Chapter 3
Signal Connections
3-6
©
National Instruments Corporation
Caution
N
EVER
connect a signal to screw terminals CH0–CH63 that violates their
overvoltage protection limits. When the AMUX-64T is powered on, the screw
terminals CH0–CH63 overvoltage protection is ±35 V; when the AMUX-64T
is powered off, overvoltage protection is ±20 V.
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 often complex 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 following form:
where x is the thermocouple voltage in volts, T is the temperature difference
between the measuring end and the AMUX-64T screw terminals in degrees
Celsius, and a
0
through a
n
are coefficients that are specific to each
thermocouple type. To speed computation time, a polynomial should be
Table 3-2.
Thermocouple Voltage Output Extremes (mV)*
Thermocouple
Low
High
J
–8.095 at –210 °C**
69.553 at 1,200 °C**
K
–6.458 at –270 °C
54.886 at 1,372 °C
E
–9.835 at –270 °C
76.373 at 1,000 °C
T
–6.258 at –270 °C
20.872 at 400 °C
S
–0.236 at –50 °C
18.693 at 1,768 °C
R
–0.226 at –50 °C
21.101 at 1,768 °C
B
–0.000 at 0°C
13.820 at 1,820 °C
*
Source of information is NIST Monograph 175: Temperature-Electromotive Force
Reference Functions and Tables for the Letter-Designated Thermocouple Types Based on
the ITS-90, National Institute of Standards and Technology, 1993.
** All temperatures are the difference between the measuring end and the cold junction,
or AMUX-64T screw terminals in this case.
!
T
a
0
a
1
x
a
2
x
2
…
a
n
x
n
+
+
+
+
=