Apogee Instruments SF-110 Benutzerhandbuch - Seite 15
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Independent Verification of Functionality
Apogee SF-110 radiation frost detectors yield an independent resistance that is proportional to simulated leaf
temperature. A quick and easy check of functionality of the thermistor can be accomplished with an ohmmeter.
To check the simulated leaf circuit, connect the lead wires of the ohmmeter to the red and white wires from the
detector. The resistance should read 10 kΩ (10,000 ohms) at 25 C. If detector temperature is less than 25 C,
resistance will be higher. If detector temperature is greater than 25 C, resistance will be lower. Connect the lead
wires of the ohmmeter to the white and black wires from the detector. The resistance should read 24.9 kΩ and
should not vary. Connect the lead wires of the ohmmeter to the red and black wires from the sensor. The
resistance should be the sum of the resistances measured across the red and white wires, and white and black
wires (e.g., 10 kΩ plus 24.9 kΩ at 25 C).
Compatible Measurement Devices (Dataloggers/Controllers/Meters)
Measurement of thermistor resistance requires an input excitation voltage, where 2.5 V DC is recommended. A
compatible measurement device should have the capability to supply the necessary voltage.
The sensitivity (mV output from thermistor per C) of the temperature measurements varies with the excitation
voltage and varies as a function of temperature. With an excitation voltage of 2.5 V DC, the sensitivity is lowest
near the ends of the measurement range, -50 and 70 C. A compatible measurement device (e.g., datalogger or
controller) should have resolution of at least 0.6 mV, in order to produce temperature resolution of less than 0.1 C
across the entire temperature measurement range (less than 0.05 C from -35 to 45 C).
An example datalogger program for Campbell Scientific dataloggers can be found on the Apogee webpage at
http://www.apogeeinstruments.com/content/Radiation-Frost-Detection-Sensor.CR1.
Modifying Cable Length
When the detector is connected to a measurement device with high input impedance, detector output signals are
not changed by splicing on additional cable in the field. Tests have shown that if the input impedance of the
measurements device is 1 mega-ohm or higher then there is negligible effect on the SF-110 leaf and bud
temperature sensor, even after adding up to 100 m of cable. See Apogee webpage for details on how to extend
sensor cable length (http://www.apogeeinstruments.com/how-to-make-a-weatherproof-cable-splice/). For cable
extensions, shielded, twisted pair cable is recommended, in order to minimize electromagnetic interference. This is
particularly important for long lead lengths in electromagnetically noisy environments.
The precision bridge resistors are located at the pigtail end of the cable. Thus, the SF-110 cable should not be
shortened, otherwise the bridge resistor will be removed.