Measuring Temperature With Rtds And Thermistors And Measuring Resistance - National Instruments NI 4350 User Manual

Chapter 3 NI 4350 Operation
Measuring Temperature with RTDs and Thermistors
and Measuring Resistance
Introduction to RTDs
NI 4350 User Manual
RTDs and thermistors are essentially resistors whose resistance varies
with temperature. Therefore, measurement techniques for RTDs,
thermistors, and resistors are quite similar. All techniques involve
exciting the resistor with a current or a voltage source and measuring
the resulting voltage or current, respectively, developed in the resistor.
With the NI 4350, you can excite your resistor with the built-in
precision current source and measure the resulting voltage. When using
LabVIEW, set the measurements mode to 4-wire ohms.When using the
NI435X instrument driver, set the measurement mode to Resistance.
These modes will return the measurements in units of resistance (ohms)
by dividing the measured voltage with the calibrated value of the
precision current source stored onboard. The following sections explain
the various measurement techniques in detail.
An RTD is a temperature-sensing device whose resistance increases
with temperature. An RTD consists of a wire coil or deposited film
of pure metal. RTDs can be made of different metals and can have
different resistances, but the most popular RTD is made of platinum
and has a nominal resistance of 100 Ω at 0° C.
RTDs are known for their excellent accuracy over a wide temperature
range. Some RTDs have accuracy as high as 0.01 Ω (0.026° C) at 0° C.
RTDs are also extremely stable devices. Common industrial RTDs drift
less than 0.1° C/year and some models are stable to within
0.0025° C/year.
RTDs can be difficult to measure because they have relatively low
resistance (100 Ω) that changes only slightly with temperature
(less than 0.4 Ω/° C). To accurately measure these small changes in
resistance, you may need to use special configurations that minimize
errors from lead wire resistance.
Relationship of Resistance and Temperature
in RTDs
Compared to other temperature devices, the output of an RTD
is relatively linear with respect to temperature. The temperature
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