National Instruments NI 435 Series, User Manual, Page: 34 / 77

Chapter 2 Operating the NI 435x Device
NI 435x User Manual 2-16 ni.com
Note Software packages such as VirtualBench-Logger, NI 435 x 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
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 435
x accessory that also supplies a constant current source I
EX
to excite
the RTD. As shown in Figure 2-4, the voltage drop across the lead
resistance R
L
, adds to the measured voltage.
Figure 2-4. 2-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.
Table 2-4. Callendar-Van Dusen Coefficients Corresponding to Common RTDs
Standard
Temperature
Coefficient
α
A B C
*
IEC751 0.00385055 3.9083
×
10
–3
–5.775
×
10
–7
–4.183
×
10
–1
DIN 43760 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.
I
EX+
, I
EX0+
, or I
EX1+
RTD
R
L
R
L
CH+
CH–
I
EX–
, I
EX0–
, or I
EX1–