Chapter 2
Operating the NI 435x Device
©
National Instruments Corporation
2-19
A main advantage of thermistors for temperature measurement is their high
sensitivity. For example, a 2,252
Ω
thermistor has a sensitivity of
–100
Ω
/°C at room temperature. Higher resistance thermistors can exhibit
temperature coefficients of –10 k
Ω
/°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 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 4-wire or 3-wire connections to reduce
errors caused by lead wire resistances, 2-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 2-8 shows the resistance-temperature curve for a
5,000
Ω
thermistor. The curve of a 100
Ω
RTD also is shown for
comparison.
Figure 2-8.
Resistance-Temperature Curve of a Thermistor
The thermistor has been used primarily for high-resolution measurements
over limited temperature ranges. Continuous improvements in thermistor
stability, accuracy, and availability of interchangeable thermistors have
prompted increased usage of thermistors in all types of industries.
10 M
1 M
100 k
10 k
1 k
100
10
–200 –150 –100
–50
0
50
100
150
200
250
300
350
400
Resistance (
Ω
)
Thermistor
(5,000
Ω
at 25
°
C)
RTD
(PT 100
Ω
at 0
°
C)
Temperature (
°
C)