TROUBLESHOOTING AND CUSTOMER SUPPORT
Independent Verification of Functionality
Apogee ST Series thermistor temperature sensors yield a resistance proportional to temperature. A quick and easy
check of thermistor/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 k
Ω
(10,000 ohms) at 25 C. If
sensor temperature is less than 25 C, resistance will be higher. If sensor 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 sensor.
The resistance should read 24.9 k
Ω
, 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 resistances measured across the red and white
wires, and white and black wires (e.g., 10 k
Ω
plus 24.9 k
Ω
at 25 C).
Apogee ST-300 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
blue/yellow and green/black wires from the sensor.
The resistance should read 100Ω
(0.1 k
Ω
) at 0 C. If the sensor
temperature is less than 0 C, the resistance will be lower. If the sensor temperature is greater than 0 C, the
resistance will be higher. The blue and yellow wires are connected internally, without a resistor between them;
continuity between blue and yellow wires indicates both wires are functional. Similarly, the green and black wires
are connected internally, without a resistor between them; continuity between green and black wires indicates
both wires are functional. Connect the lead wires of the ohmmeter to the green/black and white wires from the
sensor. The resistance should read 100
Ω
, and should not vary. Connect the lead wires of the ohmmeter to the
white and red wires from the sensor. The resistance should read 10 k
Ω
, and should not vary. Connect the lead
wires of the ohmmeter to the blue/yellow and red wires from the sensor. The resistance should be the sum of the
resistances measured across the blue/yellow and green/black wires, green/black and white wires, and white and
red wires (e.g., 100
Ω
plus 100
Ω
plus 10 k
Ω
at 0 C).
Compatible Measurement Devices (Dataloggers/Controllers/Meters)
Measurement of thermistor resistance requires an input excitation voltage, where 2.5 V DC is recommended.
Measurement of ST-300 PRT resistance also requires an input excitation voltage, where 2.1 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 measurement 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, -60 and 80 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).
The sensitivity (mV output from PRT per C) of the temperature measurement from the ST-300 PRT is
approximately constant across the entire measurement range. With an excitation voltage of 2.1 V DC, a compatible
measurement device should have resolution of at least 0.008 mV, in order to produce temperature resolution of
less than 0.1 C across the entire temperature measurement range.
An example datalogger program for Campbell Scientific dataloggers can be found on the Apogee webpage at
http://www.apogeeinstruments.com/content/Thermistor-Temperature-Sensor.CR1
