8.5.2 Preventive Maintenance
Wind
Speed
The
anemometer has just one mechanical system which will benefit from preventive maintenance. That is the bearing assembly. There
are two strategies from which to choose. One is to change the bearings (or
the entire instrument if a spare is kept for that purpose) on a scheduled
basis and the other is to make the change when torque measurements suggest
change is in order. The former is most conservative with respect to data
quality assuming that any time a torque measurement indicates a bearing
problem, the bearing will be changed as a corrective maintenance action.
As
routine calibrations become less frequent (8.3.5), the probability increases
that a starting torque measurement will be made which indicates the
anemometer is outside its performance specification. This will effect both
the threshold (by increasing it) and the transfer function (by moving the
non-linear threshold toward high speeds). It is unlikely that corrections
can be properly made to the data in this case. The consequence might be the
loss of a half-year's data, if that is the period for routine
calibration. If experience indicates that the anemometer bearing assembly
shows serious wear at the end of one year or two years (based on torque
measurements), a routine change of bearings at that frequency is
recommended.
Wind
Direction
The
wind vane usually has two mechanical systems which will benefit from
preventive maintenance. The bearing assembly is one and can be considered in
the same way as the anemometer bearing assembly described above. The other
is the potentiometer which will certainly "wear out" in time. The
usual mode of failure for a potentiometer is to become noisy for certain
directions and then inoperative. The noisy stage may not be apparent in the
average direction data. If
is calculated, the noise will bias the value toward a higher value. It will
probably not be possible to see early appearance of noise in the
data. When it becomes obvious that the
is too high, some biased data may already have been validated and archived.
Systems with time constant circuits built into the direction output will
both mask the noise from the potentiometer (adding to the apparent
potentiometer life) and bias the
toward a lower value. Such circuits should not be used if they influence the
actual output capability of the sensor. Each manufacturer may be different
in their selection of a source and specifications used in buying
potentiometers. The operator needs to get an expected life for the
potentiometer from the manufacturer and monitor the real life with a noise
sensitive test. An oscilloscope is best and can be used without disrupting
the measurement. When potentiometer life expectations have been established,
a preventive maintenance replacement on a conservative time basis is
recommended.
Temperature
and Temperature Difference
Aspirated
radiation shields use fans which will also fail in time. The period of this
failure should be several years. The temperature error resulting from this
failure will be easily detected by a QC meteorologist inspecting the data.
Some aspirated radiation shields include an air flow monitoring device or a
current check which will immediately signal a disruption in aspiration.
Preventive maintenance is not required but spare fans should be on the shelf
so that a change can be made quickly when failure does occur.
Dew
Point Temperature
Field
calibration checks of the dew point temperature measurement system can be
made with a high-quality Assmann-type or portable, motor-aspirated
psychrometer. Sling psychrometers should
not be used. Several readings should be taken at the intake of the aspirator
or shield at night or under cloudy conditions during the day. These field
checks should be made at least monthly, or in accordance with manufacturer's
suggestions, and should cover a range of relative humidity values.
Periodically
(at least quarterly) the lithium chloride in dew cells should be removed and
recharged with a fresh solution. The sensor should be field-checked as
described above before and at least an hour after the lithium chloride
solution replacement.
If
cooled-mirror type dew point systems are used, follow the manufacturer's
service suggestions initially. The quality of the data from this method of
measurement is dependent upon the mirror being kept clean. The frequency of
service required to keep the mirror clean is a function of the environment
in which the sensor is installed. That environment may vary with seasons or
external weather conditions. If changes in dew point temperature of a
magnitude larger than can be tolerated are found after service scheduled
according to the manufacturer's suggestion, increase the service frequency
until the cleaning becomes preventive maintenance rather than corrective
service. This period will vary and can be defined only by experience.
Station log data must include the "as found" and the "as
left" measurements. Dew point temperature does not change rapidly (in
the absence of local sources of water) and the difference between the two
measurements will usually be the instrument error due to a dirty mirror.
Precipitation
The
gauge should be inspected at regular intervals using a bubble level to
see that the instrument base is mounted level. Also, the bubble level
should be placed across the funnel orifice to see that it is level. The wind
screen should also be checked to see that it is level, and that it is
located l/2 inch above the level of the orifice, with the orifice centered
within the screen.
Pressure
The
output of the pressure sensor should be regularly checked against a
collocated instrument. A precision aneroid barometer can be used for this
check. The collocated barometer should be occasionally checked against a
mercurial barometer reading at a nearby NWS station.
Radiation
The
optical hemispheres on pyranometers and net radiometers should be cleaned
frequently (preferably daily) with a soft, lint-free cloth. The surfaces of
the hemispheres should be regularly inspected for scratches or cracks. The
detectors should be regularly inspected for any discoloration or
deformation. The instruments should be inspected during cool temperatures
for any condensation which may form on the interior of the optical surfaces.
While
calibrations must be done by the manufacturer, radiation can be
field-checked using a recently-calibrated, collocated instrument. Since
signal processing is particularly critical for these sensors, the collocated
instrument should also use its own signal conditioner and data recording
system for the check. This kind of field check should be done every six
months. It is mandatory to log "as found" and "as
left" information about the parts of the system which seem to require
work. Without this information it becomes difficult, if not impossible, to
assess what data are usable and what are not.
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