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TROUBLESHOOTING AND CUSTOMER SUPPORT
Independent Verification of Functionality
Apogee S2 series NDVI sensors are self-powered devices and output a voltage signal proportional to irradiance
(upward-looking sensors) and radiance (downward-looking sensors). A quick and easy check of sensor functionality
can be determined using a voltmeter with millivolt resolution. Connect the positive lead wire from the voltmeter
to the yellow wire from the sensor and the negative (or common) lead wire from the voltmeter to the blue wire
from the sensor. Direct the sensor head toward a light source and verify the sensor provides a signal. Increase and
decrease the distance from the sensor head to the light source to verify that the signal changes proportionally
(decreasing signal with increasing distance and increasing signal with decreasing distance). Blocking all radiation
from the sensor should force the sensor signal to zero. Repeat the process by connecting the positive lead wire
from the voltmeter to the white wire from the sensor and the negative (or common) lead wire from the voltmeter
to the black wire from the sensor.
Compatible Measurement Devices (Dataloggers/Controllers/Meters)
S2-100 series NDVI sensors have small sensitivities, outputting a few mV per W m⁻². Typical irradiance from the
-2
sun on a clear day is between 1-2 W m
at the center wavelengths measured by the sensors. It is recommended
that the measurement device (e.g., datalogger or controller) have resolution of at least 0.001 mV.
Example datalogger programs for Campbell Scientific dataloggers can be found on the Apogee webpage at:
https://www.apogeeinstruments.com/content/NDVI-Analog.CR1
Cable Length
When the sensor is connected to a measurement device with high input impedance, sensor output signals are not
changed by shortening the cable or splicing on additional cable in the field. Tests have shown that if the input
impedance of the measurements device is greater than 1 mega-ohm there is negligible effect on the calibration,
even after adding up to 100 m of cable. All Apogee sensors use shielded, twisted pair cable to minimize
electromagnetic interference. For best measurements, the shield wire must be connected to an earth ground. This
is particularly important when using the sensor with long lead lengths in electromagnetically noisy environments.
Modifying Cable Length
See Apogee webpage for details on how to extend sensor cable length:
(http://www.apogeeinstruments.com/how-to-make-a-weatherproof-cable-splice/).