MAINTENANCE AND RECALIBRATION
Moisture or debris on the diffuser is a common cause of low readings. The meter has a domed diffuser and housing
for improved self-cleaning from rainfall/irrigation, but materials can accumulate on the diffuser (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 diffuser.
Although Apogee meters are very stable, nominal accuracy drift is normal for all research-grade instruments. To
ensure maximum accuracy, we generally recommend meters are sent in for recalibration every two years,
although you can often wait longer according to your particular tolerances.
To determine if your meter needs recalibration, the Clear Sky Calculator (
) website
and/or smartphone app can be used to indicate the PPFD and ePPFD 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
PPFD to increase above the value predicted by the Clear Sky Calculator. Measured values of PPFD can exceed
values predicted by the Clear Sky Calculator due to reflection from high, thin clouds and edges of clouds, which
enhances incoming PPFD. 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 PPFD measurements to
calculated values for a clear sky. If meter PPFD measurements over multiple days near solar noon are consistently
different than calculated values (by more than 6 %), the meter 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).
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