User Manual

TROUBLESHOOTING AND CUSTOMER SUPPORT
Independent Verification of Functionality
Apogee models SQ-214 and SQ-224 quantum sensors provide a 4-20 mA output that is proportional to incident
PPFD. A quick and easy check of sensor functionality can be determined using a DC power supply and an ammeter.
Power the sensor with a DC voltage by connecting the positive voltage signal to the red wire from the sensor and
the negative (or common) to the black wire from the sensor. Use the ammeter to measure across the white wire
(signal output) and black wire (signal ground). 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 4 mA.
Compatible Measurement Devices (Dataloggers/Controllers/Meters)
SQ-214 and SQ-224 quantum sensors are calibrated with a standard calibration factor of 250 µmol m
-2
s
-1
per mA,
yielding a sensitivity of 0.004 mA per µmol m
-2
s
-1
. Thus, a compatible measurement device (e.g., datalogger or
controller) should have resolution of at least 0.004 mA, in order to provide PPFD resolution of 1 µmol m
-2
s
-1
.
The 4-20 mA circuit design allows the output to drive a resistive load (R
L
) to within 2 volts of the supply voltage to
the sensor (V
S
), at 20 mA (0.02 A). The equation to calculate resistive load is R
L
= [V
S
– 2 V] / 0.02 A. For example, a
sensor with a supply voltage of 12 V DC can drive a maximum load of 500 Ω (R
L
= [12 V – 2 V] / 0.02 A = 500 Ω). The
output voltage from the sensor is calculated by adding the wire resistance to the input resistance of the data
collection system, and then multiplying by 0.02 A.
Effect of Cable Length
Shortening or splicing on additional cable in the field is generally not a problem for the current output of the SQ-
214 and SQ-224. However, adding cable will result in a greater resistive load, which should be taken into
consideration when determining the maximum resistive load that the sensor will drive (see section above on
Compatible Measurement Devices). Apogee model SQ series quantum sensors use shielded, twisted pair cable,
which minimizes electromagnetic interference. This is particularly important for 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/).
Unit Conversion Charts
Apogee SQ series quantum sensors are calibrated to measure PPFD in units of µmol m
-2
s
-1
. Units other than
photon flux density (e.g., energy flux density, illuminance) may be required for certain applications. It is possible to
convert the PPFD value from a quantum sensor to other units, but it requires spectral output of the radiation
source of interest. Conversion factors for common radiation sources can be found on the Unit Conversions page of
the Support Center on the Apogee website (http://www.apogeeinstruments.com/unit-conversions/). A
spreadsheet to convert PPFD to energy flux density or illuminance is also provided on the Unit Conversions page of
the Support Center on the Apogee website (http://www.apogeeinstruments.com/content/PPFD-to-Illuminance-
Calculator.xls).