14
MAINTENANCE AND RECALIBRATION
Moisture or debris on the diffuser is a common cause of low readings. The sensor has a domed diffuser and housing for
improved self-cleaning from rainfall, 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.
The Clear Sky Calculator (
www.clearskycalculator.com
) can be used to determine the need for quantum meter recalibration.
It determines PPFD 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 PPFD can exceed values predicted by the Clear Sky Calculator due to reflection from the sides and edges of clouds.
This reflection increases the incoming 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 PPFD measurements to calculated
PPFD values for a clear sky. If sensor PPFD 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 PPFD measurements are still different
after a second test, email
to discuss test results and possible return of meter(s) for
recalibration.
Homepage of the Clear Sky Calculator. Two calculators are
available: one for quantum sensors (PPFD) and one for
pyranometers (total shortwave radiation).
Clear Sky Calculator for quantum sensors. Site data are input
in blue cells in middle of page and an estimate of PPFD is
returned on right-hand side of page.
