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ATMOS 41W
• A custom pyranometer calibration factor can be calculated using the Clear Sky Calculator.
Neither Apogee Instruments nor METER guarantee this method is as accurate as a new unit
that has recently been calibrated, but, if done properly, it should result in better than 5%
accuracy in the solar radiation measurements.
•
Precipitation: The drop counting portion of ATMOS 41W 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.
Individual calibration is performed on the tipping spoon portion of the ATMOS 41W
rain gauge. Each spoon is calibrated to tip at the equivalent of precisely 0.34 mm of
rainfall during the production process.
The functionality of both ATMOS 41W rain gauge measurements 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 to be within 0.2 °C at the time of manufacture.
•
Vapor Pressure and RH: ATMOS 41W vapor pressure and relative humidity 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 41W 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 a 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 41W 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.
