Three-wire compensation of lead resistance errors – National Instruments FP-RTD-122 User Manual
Page 8
FP-RTD-122 and cFP-RTD-122
8
ni.com
The [c]FP-RTD-122 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
]
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 2.
Table 2 lists the coefficients used in this equation for each of the
TCR values that the [c]FP-RTD-122 supports. If you have a
nonstandard RTD that does not match one of these linearization
curves, measure the resistance with the [c]FP-RTD-122 and
convert the resistance to temperature in the manner suggested by
the RTD vendor.
Three-Wire Compensation of Lead
Resistance Errors
The [c]FP-RTD-122 uses a three-wire compensation technique to
compensate for the lead resistances. The SENSE lead measures the
resistance of the return COM lead. If the EX+ lead has the same
resistance as the COM lead, the [c]FP-RTD-122 corrects for the
Table 2. Callendar-Van Dusen Coefficients Used by the [c]FP-RTD-122
TCR
m
Ω/Ω/°C
A
(
°C)
–1
B
(
°C)
–2
C
(
°C)
–4
3.750
a
3.81
Ч
10
–3
–6.02
Ч
10
–7
–6.0
Ч
10
–12
3.851
b
3.9083
Ч
10
–3
–5.775
Ч
10
–7
–4.183
Ч
10
–12
3.911
c
3.9692
Ч
10
–3
–5.8495
Ч
10
–7
–4.233
Ч
10
–12
3.916
d
3.9739
Ч
10
–3
–5.870
Ч
10
–7
–4.4
Ч
10
–12
3.920
e
3.9787
Ч
10
–3
–5.8686
Ч
10
–7
–4.167
Ч
10
–12
3.928
f
3.9888
Ч
10
–3
–5.915
Ч
10
–7
–3.85
Ч
10
–12