Chapter 3
NI 4350 Operation
©
National Instruments Corporation
3-17
Figure 3-4.
Two-Wire RTD Measurement
For example, a lead resistance R
L
of 0.3
Ω
in each wire adds a 0.6
Ω
error to the resistance measurement. For a platinum RTD with
α
= 0.00385, the resistance equals a 0.6
Ω
/(0.385
Ω
/
°
C) = 1.6
°
C error.
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 four-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 3-5 illustrates this configuration.
Figure 3-5.
Four-Wire RTD Measurement
Alternatively, you can use a three-wire RTD instead. Figure 3-6 shows
a the three-wire RTD in a Wheatstone configuration with a current
source. Another variation of the three-wire RTD configuration is shown
in Figure 3-7. In this configuration, the resistance R
L1
of only one lead
adds error to the measurement.
IEX+
RTD
R
L
R
L
CH+
CH–
IEX–
IEX+
RTD
R
L2
R
L1
R
L3
R
L4
CH+
CH–
IEX–
