Skywatch

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NIST (National Institute of Standards and Technology) Calibration report for  NRG anemometer

As the owner of Skywatch Weather Station here in McKinney Texas I try to provide the most accurate data to the public. Most stations on Wunderground and CWOP provide good data. However some due to improper station siting of sensors, installation,  maintenance, and poor calibration provide less than ideal data. When it concerns the public and public safety, accuracy is very important. I've spent some time trying to carefully observe local conditions and calibrate my readings accordingly.

 

Back in June I went out and purchased some better quality wind instruments. These are the type of instruments used by the government, universities, and scientific institutions. Manufactured by NRG systems in Vermont. Anemometers are NIST calibrated for precision accuracy. NRG sensors are also designed to never wear out. So the accuracy today would be the same if it were years ago and will be that way for years ahead. And using Rulon J Teflon bearings which never require lubrication. The 1900 40C model anemometer which I employ on my weather station also doesn't use reed switches as traditional anemometers do. Instead it generates its own electrical pulses similar to a AC generator. Uses a magnet and copper coil to generate a sine wave current that the station translates into wind speed. Anyway to cut to the chase, I would like to provide the accuracy data here.

 

 

Installation is also important. The NWS standard for an anemometer and wind vane is 33' (10m) For my installation in order to avoid confrontation with the HOA I had to settle at 26'. However I carefully figured out an offset for the anemometer to ensure it would be accurate even at a lower than standard height.

The pole is also installed the farthest away from the house as possible and in a location parallel to gaps between houses and other obstructions. This allows the wind speed and direction to not be affected by obstacles. Siting in a suburban location isn't easy but following careful guidelines I've managed to get my sensors as close to ideal as possible. 

Using Weather Display software I'm able to fix the readings to maintain their accuracy for other factors that can't be avoided.

 

NRG wind sensors Vs. Davis wind sensor Vs. R.M. Young

At first glance an anemometer seems like just that an anemometer. You see one you see them all. However quite the contrary. Design plays a huge roll in performance and accuracy. Not only the anemometer but the wind vane as well.

 

Davis anemometer, This is a good sensor and is included with Vantage Pro2 stations. This sensor makes it easy by combining both speed and direction components into one easy to mount sensor that can be installed away from the rest of the station. Anemometer is reasonably sensitive.

 

 

The anemometers like most on the market use reed switched and stainless steel bearings which both over time wear out.

 

The anemometer while sensitive was subject to the lagging effect of momentum after a sudden wind gust. The wind would get the anemometer going but after the wind has passed momentum keeps the anemometer moving long after the wind gust. This results in over-estimation of wind speed which is the average wind over a 10 minute period. Of the total wind speed 10% of that is the actual wind then 90% residual momentum. Something that effects most weather stations not just Davis stations.

 






 

 

NRG wind set. These are rugged and proven sensors. The design I believe was originally employed by Maximum Inc in the early 1960's. Then in the 1980's NRG was founded and the sensors were primarily manufactured by NRG. These sensors use a magnet inducing a current into a copper coil. Similar to a electrical AC generator. The Davis Universal Anemometer Interface. Turns this AC signal into an on/off pulse the station recognises as wind speed. The NRG anemometer also uses Rulon J 
Teflon Bearings which are said to live forever.

 

 

The vane is also more sensitive. Responding to the slightest winds.

 

 

 

R.M. Young wind monitor. These are used on Weather Bug sites many NWS sites and on RAWS stations. But are they really all that? I've personally observed these sensors in person. They're overrated sensors.


The propellers are dependant on the wind vane which the two don't always agree with each other. I've seen it where a stray wind would catch the vane throw it around and the propeller sit there and doddle. The sensors are great if the winds are consistent but in other situations they're not so great. The speed is measured from 180 degrees. Other means of wind speed measurement is measured from 360 which is far better. it's better to have a full range of speed measurement than a partial range of measurement dependant on another variable.




 

 

 





Ultrasonic anemometers. 

These wind sensors are also popular in NWS RAWS Hazmat Pollution and fire monitoring etc. They use ultrasound to measure the wind. As the wind moves between the sound generators the wind disrupts the sound. The transducer on the adjacent ends measures the sound received and this registers as wind speed and direction. Because of their principle of operation these sensors can measure winds to 0.01 mph and 0.1 resolution wind direction.


However there is some unknown about the sensor. To my knowledge it hasn't yet been determined whether or not these sensors are affected by airborne particles and debris. I'm sure if a bird or insect got in between the sensor's components this could cause problems in the measurements. These same problems might affect cup anemometers as well but usually cup anemometers if they start rotating the bird will most likely fly off. I don't know if birds or insects are sensitive to the ultrasound.


Another problem with these sensors is because of how sophisticated they are they are not well suited for many weather station applications. They require special calibration processes and programming to work properly. They can't just be installed on a weather station. Because of this they're not suited for many station applications and 0.01 speed or 0.1 degree wind direction resolution isn't necessary for most station applications.


 

 

Sensor photos

 

 

 

Davis wind sensor.

 

 

 

NRG wind sensors

 

 

 


R.M. Young wind sensor

 

 

 

 

 

                                                                                                                                                                  Ultrasonic anemometer

Accuracy specs for other sensors

Although this page is mainly to explain the anemometer I decided to include the other sensors specs as well. Accuracy is as important to all other variables as wind speed. And with that said here are the specs for the other sensors.

 

 

For that here's the accuracy of each sensor

Variable                               Range                                    Accuracy                                     Resolution

Wind speed/gust         0-220 mph (1-320km/hr)               2%                                         1.0 mph/km/hr

Wind direction                  0-360 degrees                            1%                                           1 degree

Temperature                  -40-150 F (-40-65 C)                    1 degree F (0.5 deg C)       0.1 F/C

Humidity                               0-100%                                      3%                                           1%

Barometeric pressure   16-32.5in/hg (540-1100mb)      0.03in/Hg (1.0mb)                0.01 in/Hg (0.1mb)

Solar radiation                      0-1800 W/M2                          5%                                             1 W/M2

UV index                                  0-16                                        5%                                              0.1

 

 

Everything in green means good. Yellow means OK. In the case of the wind speed resolution 0.1 resolution would be ideal. On Wunderground resolution to 0.1 is given however this is a wind speed offset factor and for the average wind speed displayed in the wind direction indicators uses the current plus average wind speed and counts in small increments depending on the wind.

 

All siting factors are met.

 

For more information

 

http://www.srh.noaa.gov/media/epz/mesonet/CWOP-Siting.pdf

http://www.comptus.com/PDF/CWOP_Guide.pdf

http://www.eos.ubc.ca/courses/atsc303/Instruments/manuals/Misc_siting.pdf

http://www.comptus.com/PDF/CWOP_Guide.pdf