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Chapter 2
Operating the NI 435x Device
Connecting the Thermistor
NI 435x User Manual
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 are 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
Where T(K) is the temperature in kelvin, equal to T(°C) + 273.15, and R
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 also can modify these conversion routines for the 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 435x. The high resistance and high sensitivity of the thermistor simplify
the necessary measurement circuitry and signal conditioning. Special
3-wire or 4-wire connections are not necessary. As shown in Figure 2-9,
the measured voltage V
1
=
------------------------------------------------------------------- -
×
(
)
a
b
ln
R t
+
+
×
R t
I
is equal to (
EX
t
2-20
3
×
(
)
c
ln
R t
).
is
t
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