Campbell AVW200 series, Instruction Manual

Page: 67 / 112

Appendix B. Thermistor Information
B.1 Converting Resistance to Temperature
The AVW200 outputs a resistance value for sensors that contain a thermistor.
Temperature is calculated by applying the resistance to a known equation (e.g.,
Steinhart-Hart equation) which converts resistance to temperature.
The Steinhart-Hart equation for converting resistance to degree Celsius is as
follows:
Temperature = 1/(A + B*LN(resistance) + C*(LN(resistance))^3) - 273.15
Where A, B, and C are coefficients for the Steinhart-Hart equation.
The coefficients for the Steinhart-Hart equation are specific to the thermistor
contained in your sensor and are obtained from the sensor manufacturer.
Please see your manufacturer to get the coefficients for their
thermistor. NOTE
B.1.1 Resistance Conversion Example – Geokon Sensor
If the coefficients for Steinhart-Hart equation are as follows
A=0.0014051
B=0.0002369
C=0.0000001019
The equation for converting the resistance measurement to degrees Celsius is:
Temperature = 1/(0.0 0.0002369*LN(resistance) +
0.0000001019*(LN(resistance))^3) - 273.15
If the measured resistance is 2221 ohms, the calculated temperature in degree
Celsius is:
Temperature = 1/(0.0 0.0002369*LN(2221) +
0.0000001019*(LN(2221))^3) - 273.15
Temperature = 31.98°C
B.2 Accuracy and Resolution
The accuracy of the temperature measurement is a function of the following
factors:
1. Thermistor's interchangeability
2. Resistance of the wire
3. Steinhart-Hart Equation error
B-1