©
National Instruments Corporation
19
NI WSN-3226 User Guide and Specifications
Different types of platinum RTDs have different TCRs. The NI WSN-3226 supports the following
TCRs: 3750, 3851, 3911, 3916, 3920, and 3928
μΩ
/
Ω
/°C. The most common TCR for RTDs is
3851
μΩ
/
Ω
/°C, as defined in IEC-751, DIN 43760, BS 1904, and ASTM E1137 international standards.
The TCR of 3928
μΩ
/
Ω
/°C is used in the reference function for platinum thermometers in the
International Temperature Scale of 1990 (ITS-90) for high-accuracy metrology applications.
Unfortunately, not all TCR values are as well defined by standards organizations, and the behavior of
RTDs with the same TCR value may vary from vendor to vendor. The variations are usually small. The
built-in linearization algorithms of the NI WSN-3226 are appropriate for nearly all applications.
The NI WSN-3226 uses a linearization curve known as the Callendar-Van Dusen equation to measure
the temperature of RTDs. The equation is as follows:
Temperatures below 0
°
C:
R
T
=
R
0
[1 + A
×
T
+ B
×
T
2
+ C
×
T
3
×
(
T
– 100 °C)]
Temperatures above 0
°
C:
R
T
=
R
0
[1 + A
×
T
+ B
×
T
2
]
where T
= temperature in
°
C
R
T
= RTD resistance at temperature
T
R
0
= RTD nominal resistance at 0
°
C
A, B, C are coefficients given in Table 3
Table 3 lists the coefficients used in this equation for the TCR values that the NI WSN-3226 supports.
If you have a nonstandard RTD that does not match one of these linearization curves, measure the RTD
in the resistance mode of NI WSN-3226, and convert the resistance to temperature in the manner
suggested by the RTD vendor.
Controlling Digital Inputs and Outputs
The following sections discuss how to configure the NI WSN-3226 device for different types of input
and output, and how to connect external devices to the NI WSN-3226 DIO terminals.
On the 18-position screw terminal connector, terminals 13–17 are the DIO section. The DIO section has
the following terminals:
•
DIO
x
—signal terminal
•
DGND—ground terminal
•
DIO PWR—DIO Power positive (required when one of the DIO channels is configured for
or
Drive High and Low (Sinking and Sourcing)
Table 3.
NI WSN-3226 Callendar-Van Dusen Coefficients
TCR
μΩ
/
Ω
/
°
C
A
(
°
C)
–1
B
(
°
C)
–2
C
(
°
C)
–4
3750
3.81
×
10
–3
–6.02
×
10
–7
–6.0
×
10
–12
3851
3.9083
×
10
–3
–5.775
×
10
–7
–4.183
×
10
–12
3911
3.9692
×
10
–3
–5.8495
×
10
–7
–4.233
×
10
–12
3916
3.9739
×
10
–3
–5.870
×
10
–7
–4.4
×
10
–12
3920
3.9787
×
10
–3
–5.8686
×
10
–7
–4.167
×
10
–12
3928
3.9888
×
10
–3
–5.915
×
10
–7
–3.85
×
10
–12
