GHowSAW Wind Direction
The wind direction is easily seen using a wind vane. A wind vane is a horizontally rotating shaft which has a larger cross-sectional area one side whilst maintaining a mass balance. The side with the lower sectional area will always point to the direction the wind is coming from. All that is required now is a sensor to measure the direction the vane is pointing without interferring with its rotation.
The low cost wind vanes use a magnet and an array of 8 magnetic reed switches or hall effect sensors to determine the pointer position to a resolution of 22.5° . My personal preference is the Inspeed E-vane which used a sealed Hall Effect sensor to provide a 0.25 - 4.75VDC analogue output from an input of 5 VDC. This unit has several advantages over potentiometer vanes, including:
- Zero dead band. Many potentiometer wind vanes have a dead spot of several degrees. This device has no dead band at all.
- Near-zero friction. Since the magnet is not in contact with the Hall sensor, there is no friction from the sensor.
- Virtually infinite life. Unlike potentiometers that wear out, the magnetic Hall sensor is non-contacting and should theoretically last forever.
With a 12-bit ADC a resolution of 0.025 degrees it possible with and accuracy of +/-0.3 to 0.5% of signal range. Provided of course that a stable input voltage is used.
The Inspeed e-Vane includes a locking feature to lock the sensor so you can twist the vane and set "North" (or zero output) to wherever you wish. This means that the offset bracket can be installed in any direction. With the e-Vane, just mount the vane, lock the sensor when the software (or your device) says North, twist the vane until it points North, then release the sensor so it is free to rotate.
The schematic of this arrangement with a DS2438 one-wire slave is shown below.
To obtain the best results from your wind vane careful siting is essential. In general the wind vane should be located close to and below the anemometer but not so close as to interfere with the anemometer.
- For best results use a spirit level to make sure that the vane rotates in the horizontal plane.
Because the preferred location of an wind vane is not readily accessible it is important to thoroughly check the anemometer before installation. The following points should be attended to prior to final installation:-
- If you can access them readily then oil the main bearings
- Balance the rotor for longer life.
- Hold the unit horizontally either by clamping it to a table or using a bench vice making sure that the vane can rotate freely.
- Gently rotate the vane and note which end is heavier and falls to the bottom. Apply a small amount of metal tape to the uppermost end.
- Gently rotate the vane again. Continue adding tape to the top end until the rotor seems balanced. You don't need to balance it perfectly but the rotor should spin slowly and stop smoothly without swinging back and forth.
- Because the wind vane will be exposed to the elements, especially rain, make sure that any joints in the housing and cable entry points are sealed properly. The cable should be tied to the mast with a drip loop to prevent water collecting at the cable gland. Regardless of how well the unit itself may be sealed it is always advisable to protect any internal circuit boards with PCB lacquer spray
It is normally essential to protect the meteorological mast installation from lightning strike. Due to their nature, masts cannot avoid being struck by lightning and the challenge therefore is to ensure that a strike does not damage the test equipment. A number of key precautions can be taken:
- a lightning finial (attractor) should be mounted at the top of the mast, in such a position that it affords the tower top anemometer with protection (normally a 60º protection umbrella can be assumed) - it is normally adequate to use the tower as the path to ground, but added protection can be afforded by running a separate cable
- an adequately sized earth connection (earthing rod) should be strapped to the tower base
- the instrumentation system should be designed so that it does not provide a low resistance path to earth, the aim being to encourage the strike to pass via the structure
- lightning surge arrestors should be used should the data system not have in-built protection.
Sometimes the probability of lightning strike will be very low, and it may be decided not to use a lightning protection finial so as not to disturb the mast top anemometer.