basic transmission rate.

#### Old Tele man

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##### Re: Anemometer basics
« Reply #5 on: January 12, 2016, 06:31:24 PM »
Ah, it's being limited by the ISS's intrinsic sample-rate/interval.
• SYS: Davis VP2 Vue/WL-IP & Envoy8X/WL-USB;
• DBX2 & DBX1 Precision Digital Barographs
• CWOP: DW6988 - 2 miles NNE of Cortaro, AZ
• WU - KAZTUCSO202, Countryside

#### miraculon

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##### Re: Anemometer basics
« Reply #6 on: January 13, 2016, 09:25:57 AM »
My dad bought the NetAtmo couple of months ago so for Christmas I bought him the brand new anemometer for NetAtmo station. The interesting thing is that it uses ultrasonic technology, so I am curious how well it will perform.

I am curious about this as well. It is low cost for an ultrasonic wind sensor. Let us know what you find out. I wonder about battery life as well....

Greg H.

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