Rtd Measurement Errors; The Relationship Between Resistance And Temperature In Rtds; Figure 4-2. 2-Wire Rtd Measurement - National Instruments Module SCXI-1503 User Manual

Chapter 4 Theory of Operation
SCXI-1503 User Manual
changes in resistance, you must use special configurations that minimize
measured errors caused by lead-wire resistance.

RTD Measurement Errors

Because the RTD is a resistive device, you must pass a current through the
device and monitor the resulting voltage. However, any resistance in the
lead wires that connect the measurement system to the RTD adds error to
the readings. For example, consider a 2-wire RTD element connected to a
measurement system that also supplies a constant current, I
RTD. As shown in Figure 4-2, the voltage drop across the lead resistances
(labeled R ) adds an error voltage to the measured voltage.
L
V
0

Figure 4-2. 2-Wire RTD Measurement

For example, a lead resistance of 0.3
the resistance measurement. For a platinum RTD at 0 °C with = 0.00385,
the lead resistance equates to an error of approximately
Chapter 2, Connecting
resistive devices to the SCXI system.
The Relationship Between Resistance and
Temperature in RTDs
Compared to other temperature-measurement devices, the output of an
RTD is relatively linear with respect to temperature. The temperature
coefficient, called alpha ( ), differs between RTD curves. Although
various manufacturers specify alpha differently, alpha is most commonly
I
EX
R
L
+
–
R
L
in each wire adds a 0.6
0.6
1.6 C
---------------------------- - =
0.385 / C
Signals, describes different ways of connecting
4-6
, to excite the
EX
R
T
error to
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