TROUBLESHOOTING AND CUSTOMER SUPPORT
Independent Verification of Functionality
Apogee ST-110 temperature sensors yield a resistance proportional to temperature. A quick and easy check of
sensor functionality can be accomplished with an ohmmeter. Connect the lead wires of the ohmmeter to the red
and white wires from the sensor. The resistance should read 10 kΩ (10,000 ohms) 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 white and black 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
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).
A quick and easy check of fan functionality can be determined using a DC power supply. Power the fan with 12 V
DC by connecting the positive voltage signal to the red wire from the fan and the negative (or common) to the
black wire from the fan. When powered, the fan blades should spin. Make sure to keep fingers clear of blades
before connecting fan to 12 V DC.
EE08 probes output voltage signals linearly proportional to air temperature and relative humidity. A quick and easy
check of probe functionality can be accomplished with a voltmeter and 9 V battery. Connect the red wire from the
probe to the positive terminal on the 9 V battery, and connect the black wire from the probe to the negative
terminal on the 9 V battery. Connect the positive lead wire of the voltmeter to the green wire from the probe and
the negative lead wire of the voltmeter to the black wire from the probe. The voltage measurement should be 1.25
V DC at 20 C and 1.350 V DC at 25 C (20 to 25 C is the approximate room temperature range). Move the positive
lead wire of the voltmeter to the white wire from the probe. The voltage measurement should be 0.5 V DC at 20 %,
1.25 V DC at 50 %, and 2.0 V DC at 80 %.
Compatible Measurement Devices (Dataloggers/Controllers/Meters)
Measurement of ST-110 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 for
the specific sensor.
The sensitivity (mV output from thermistor per C) of the temperature measurement from the ST-110 thermistor
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).
To operate the fan, a 12 V DC power supply capable of supplying at least 80 mA is required. All fans also include
pulse width modulation (PWM; allows for reduction of fan speed under certain conditions) and a tachometer
(allows for monitoring of fan speed). In order to use the PWM option, the fan must be connected to a datalogger
or controller that has a PWM output of approximately 20 kHz, a duty cycle of 50 to 100 %, and a 0 to 5 V DC square
waveform. To measure the output from the tachometer, a pulse counter is required. The maximum voltage input
from the tachometer to the pulse counter is determined by the input voltage to the tachometer, which should be
fixed at a given voltage in the range of 0 to 30 V DC. The measurement device should be capable of supplying a
fixed voltage in this range.
An example datalogger program for Campbell Scientific dataloggers can be found on the Apogee webpage at
http://www.apogeeinstruments.com/content/Aspirated-Radiation-Shield-Thermistor.cr1