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5 SYSTEM PERFORMANCE

5.1 System Accuracies

Accuracy is the amount by which a measured variable deviates from a value accepted as true or standard. Accuracy can be thought of in terms of individual component accuracy or overall system accuracy. For example, the overall accuracy of a wind speed measurement system includes the individual component accuracies of the cup or propeller anemometer, signal conditioner, analog-to-digital converter, and data recorder.

The accuracy of a measurement system can be estimated if the accuracies of the individual components are known. The system accuracy would be the square root of the sum of the squares of the random component accuracies [17]. The accuracies recommended for meteorological monitoring systems are listed in Table 5-1. These are stated in terms of overall system accuracies, since it is the data from the measurement system which are used in air quality modeling analyses. Recommended measurement resolutions, i.e., the smallest increments that can be distinguished, are also provided in Table 5-1. These resolutions are considered necessary to maintain the recommended accuracies, and are also required in the case of wind speed and wind direction for computations of standard deviations.

Table 5-1
Recommended System Accuracies and Resolutions
Meteorological 
Variable
System
Accuracy
Measurement
Resolution
Wind Speed
(horizontal and vertical)
(0.2 m/s + 5% of observed) 0.1 m/s
Wind Direction
(azimuth and elevation)
5 degrees 1.0 degree
Ambient Temperature 0.5 C 0.1 C
Vertical Temperature 
Difference
0.1 C 0.02 C
Dew Point Temperature 1.5 C 0.1 C
Precipitation 10% of observed or 5 mm 0.3 mm
Pressure 3 mb (0.3kPa) 0.5 mb
Solar Radiation 5% of observed 10 W/m2

The recommendations provided in Table 5-1 are applicable to microprocessor-based digital systems (the primary measurement system). For analog systems, used as backup, these recommendations may be relaxed by 50 percent. The averaging times associated with the recommended accuracies correspond to the averaging times associated with the end use of the data (nominally, 1-hour averaging for regulatory modeling applications) and with the audit methods recommended to evaluate system accuracies.

5. SYSTEM PERFORMANCE
 5.1 System Accuracies
 5.2 Response Characteristics of Meteorological Sensors
 
5.3 Data Recovery
      5.3.1 Length of Record
      5.3.2 Completeness Requirement
  5.4 Recommendations


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