Chapter 2
Operating the NI 435x Device
©
National Instruments Corporation
2-15
Figure 2-3.
Resistance-Temperature Curve for a 100
Ω
Platinum RTD
Although the resistance-temperature curve is relatively linear, converting
measured resistance to temperature accurately requires curve fitting. The
Callendar-Van Dusen equation is commonly used to approximate the RTD
curve:
R
RTD
= R
0
[1 +
A
×
t +
B
×
t
2
+
C
×
(t – 100)
×
t
3
]
where R
RTD
is the resistance of the RTD at temperature T
RTD
; R
0
is the
resistance of the RTD in
Ω
at 0
°
C;
A
,
B
, and
C
are the
Callendar-Van Dusen coefficients shown in Table 2-4; and T
RTD
is the
temperature in
°
C. For temperatures above 0
°
C, coefficient
C
equals 0.
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),
which is used with wire-wound RTDs (
α
= 0.003925). Table 2-4 lists the
Callendar-Van Dusen coefficients for each of these three platinum RTD
curves.
1000
100
10
–200 –150 –100
–50
0
50
100
150
200
250
300
350
400
Resistance (
Ω
)
Temperature (
°
C)
RTD
(PT 100
Ω
)
T
RTD
2
R
RTD
R
0
-------------
1
–
A
A
2
4
B
R
RTD
R
0
-------------
1
–
+
+
--------------------------------------------------------------
=