RXW Davis Wind Speed and Direction Sensor (RXW-WCF-xxx) Manual
1-800-LOGGERS 6
www.onsetcomp.com
3.
Drive the 3-inch lag screws through the holes in the
anemometer base and into the wood.
Securing the Sensor Cables
To prevent fraying or cutting the anemometer cables where it is
exposed to weather, secure it so it does not whip about in the
wind. Use cable clips or weather resistant cable ties to secure
the cable. Place clips or cable ties approximately every 1 to 1.6
m (3 to 5 ft). Do not use metal staples to secure cables as they
can cut the cables.
Measurement Operation
Wind speed measurements are averaged automatically every 3
seconds within each logging interval. Wind speed is reported as
the average speed for the entire logging interval. For example,
if the logging interval is set to 1 minute, then 20 measurements
(1 measurement every 3 seconds for 60 seconds) are averaged
together and the result is logged each minute. The gust speed is
the highest 3-second wind recorded during the logging interval.
Unit vector averaging is used to determine wind direction
because traditional averaging would produce inaccurate
results. For example, three measurements of 350, 11, and 12
degrees—which are all winds from the north—averaged
together would result in 126 degrees, which incorrectly
indicates a southeasterly wind. Instead, the vector components
(North/South and East/West) for each wind measurement are
calculated every three seconds for the duration of the logging
interval. At the conclusion of the logging interval, the
North/South and East/West components are averaged and
then re-combined to calculate the average wind direction for
the logging interval.
North Alignment
The sensor must be oriented to true north to obtain meaningful
data. There are two methods to align the sensor:
•
Compass Alignment
•
Global Positioning Satellite (GPS) alignment
Note:
The magnetic declination must be known to align the
direction sensor to true north using a magnetic compass.
Worldwide declination information is available from the
National Space Science Data Center at:
http://nssdc.gsfc.nasa.gov/space/cgm/cgm.html.
Compass Alignment
Tools required:
•
Compass
•
Binoculars
•
Tape (such as electrical, packing, or duct tape)
Two people are required to complete this procedure.
1.
Position the wind vane so that it points in the same
direction as the mounting arm. Secure the vane on the arm
with a piece of tape so that the vane cannot rotate.
2.
While standing 150 to 200 feet south of the sensor, use the
compass to determine magnetic north. If true north is the
same as magnetic north, align yourself so the compass
points north and directly at the sensor. If you are in area
with an east variation, align yourself so that the station is
that number of degrees to the east of magnetic north. If
you are in an area with a west variation, align yourself so
that the station is that number of degrees to the west of
magnetic north.
3.
While viewing the sensor through binoculars, instruct
another person to rotate the arm to point the vane north.
The vane should seem to disappear from sight when
properly aligned.
4.
Once you’ve obtained the correct position, secure the
sensor base and remove the tape.
GPS Alignment
Tools required:
•
Handheld GPS with WAAS-enabled receiver or any
similar high accuracy GPS device
•
Flag, orange cone, or other temporary marker
•
Access to your HOBOlink account
This procedure requires only one person, but is easier to
complete with two people. In this procedure, you will be using
the GPS receiver first to create an arbitrary waypoint and then
to determine the bearing from the sensor to that waypoint. You
will then align the sensor so that when the vane is pointed at
the waypoint, the direction reported by HOBOlink matches the
GPS receiver’s bearing to the waypoint.
1.
In HOBOlink, temporarily set the logging interval to 1 minute
or the fastest logging interval supported by the device
communication plan (if applicable). Refer to the online help
for details on operating HOBOlink.
2.
Pick a visible location that is at least 100 meters (110 yards)
away from the sensor and walk to it. Establish a waypoint
with the handheld GPS receiver. You may want to use
averaging to minimize the waypoint position error if your
GPS receiver is so equipped. (For best results, the estimated
position error of the waypoint should be less than 10 feet if
the distance to the sensor is 100 meters, and less than 20
feet for a distance of 200 meters. Mark the waypoint with a
flag, orange cone, or other suitable marker. Walk back to
the sensor and determine the bearing to the waypoint you
just created with the GPS receiver. Again you may need to
determine the average value of the bearing to keep the
errors to a minimum.