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.
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