National Instruments NI 435 Series, User Manual, Page: 35 / 77

Chapter 2 Operating the NI 435x Device
© National Instruments Corporation 2-17 NI 435x User Manual
If you are using lead lengths greater than 10 ft, you may need to
compensate for this lead resistance in order to increase accuracy. The
preferred RTD measurement method is to use a 4-wire RTD. One pair of
wires carries the current through the RTD; the other pair senses the voltage
across the RTD. Because only negligible current flows through the sensing
wires, the lead resistance error of R
L2
and R
L3
is negligible. Figure 2-5
illustrates this configuration.
Figure 2-5. 4-Wire RTD Measurement
Alternatively, you can use a 3-wire RTD. Figure 2-6 shows a 3-wire RTD
configuration with a current source. In this configuration, the resistance R
L1
of only one lead adds error to the measurement.
Figure 2-6. 3-Wire RTD Measurement
Another variation of the 3-wire RTD configuration is shown in Figure 2-7.
In this configuration, the effects of the lead wire resistance cancel out as
long as all three wires have the same lead resistance.
I
EX+
, I
EX0
+, or I
EX1+
RTD
R
L2
R
L1
R
L3
R
L4
CH+
CH–
I
EX–
, I
EX0–
, or I
EX1–
I
EX+
, I
EX0+
, or I
EX1+
RTD
R
L1
R
L2
R
L3
CH+
CH–
I
EX–
, I
EX0–
, or I
EX1–