Apogee SP-214 Manuale d'uso - Pagina 17
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TROUBLESHOOTING AND CUSTOMER SUPPORT
Independent Verification of Functionality
Apogee model SP-214 pyranometers provide a 4-20 mA output that is proportional to incident total shortwave
radiation. 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)
-2
SP-214 pyranometers are calibrated with a standard calibration factor of 125 W m
per mA, yielding a sensitivity of
-2
0.008 mA per W m
. Thus, a compatible measurement device (e.g., datalogger or controller) should have
-2
resolution of at least 0.008 mA, in order to provide shortwave radiation resolution of 1 W m
.
The 4-20 mA circuit design allows the output to drive a resistive load (R
) to within 2 volts of the supply voltage to
L
the sensor (V
), at 20 mA (0.02 A). The equation to calculate resistive load is R
= [V
– 2 V] / 0.02 A. For example, a
S
L
S
sensor with a supply voltage of 12 V DC can drive a maximum load of 500 Ω (R
= [12 V – 2 V] / 0.02 A = 500 Ω). The
L
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 SP-
214. 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 SP series pyranometers 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/).