The wind vane converts the direction of the wind into a DC voltage which is measured by the station. The output voltage increases with increasing wind direction.
In order to not have to fix the wind tree in a particular orientation and to allow for different designs of vanes, the station software has a calibration feature built in. It detects the voltage range through the 360 degrees of motion and records the value representing true north.
Wind for meteorological purposes is referenced to geographic or true North and expressed in miles per hour. Wind direction broadcasted at airports however is called runway wind and is referenced to magnetic north.
Calibration of the vane is best done with the help of a smart phone or a tablet while standing next to the wind tree.
- Go to the stations web interface and click on Calibration, enter the credentials and then click on Sensors.
- Click on the button “CALIBRATE” at the bottom of the page, then observe the reading as you slowly turn the vane.
- Find the spot where the reading jumps from low to high or high to low and move the vane slowly through this point a few times to get the min and max recorded accurately.
- Using the compass on your smartphone/tablet figure out which way north is. Keep the compass a couple of feet away from metal poles and the vane itself or else you will get a distorted reading. Note that a compass points to magnetic north and that you have to add or subtract the local variance from the compass reading to find true (geographic) north.
- Point the vane to true North and click SAVE to store the readings.
It is possible that during calibration the value stops updating before you get a chance to click SAVE. This is due to a timeout in the connection with your tablet or phone, possibly because of poor WiFi coverage or latencies in the connection. Simply try again.
The calibration takes into account the voltage drop in the cabling to and from the station as well as variations in the power supply voltage. Calibration therefore should be repeated whenever the cabling or the power supply changes.
The vane offset to North (in degrees) can also be entered manually under Wind Vane Offset.
An other way of finding true north is by looking a Google map and draw a vertical line from your location to a landmark on the map. Then find that same landmark outside when you calibrate the vane. make the point of the vane point to the landmark and click SAVE.