Resistance-Temperature Characteristic Of Thermistors - National Instruments NI 4350 User Manual

Chapter 3 NI 4350 Operation
Connecting Your Thermistor
NI 4350 User Manual
The thermistor has been used primarily for high-resolution
measurements over limited temperature ranges. Continuous
improvements in thermistor stability, accuracy, and the availability
of interchangeable thermistors have prompted increased usage of
thermistors in all types of industries.
Resistance-Temperature Characteristic of
Thermistors
The resistance-temperature behavior of thermistors is highly dependent
upon the manufacturing process. Therefore, thermistor manufacturers
have not standardized thermistor curves to the extent that thermocouple
or RTD curves have been standardized.
Typically, thermistor manufacturers supply the resistance-versus-
temperature curves or tables for their particular devices. The thermistor
curve, however, can be approximated relatively accurately with the
Steinhart-Hart equation:
( )
T K = ----------------------------------------------
•
a + b
Where T(K) is the temperature in kelvin, equal to T(° C) + 273.15, and
Rt is the resistance of the thermistor. The coefficients a, b, and c can be
provided by the thermistor manufacturer, or calculated from the
resistance-versus-temperature curve.
Software packages such as LabVIEW and LabWindows/CVI include
routines that perform these conversions for some types of thermistors.
You can also modify these conversion routines for your particular type
of thermistor.
Because the thermistor is a resistive device, you must pass a current
through the thermistor to produce a voltage that can be measured by the
NI 4350 instrument. The high resistance and high sensitivity of the
thermistor simplify the necessary measurement circuitry and signal
conditioning. Special three-wire, four-wire, or Wheatstone bridge
connections are not necessary. As shown in Figure 3-9, the measured
voltage Vt will be equal to (Rt • IEX).
1
3
ln Rt c ln Rt
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