Figure 3-8. Resistance-Temperature Curve Of A Thermistor - National Instruments NI 4350 User Manual

10 M
1 M
100 k
10 k
1 k
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© National Instruments Corporation
commonly used than PTC thermistors, especially for temperature
measurement applications.
A main advantage of thermistors for temperature measurement is their
extremely high sensitivity. For example, a 2252 Ω thermistor has a
sensitivity of –100 Ω/° C at room temperature. Higher resistance
thermistors can exhibit temperature coefficients of –10 kΩ/° C or more.
In comparison, a 100 Ω platinum RTD has a sensitivity of only
0.4 Ω/° C. The small size of the thermistor bead also yields a very
fast response to temperature changes.
Another advantage of the thermistor is its relatively high resistance.
Thermistors are available with base resistances (at 25° C) ranging from
hundreds to millions of ohms. This high resistance diminishes the effect
of inherent resistances in the lead wires, which can cause significant
errors with low resistance devices such as RTDs. For example, while
RTD measurements typically require four-wire or three-wire
connections to reduce errors caused by lead wire resistances, two-wire
connections to thermistors are usually adequate.
The major trade-off for the high resistance and sensitivity of the
thermistor is its highly nonlinear output and relatively limited operating
range. Depending on the type of thermistors, upper ranges are typically
limited to around 300° C. Figure 3-8 shows the resistance-temperature
curve for a 5,000 Ω thermistor. The curve of a 100 Ω RTD is also shown
for comparison.
(5,000 Ω at 25˚ C)
Temperature (˚C)

Figure 3-8. Resistance-Temperature Curve of a Thermistor

Thermistor
(PT 100 Ω at 0˚ C)
3-19
Chapter 3 NI 4350 Operation
RTD
NI 4350 User Manual