National Instruments Module SCXI-1503 User Manual
Page 42
Chapter 4
Theory of Operation
4-12
ni.com
The maximum resistance of the thermistor is determined from the current
excitation value and the maximum voltage range of the input device. When
using the SCXI-1503, the maximum measurable resistance is 100 k
Ω.
The level of the voltage output signal depends directly on the thermistor
resistance and magnitude of the current excitation. Do not use a higher level
of current excitation in order to produce a higher level output signal
because the current causes the thermistor to heat internally, leading to
temperature-measurement errors. This phenomena is called self-heating.
When current passes through the thermistor, power dissipated by the
thermistor equaling (I
EX
2
R
T
), heats the thermistor.
Thermistors, with their small size and high resistance, are particularly
prone to these self-heating errors. Manufacturers typically specify this
self-heating as a dissipation constant, which is the power required to heat
the thermistor 1 °C from ambient temperature (mW/°C). The dissipation
constant depends heavily on how easily heat is transferred away from the
thermistor, so the dissipation constant can be specified for different
media—in still air, water, or oil bath. Typical dissipation constants range
anywhere from less than 0.5 mW/°C for still air to 10 mW/°C or higher for
a thermistor immersed in water. A 2,252
Ω thermistor powered by a
100
μA excitation current dissipates:
If this thermistor has a dissipation constant of 10 mW/°C, the thermistor
self-heats 0.00225 °C so the self-heating from the 100
μA source of the
SCXI-1503 is negligible for most applications. It is still important to
carefully read self-heating specifications of the thermistors.
Resistance/Temperature Characteristic of
Thermistors
The resistance-temperature behavior of thermistors is highly dependent
upon the manufacturing process. Therefore, thermistor curves are not
standardized 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.
You can, however, approximate the thermistor curve relatively accurately
with the Steinhart-Hart equation:
I
2
R
100
μA
2
2,252
Ω
×
0.0225 mW
=
=
T(
°K )
1
a b ln R
T
( )
[
] c ln R
T
( )
[
]
3
+
+
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=