Cold junction compensation
Thermocouple Cold Junctions
The terms hot junction and cold junction, as applied to thermocouple devices, are mostly historical. You don't need to have any junctions to get thermocouple effects. If you heat one end of a metal conductor and hold the other end at a constant reference temperature, two important things occur.
In practice, it is difficult to measure the Seebeck effect directly. When you attach measurement probes, there is a thermal difference across the probe leads, producing additional thermocouple effects that interfere with the measurements.
Classical thermocouple loop configuration
To make the thermal effects measurable, two different metal conductors are used. They must be chemically, electrically, and physically compatible. They produce different electric potentials when subjected to the same thermal gradient.
In the classical configuration, the dissimilar thermocouple wires are welded together at the measurement end (hot junction), and again at the reference end (cold junction), forming a loop. The hot junction assures that the potential at that point matches in the two metals. Immersing the reference-end junction in an ice-water slurry assures that the temperature gradients are the same across both materials. The ice-water slurry establishes a reference temperature at 0 degrees C.
Welding the thermocouple wires at the cold junction also equalizes the potentials there. To make the potential difference observable again, it is necessary to break the loop. Pick a location in one of the thermocouple wires where the temperature matches the temperature of the measurement leads. Break the loop there, and attach matching leads to the two sides of the gap to measure the potential.
Cold junction in practice
Maintaining an ice water slurry and actual cold junction is rarely feasible. Typically, the cold junction is omitted, and the potential is measured directly across the two terminal ends of the thermocouple wires at ambient temperature. For historical reasons, we speak of the terminal ends of the thermocouple wires as the cold junction, despite the fact that there is no longer an intentional junction. (For the same historical reasons, we refer to the measurement junction of the thermocouple as the hot junction even if it is used to measure below-zero temperatures.) The measured potential indicates the temperature difference between the hot junction point and the unknown cold junction terminals. To complete the temperature measurement, you must determine the terminal temperature in some manner.
Cold Junction Compensation
There are two commonly used approaches.
LT1025A solid state temperature measurement devices are
available on MSTB009 termination boards and optionally
available on MSXB037 analog expansion boards for measuring
the cold junction temperature when thermocouple wires
connect directly to the terminals on the boards. Any errors
in reading the cold junction temperature will appear
directly as errors in the final temperature measurement.
The readings are 10 millivolts per degree centigrade
absolute temperature. Install a header plug on your
board to activate temperature measurements that you can
route directly into the