Appendix D. Measuring
thermistors and PRTs
Vibrating wire sensors may include an embedded thermistor or PRT (platinum resistance
thermometer).
D.1 Converting PRT resistance to
temperature
The VWIRE 305 analyzer does not support direct PRT measurement conversion to temperature,
but it does measure PRT resistance. You can add lines of code to the CRBasic data logger
program to convert PRT resistance to temperature, or you can convert the resistance as part of a
post-processing operation in a spread sheet or other software. You will need to get PRT
conversion factors from the sensor manufacturer.
D.2 Converting thermistor resistance to
temperature
Convert resistance of most thermistors using the Steinhart-Hart equation as follows:
°C = 1 / (A+B(ln(R)) + C(ln(R))
3
) – 273.15
Where ln(R) is the natural log of the resistance measurement and A, B, and C are Steinhart-Hart
equation coefficients. Each thermistor type has unique coefficients. Sensor manufacturers
provide the coefficients. Our C coefficient is DGSI’s D coefficient.
Example:
The Steinhart-Hart coefficients for the thermistor embedded in a standard Geokon vibrating wire
sensor are as follows:
A = 1.4051E–03
B = 2.369E–04
C = 1.019E–07
The Steinhart-Hart equation to convert thermistor resistance to degrees Celsius is as follows:
°C = 1/ (1.4051E–03 + 2.369E–04(ln(R)) + 1.019E–07(ln(R))
3
) – 273.15
VWIRE 305
55
