Connecting The Rtd - National Instruments NI 435 Series User Manual

Chapter 2 Operating the NI 435x Device
Table 2-4. Callendar-Van Dusen Coefficients Corresponding to Common RTDs
Temperature
Coefficient α
Standard
IEC751
DIN 43760
American
ITS-90
* For temperatures below 0 °C only; C = 0.0 for temperatures above 0 °C.
Note Software packages such as VirtualBench-Logger, NI 435x instrument driver, MAX
Create New 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 the RTD

NI 435x User Manual
3.9083 × 10
0.00385055
3.9080 × 10
0.003850
3.9692 × 10
0.003911
3.9848 × 10
0.003925
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 the measurement system to the RTD adds errors to
the readings. For example, consider a 2-wire RTD element connected to the
NI 435x accessory that also supplies a constant current source I
the RTD. As shown in Figure 2-4, the voltage drop across the lead
resistance R , adds to the measured voltage.
L
For example, a lead resistance R
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.
A
–5.775 × 10
–3
–5.8019 × 10
–3
–5.8495 × 10
–3
–5.870 × 10
–3
R
L
RTD
R
L
Figure 2-4. 2-Wire RTD Measurement
of 0.3 Ω in each wire adds a 0.6 Ω error
L
2-16
B C
–4.183 × 10
–7
–4.2735 × 10
–7
–4.2325 × 10
–7
–4.0000 × 10
–7
EX
I , I , or I
EX+ EX0+ EX1+
CH+
CH–
I , I , or I
EX– EX0– EX1–
*
–1
–12
–12
–12
to excite
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