Chapter 3
NI 4350 Operation
©
National Instruments Corporation
3-15
coefficient, called alpha (
α
) differs between RTD curves. Although
various manufacturers may specify
α
differently,
α
is most commonly
defined as the change in RTD resistance from 0
°
to 100
°
C, divided by
the resistance at 0
°
C, divided by 100
°
C:
α
(
Ω
/
Ω
/
°
C) = [(R
100
- R
0
)/R
0
]/100
°
C
where R
100
is the resistance of the RTD at 100
°
C, and R
0
is the
resistance of the RTD at 0
°
C.
For example, a 100
Ω
platinum RTD with
α
= 0.00385 will measure
138.5
Ω
at 100
°
C. Figure 3-3 shows a typical resistance-temperature
curve for a 100
Ω
platinum RTD.
Figure 3-3.
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 3-4, and T
RTD
is the
temperature in
°
C. For temperatures above 0
°
C, coefficient C equals 0.
RTD
(PT 100
Ω
)
Resistance (
Ω
)
Temperature (˚C)
1 k
100
10
–200
–150
–100
–50
0
50
100
150
200
250
300
350
400
