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SC-2070 Board
Chapter 2
SC-207X Series User Manual
2-16
© National Instruments Corporation
Working with Thermocouples
One of the most frequently used temperature transducers is the thermocouple. A thermocouple is
created when two dissimilar metals are brought in contact, and the contact produces a small voltage as
a function of temperature. In most measurement situations, a thermocouple is placed where the
temperature is to be measured; then two wires, consisting of the same type of metal as each side of the
thermocouple, are run back to a termination panel such as the SC-2070. Additional dissimilar
junctions are formed where these wires contact the screw terminals, and each of these junctions
generates its own temperature-dependent voltage. Other temperature-dependent voltages are generated
at ribbon cable junctions and solder joints. However, effects of these junctions are negligible and tend
to cancel in most cases. The voltage measured by the DAQ board is the sum of the voltages of all
dissimilar metal junctions in the circuit, not just the thermocouple junction used to measure
temperature.
You only need to know the temperature of the screw terminal junctions to compensate for all dissimilar
metal junctions in a system with constant temperature (Thermocouple Law of Intermediate Metals).
Therefore, if you know the temperature at the screw terminals, you can easily calculate the temperature
at the point of interest from the voltage measured. A National Semiconductor LM-35CZ is used on the
SC-2070 board to determine the temperature of the screw terminals. The dissimilar metal junctions at
the screw terminals are also referred to as the cold junction. The process of using the known
temperature at that point to find the temperature at the unknown point is called cold-junction
compensation (CJC).
Thermocouples are usually more desirable than direct-reading sensors for several reasons.
Thermocouples are more rugged than most other sensors and can tolerate several hundred degree
temperatures without degradation. Semiconductor sensors, the easiest to use, usually cannot be used
above 70
°
C. Thermocouples are inexpensive and physically small, so they can track temperature
changes rapidly.
Thermocouple output voltages are very low, on the order of 50
µ
V/
°
C. The temperature coefficient
changes with temperature, resulting in a nonlinear curve. The temperature can be determined from
tables available from thermocouple vendors or calculated from polynomial equations such as those
given in the next section.
Summary of Contents for SC-207 Series
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