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. 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