Apogee Instruments SIF-121-SS Manuale d'uso - Pagina 17
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TROUBLESHOOTING AND CUSTOMER SUPPORT
Independent Verification of Functionality
The radiation detector in Apogee SIF-100 series infrared radiometers is a self-powered device that outputs a
voltage signal proportional to the radiation balance between the detector and target surface. A quick and easy
check of detector functionality can be accomplished using a voltmeter with microvolt (µV) resolution. Connect the
positive lead of the voltmeter to the white wire from the sensor and the negative lead (or common) to the black
wire from the sensor. Direct the sensor toward a surface with a temperature significantly different than the
detector. The µV signal will be negative if the surface is colder than the detector and positive if the surface is
warmer than the detector. Placing a piece of tinfoil in front of the sensor should force the sensor µV signal to zero.
The thermistor inside Apogee SIF-100 series radiometers yields a resistance proportional to temperature. A quick
and easy check of thermistor functionality can be accomplished with an ohmmeter. Connect the lead wires of the
ohmmeter to the red and green wires from the sensor. The resistance should read 10 kΩ at 25 C. If the sensor
temperature is less than 25 C, the resistance will be higher. If the sensor temperature is greater than 25 C, the
resistance will be lower. Connect the lead wires of the ohmmeter to the green and blue wires from the sensor. The
resistance should read 24.9 kΩ, and should not vary. Connect the lead wires of the ohmmeter to the red and blue
wires from the sensor. The resistance should be the sum of the resistances measured across the red and green
wires, and green and blue wires (e.g., 10 kΩ plus 24.9 kΩ at 25 C).
Compatible Measurement Devices (Dataloggers/Controllers/Meters)
SIF-100 series radiometers have sensitivities in the microvolt range, approximately 5 to 15 µV per C difference
between target and detector (depending on specific model). Thus, a compatible measurement device (e.g.,
datalogger or controller) should have resolution of at least 0.75 µV (0.00075 mV), in order to produce temperature
resolution of 0.05 C.
Measurement of detector temperature from the internal thermistor requires an input excitation voltage, where
2500 mV is recommended. A compatible measurement device should have the capability to supply the necessary
voltage. An example datalogger program for Campbell Scientific dataloggers can be found on the Apogee webpage
at
http://www.apogeeinstruments.com/content/Infrared-Radiometer-Analog.CR1.
Modifying 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 measurement device is 10 MΩ or higher, there is negligible effect on the radiometer calibration,
even after adding up to 50 m of cable. Apogee model SIF series infrared radiometers use shielded, twisted pair
cable, which minimizes electromagnetic interference. This is particularly important for long lead lengths in
electromagnetically noisy environments. See Apogee webpage for details on how to extend sensor cable length
(http://www.apogeeinstruments.com/how-to-make-a-weatherproof-cable-splice/).
Signal Interference
Due to the small voltage signals from the detector, care should be taken to provide appropriate grounding for the
sensor and cable shield wire, in order to minimize the influence of electromagnetic interference (EMI). In instances
where SIF-100 series radiometers are being used in close proximity to communications (near an antenna or
antenna wiring), it may be necessary to alternate the data recording and data transmitting functions (i.e.,
measurements should not be made when data are being transmitted wirelessly). If EMI is suspected, place a tinfoil
cap over the front of the sensor and monitor the signal voltage from the detector. The signal voltage should
remain stable at (or very near) zero.