MAINTENANCE AND RECALIBRATION
Moisture or debris on the filters is a common cause of errors. The upward-looking sensor has a domed housing for
improved self-cleaning from rainfall, but materials can accumulate on the diffuser or window (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. Materials can also accumulate on the downward-looking detector. 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
diffuser.
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.
Upward-Looking Pyranometer (Shortwave Radiation)
The Clear Sky Calculator (
) can be used to determine the need for pyranometer
recalibration. It determines total shortwave radiation 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. For best accuracy, the sky must be completely clear, as reflected radiation from clouds causes incoming
radiation to increase above the value predicted by the clear sky calculator. Measured values of total shortwave
radiation can exceed values predicted by the Clear Sky Calculator due to reflection from thin, high clouds and
edges of clouds, which enhances incoming shortwave radiation. The influence of high clouds typically shows up as
spikes above clear sky values, not a constant offset greater than clear sky values.
To determine recalibration need, input site conditions into the calculator and compare total shortwave radiation
measurements to calculated values for a clear sky. If sensor shortwave radiation measurements over multiple days
near solar noon are consistently different than calculated values (by more than 6 %), 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).
