Chapter 2
SC-2070 Board
© National Instruments Corporation
2-19
SC-207X Series User Manual
Table 2-3. Thermocouple Voltage Output Extremes (mV)
1
Thermocouple
Low
High
J
-8.096 at -210
°
C
2
42.922 at 760
°
C
2
K
-6.548 at -270
°
C
54.807 at 1370
°
C
E
-9.835 at -270
°
C
76.358 at 1000
°
C
T
-6.258 at -270
°
C
20.869 at 400
°
C
S
0.000 at 0
°
C
18.612 at 1760
°
C
R
0.000 at 0
°
C
21.006 at 1760
°
C
B
-0.003 at 20
°
C
13.814 at 1820
°
C
1
Source of information is Omega Temperature Measurement Handbook & Encyclopedia, Omega
Engineering (Stamford, CT).
2
All temperatures are the difference between the measuring end and the cold junction, or SC-2070
screw terminals in this case.
Linearizing the Data
Thermocouple output voltages are highly nonlinear. The Seebeck coefficient, or voltage change per
degree of temperature change, can vary by a factor of three or more over the operating temperature
range of some thermocouples. For this reason, the temperature from thermocouple voltages must
either be approximated by polynomials or matched against a look-up table. The polynomial approach
is easier to use, but it trades measurement time for memory usage. The polynomials are in the form:
T = a
0
+ a
1
x + a
2
x
2
+ ... + a
n
x
n
(Formula 2-4)
where x is the thermocouple voltage in volts, T is the temperature difference between the measuring
end and the SC-2070 screw terminals in degrees Celsius, and a
0
through a
n
are coefficients specific to
each thermocouple type. To speed computation time, a polynomial should be computed in nested
form. Consider the fourth order polynomial:
T = a
0
+ a
1
x + a
2
x
2
+ a
3
x
3
+ a
4
x
4
(Formula 2-5)
If this polynomial is evaluated as it is written, several unnecessary multiplications are performed to
raise x to the various powers. On the other hand, if the polynomial is written and evaluated as follows,
then no powers are computed, and execution proceeds much faster:
T = a
0
+ x(a
1
+
x(a
2
+ x(a
3
+ xa
4
)))
(Formula 2-6)
Table 2-4 lists the National Bureau of Standards (NBS) polynomial coefficients for several popular
thermocouples.
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