8.1.2 Wind Direction

This section provides guidance for procurement of wind vanes; i.e., mechanical wind direction sensors which rely on the force of the wind to turn a shaft. Guidance for the procurement of remote sensors for the measurement of wind direction is provided in Section 9.

The wind direction measurement with a wind vane is a relative measurement with respect to the orientation of the direction sensor. There are three parts to this measurement which must be considered in quality assurance. These are:

1. the relative accuracy of the vane performance in converting position to output,



2. the orientation of the vane both horizontal (with respect to "true north") and vertical (with respect to a level plane), and



3. the dynamic response of the vane and conditioning circuit to changes in wind direction.

The procurement document should ask for:

1. the starting torque of the vane shaft (with the vane removed) which represents a new bearing (and potentiometer) condition, and



2. the starting torque above which the vane will be out of specification.; when the latter value is exceeded, the bearings should be replaced.

An example performance specification for a wind vane is shown in Table 8-2.

Table 8-2 Example Performance Specification for a Wind Vane
Range	1 to 360 or 540 degrees
Threshold1	0.5 m/s
Accuracy (error)1	3 degrees relative to sensor mount or index 5 degrees absolute error for installed system
Delay Distance 1	5 m at 1.2 kg/m3 (at std sea-level density)
Damping Ratio1	0.4 at 1.2 kg/m3 or
Overshot1	25% at 1.2 kg/m3
1 As determined by wind tunnel test conducted on production samples in accordance with ASTM D-22.11 test methods.8-4

The range of 1 to 540 degrees was originally conceived to minimize strip chart "painting" when the direction varied around 360 degrees. It also minimizes errors (but does not eliminate them) when sigma meters are used. It may also provide a means of avoiding some of the "dead band" errors from a single potentiometer. In these days of "smart" data loggers, it is possible to use a single potentiometer (1 to 360 degrees) system without excessive errors for either average direction or _A.

If the wind direction samples are to be used for the calculation of _A, the specification should also include a time constant requirement for the signal conditioner. Direction samples should be effectively instantaneous. At 5 m/s, a 1m delay distance represents 0.2 seconds. A signal conditioner specification of a time constant of <0.2 seconds would insure that the _A value was not attenuated by an averaging circuit provided for another purpose.

8. QUALITY ASSURANCE AND QUALITY CONTROL
8.1 Instrument Procurement 
     8.1.1 Wind Speed 
     8.1.2 Wind Direction  
     8.1.3 Temperature and Temperature Difference 
     8.1.4 Dew Point Temperature 
     8.1.5 Precipitation 
     8.1.6 Pressure 
     8.1.7 Radiation  
 8.2 Installation and Acceptance Testing
     8.2.1 Wind Speed 
     8.2.2 Wind Direction  
     8.2.3 Temperature and Temperature Difference 
     8.2.4 Dew Point Temperature 
     8.2.5 Precipitation 
     8.2.6 Pressure
     8.2.7 Radiation
  8.3 Routine Calibrations  
      8.3.1 Sensor Check  
      8.3.2 Signal Conditioner and Recorder Check  
     8.3.3 Calibration Data Logs 
     8.3.4 Calibration Report  
     8.3.5 Calibration Schedule/Frequency 
     8.3.6 Data Correction Based on Calibration Results  
  8.4 Audits 
     8.4.1 Audit Schedule and Frequency  
     8.4.2 Audit Procedure 
     8.4.3 Corrective Action and Reporting 
  8.5 Routine and Preventive Maintenance 
     8.5.1 Standard Operating Procedures  
     8.5.2 Preventive Maintenance  
 8.6 Data Validation and Reporting  
     8.6.1 Preparatory Steps 
     8.6.2 Levels of Validation  
     8.6.3 Validation Procedures  
     8.6.4 Schedule and Reporting
  8.7 Recommendations