Connecting Your Rtd - National Instruments NI 4350 User Manual

Chapter 3 NI 4350 Operation
Table 3-4. Callendar-Van Dusen Coefficients Corresponding to Common RTDs
Temperature
Coefficient α
Standard
DIN 43 760 0.003850
American 0.003911
ITS-90 0.003925
* For temperatures below 0° C only; C = 0.0 for temperatures above 0° C.
Note:

Connecting Your RTD

NI 4350 User Manual
Therefore, for temperatures above 0° C, this equation reduces to a
quadratic:
T = -------------------------------------------------------------------
RTD
A +
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.
3.9080 • 10
3.9692 • 10
3.9848 • 10
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.
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
measured voltage.
R
 
RTD
2 ------------ - 1 –
 
R
0
R
 
2
• RTD
A + 4B ------------ - 1 –
 
R
0
A
–5.8019 • 10
–3
–5.8495 • 10
–3
–5.870 • 10
–3
3-16
B C*
–4.2735 • 10
–7
–4.2325 • 10
–7
–4.0000 • 10
–7
, adds to the
L
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