Sonic Senor Alignment

 

Purpose:  This procedure insures that the wind speed and wind direction are accurate by aligning the sonic sensor due north.

 

Required Equipment:  Geologic Transit

                                      Tripod

                                      Binoculars

                                  

Reference:  SOPMAN

 

  1. Down all met channels (down all trace gas channels if on the same tower) and lower the tower.
  2. Install (if not already installed) the RM Young Model 18301 Vane Alignment rod between the mounting ring and the Sonic anemometer.  Set the Vane Alignment rod to a nominal North/South orientation and tighten in place.
  3. Secure the Sonic anemometer to the Vane Alignment Rod with the label facing as close to due north as possible.
  4. Raise tower and place the Brunton Geologic Transit (transit) (see Figure 1) on its non-magnetic tripod on the ground adjacent to the tower directly underneath the Vane Alignment Rod.
  5. Using the mirror to sight the Vane Alignment Rod above you rotate the transit until the line in the mirror is parallel with the Vane Alignment Rod.  Note the bearing indicated on the compass. 
  6. Look up the current magnetic declination to the nearest degree for your site at http://www.ngdc.noaa.gov/seg/geomag/jsp/Declination.jsp
  7. Adjust your expected compass heading for north by adding the magnetic declination if the declination is west or subtracting the magnetic declination to the nearest degree if the declination is east.  For example:  At JST on 9/28/04 the magnetic declination was 4 Degrees 3 Minutes West so our adjusted North bearing would be 360 degrees + 4 degrees magnetic declination for a bearing of 4 degrees.
  8. If your Vane Alignment Rod is more the +/- 2 degrees out of the proper bearing lower the tower and adjust the rod alignment until it is within +/- 2 degrees of proper bearing.
  9. Now raise the tower and secure.  Take the transit with you and walk ~ 150 feet west of the tower.  Sighting through the SW and SE SONIC support arms with your binoculars walk to the point where the east most vertical arm is obscured by the west most vertical arm.  Take a bearing on the tower with your transit and note that bearing.  Be sure to avoid large metal objects that may influence the compass reading.
  10. Adjust the expected bearing for local magnetic declination as before – this time adding or subtracting the magnetic declination from 90 degrees.
  11. Compare the actual compass bearing to the expected bearing calculated in step 9 and record the error in alignment (if any).
  12. Down tower and realign if necessary.  Repeat steps 10 & 11 until bearing is within +/- 1 degree of expected bearing.
  13. Move tripod to due east of the tower (~150’) and sight along the SW and SE bars as before.  Adjust the expected bearing for local magnetic declination as before – this time adding or subtracting the magnetic declination from 270 degrees.
  14. Record the error in alignment (if any).
  15. Raise tower.
  16. Up all downed channels.

 

Figure 1:  Brunton Geological Transit

 

Figure 2:  Diagram of east/west sighting through SONIC support arms to accurately determine the bearing of the sensor.