Apogee Instruments SQ-624-SS Посібник користувача - Сторінка 13
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TROUBLESHOOTING AND CUSTOMER SUPPORT
Independent Verification of Functionality
Apogee SQ-624 Extended Range PFD Sensors provide a 4-20 mA output that is proportional to incident photon flux
density for the 340-1040 nm wavelength range. 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 green 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)
-2
-1
SQ-624 Extended Range PFD Sensors are calibrated with a standard calibration factor of 250 µmol m
s
per mA,
-2
-1
yielding a sensitivity of 0.004 mA per µmol m
s
. Thus, a compatible measurement device (e.g., datalogger or
controller) should have resolution of at least 0.004 mA in order to provide photon flux density resolution of 1 µmol
-2
-1
m
s
.
The 4-20 mA circuit design allows the output to drive a resistive load (RL) to within 2 volts of the supply voltage to
the sensor (VS), at 20 mA (0.02 A). The equation to calculate resistive load is RL = [VS – 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 (RL = [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.
An example datalogger program for Campbell Scientific dataloggers can be found on the Apogee webpage at
https://www.apogeeinstruments.com/downloads/#datalogger.
Cable Length
Shortening or splicing on additional cable in the field is generally not a problem for the current output of the SQ-
624. 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). 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/).