Apogee Instruments SU-220 Manuale d'uso - Pagina 12
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MAINTENANCE AND RECALIBRATION
Moisture or debris on the sensor is a common cause of low readings. The sensor has a domed-shaped housing for
improved self-cleaning from rainfall, but materials can accumulate on the photo-sensitive area (e.g., dust during
periods of low rainfall, salt deposits from evaporation of sea spray or sprinkler irrigation water) and partially block
the optical path. Dust or organic deposits are best removed using water, or window cleaner and a soft cloth or
cotton swab. Salt deposits should be dissolved with vinegar and removed with a soft cloth or cotton swab. Never
use an abrasive material or cleaner on the sensor.
Although Apogee sensors are very stable, nominal accuracy drift is normal for all research-grade sensors. To
ensure maximum accuracy, we generally recommend sensors are sent in for recalibration every two years,
although you can often wait longer according to your particular tolerances.
The Clear Sky Calculator (www.clearskycalculator.com) determines total shortwave radiation or photosynthetic
photon flux (PPF) incident on a horizontal surface at any time of day at any location in the world. It is most
accurate when used near solar noon in spring and summer months, where accuracy over multiple clear and
unpolluted days is estimated to be ± 4 % in all climates and locations around the world.
Although the Clear Sky Calculator does not specifically report an estimated value for UV radiation, it can still be
used to help determine the need for UV sensor recalibration by approximating the ratio of UV to total shortwave
or the ratio of UV to PPF. However, due to continuous changes in atmospheric conditions and their effect on UV
radiation, the comparison of the UV sensor to the Clear Sky Calculator should only be made in the summer months
near solar noon, and under completely clear skies.
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To calculate a reference value of UV radiation in units of energy flux (W m
), input site conditions into the
calculator to determine the estimated total shortwave radiation. Then multiply the estimated total shortwave
value by an approximated ratio value between 0.05 and 0.06 to convert the total shortwave radiation to total UV
radiation.
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To calculate a reference value of UV radiation in units of photon flux density (μmol m
s
), input site conditions
into the calculator to determine the estimated PPFD. Then multiply the estimated PPFD by an approximated ratio
value between 0.075 and 0.085 to convert the PPFD to total UV radiation.
If UV sensor measurements over multiple days near solar noon are consistently different than calculated values (by
more than 10 %), the sensor should be cleaned and re-leveled. If measurements are still different after a second
test, email
to discuss test results and possible return of sensor(s).