Chapter 3
NI 4350 Operation
3-16
©
National Instruments Corporation
Therefore, for temperatures above 0
°
C, this equation reduces to a
quadratic:
Most platinum RTD curves follow one of three standardized curves:
the DIN 43760 standard (
α
= 0.00385), the U.S. Industrial or American
standard (
α
= 0.003911), or the International Temperature Scale
(ITS-90) that is used with wire-wound RTDs (
α
= 0.003925). Table 3-4
lists the Callendar-Van Dusen coefficients for each of these three
platinum RTD curves.
Note:
Software packages, such as VirtualBench, NI435X instrument driver, DAQ
Channel Wizard, LabVIEW, and LabWindows/CVI include routines that
perform these conversions for different types of RTDs based on the various
commonly used standards.
Connecting Your RTD
Because the RTD is a resistive device, you must pass current through
the device and measure the resulting voltage. However, any resistance
in the lead wires that connect your measurement system to the RTD will
add errors to your readings. For example, consider a two-wire RTD
element connected to the NI 4350 instrument accessory that also
supplies a constant current source IEX to excite the RTD. As shown in
Figure 3-4, the voltage drop across the lead resistance R
L
, adds to the
measured voltage.
Table 3-4.
Callendar-Van Dusen Coefficients Corresponding to Common RTDs
Standard
Temperature
Coefficient
α
A
B
C*
DIN 43 760
0.003850
3.9080
•
10
–3
–5.8019
•
10
–7
–4.2735
•
10
–12
American
0.003911
3.9692
•
10
–3
–5.8495
•
10
–7
–4.2325
•
10
–12
ITS-90
0.003925
3.9848
•
10
–3
–5.870
•
10
–7
–4.0000
•
10
–12
* For temperatures below 0
°
C only; C = 0.0 for temperatures above 0
°
C.
T
RTD
2
R
RTD
R
0
-------------
1
–
A
A
2
4B
R
RTD
R
0
-------------
1
–
•
+
+
-------------------------------------------------------------------
=
