METER ATMOS 41 - 4. SERVICE; 4.1 CALIBRATION

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ATMOS 41
4. SERVICE
This section contains calibration and recalibration information, calibration frequencies,
cleaning and maintenance guidelines, troubleshooting guidelines, customer support contact
information, and terms and conditions.
4.1 CALIBRATION
• Solar Radiation: ATMOS41 pyranometers are calibrated through side-by-side comparison
to the mean of four Apogee model SP-110 transfer standard pyranometers (shortwave
radiation reference) under high-intensity discharge metal halide lamps. The transfer
standard pyranometers are calibrated through side-by-side comparison to the mean of
at least two ISO-classified reference pyranometers under sunlight (clear sky conditions)
in Logan, Utah. Each of four ISO-classified reference pyranometers are recalibrated on an
alternating yearly schedule (two instruments each year) at the National Renewable Energy
Laboratory (NREL) in Golden, Colorado. NREL reference standards are calibrated to the
World Radiometric Reference (WRR) in Davos, Switzerland.
• Precipitation: ATMOS41 rain gauges do not receive individual calibration. The critical
parameter for accurate measurement is drop size (which is controlled by the geometry
of the rain gauge nozzle), and the rain gauge nozzle is an injection molded piece with
static geometry. The accuracy of each ATMOS 41 rain gauge is verified at a single rainfall
rate during the manufacturing and quality assurance (QA) process. A subsample of rain
gauges undergoes a more in-depth accuracy verification each month as part of routine
QAprocedures.
• Air Temperature: The air temperature sensor is specified to be accurate to 0.1 °C by
the original manufacturer. All sensors are verified against a NIST-traceable standard
thermometer at the time of manufacture.
• RH/Vapor Pressure: ATMOS41 relative humidity/vapor pressure sensors receive a three-point
calibration using a METER WP4C chilled-mirror dew point sensor as the calibration standard
at 20%, 50%, and 80% RH. The WP4C is calibrated every 3 months against four salt standards
of known RH (Robinson and Stokes 1970) to be accurate to within 0.2% RH.
• Barometric Pressure: Each ATMOS41 barometric pressure sensor is calibrated against the
average of three secondary standard Measurement Specialties MS5611 pressure sensors.
The MS5611 sensors are calibrated every 3 months against a GE Druck TERPS 8100-series
pressure sensor. The GE pressure sensor is calibrated yearly at certified laboratory with
NIST traceability.
• Wind Speed: The relationship between wind speed and phase is determined by geometry
and the laws of physics. Since the geometry is tightly controlled in manufacture, no
individual anemometer calibration is needed. The phase of each ATMOS41 anemometer
is initially set to zero in a zero wind-speed condition. Extensive wind-tunnel and field
testing has shown this to result in accurate wind-speed measurements. A subsample of
anemometers is checked monthly for accuracy as part of routine QA procedures.