Appendix B
Measuring Temperature with Resistive Transducers
SCXI-1581 User Manual
B-10
ni.com
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Note
You can use the LabVIEW VI
Convert Thermistor Reading
to convert thermistor
voltage measurements into temperature readings. This VI is located in the
Data
Acquisition»Signal Conditioning
function subpalette.
Thermistor Measurement Circuits
Because the thermistor is a resistive device, you must pass a current through
the thermistor to produce a voltage that the data acquisition system can
measure. The high resistance and high sensitivity of the thermistor simplify
the necessary measurement circuitry and signal conditioning. Special
3- or 4-wire connections are not necessary. The most common technique
is to use a constant-current source, and measure the voltage developed
across the thermistor. As shown in Figure B-4, the measured voltage
V
0
equals
R
T
×
I
EX
.
Figure B-4.
Thermistor Measurement with Constant Current Excitation,
I
EX
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
+
–
V
0
V
0
= I
EX
x R
T
R
T
Thermistor
I
EX