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8.3 Routine Calibrations

A calibration involves measuring the conformance to or discrepancy from a specification for an instrument and an adjustment of the instrument to conform to the specification.

Documentation of all calibrations should include a description of the system "as found", details of any adjustments to the instrument, and a description of the system "as left"; this documentation is a vital part of the "paper-trail" for any claims of data validity. Calibrations are often confused with performance audits since both involve measuring the conformance of an instrument to a specification; the main difference has to do with the independence of the person performing the audit or calibration - the performance audit should be conducted by a person who is independent of the operating organization - calibrations, on the other hand, are often performed by individuals within the operating organization. Guidance specific to performance audits is provided in Section 8.4.

The guidance provided on calibration procedures in the following applies to in situ meteorological sensors such as would be mounted on a tower (e.g., wind vanes  and anemometers) or located at ground level (e.g., a solar radiation sensor). Ideally, a calibration should be performed in an environment as close as possible to laboratory bench-test as conditions allow. For tower mounted sensors this usually involves removing the sensor from tower. The alternative to a bench-test calibration of the in situ sensor is a calibration using a collocated transfer standard; this involves locating an identical standard instrument as close as practical to the instrument being calibrated. The collocated standard transfer method is the most complete calibration/audit method from the standpoint of assessing total system error. However it has two serious drawbacks: 1) it is limited to the conditions that prevailed during the calibration/audit, and 2) it is sensitive to siting and exposure bias.

Calibrations using a bench test or collocated transfer standard are not generally applicable to the upper-air measurement systems; the special procedures required for calibrations and audits of upper-air measurement systems are discussed in Section 9.

Documentation supplied with newly purchased instruments should include the manufacturer's recommended calibration procedures. The guidance on calibration procedures provided in the following is intended to supplement the manufacturer's recommendations; when in doubt, the instrument manufacturer should be consulted.

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

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