Dear Customer,
Apogee Instruments is committed to working with you to help
make accurate measurements of solar radiation. Several
factors contribute to long-term accuracy of radiation
sensors, including vertical orientation, occlusion of the
lens, obstruction of the field of view, errors in datalogger
programming, and drift due to aging. Under normal
conditions, Apogee sensors drift less than 2% per year;
however, we have found that in some conditions the diffusion
disk can separate from the detector and cause an abrupt
decrease in sensor output of 20 to 25%. Disk separation can
be diagnosed by 1) searching time series data for evidence
of an abrupt decrease in sensor output; and 2) by comparing
solar noon data to the clear sky solar calculation as
described below. Apogee will replace or repair sensors with
separated disks. After rigorous environmental testing, we
have eliminated this problem by switching to an epoxy
adhesive that maintains its integrity in all environments.
Our data show that disk separation has affected less than
0.4% of Apogee sensors, most of which have been from hot,
humid climates, so your sensor(s) may be fine.
You can determine if your sensor needs to be returned to
Apogee by typing conditions into the clear sky model on the
web at
www.clearskycalculator.com
and
comparing measured values with the calculated value for a
clear sky. If the output of your sensor over multiple days
at solar noon is consistently less than the model value (by
more than 8%), clean and re-level your sensor and run the
test again.
If the output is still low, contact us at
calibration@apogee-inst.com to discuss test results and the
possible return of sensors. When used near solar noon over
multiple clear, unpolluted, days during the spring and
summer months, we estimate that the accuracy of the model
can be ± 4% in all climates and locations around the world.
We greatly appreciate your business and hope to quickly
identify and repair the sensors that have experienced a
rapid decrease in output. We ask that sensors be evaluated
before 21 September 2009. We look forward to working with
you to maximize the long-term accuracy of radiation sensors.
Sincerely,
Bruce Bugbee
President
Apogee Instruments, Inc.
