One disadvantage of the thermocouple relates to its complex measuring process. Rather than reading the temperature at the probe, one temperature is taken at the hot junction (the probe) and the other at the cold junction, located at the end of the circuit. Measurement errors are sometimes associated with the copper wires that comprise the cold junction, which can interfere with the temperature reading. This has been described by Veriteq as the Achilles heel of the thermocouple, since the measurement signal is produced by the difference in temperature between the hot and cold junctions and thus is not always perfectly accurate.
Thermocouples can be constructed from a variety of metals, some of which are more durable than others. Temperature fluctuations and environmental contamination may lead to corrosion, which is not always easily detected. As Capgo highlights, it is important to maintain the integrity of the wires, since it is the "temperature gradient" that runs along the wires which produces the signal. Compromised measurements are more probable in higher-temperature applications because they are more corrosive on metals. Capgo suggests using thicker metals and stronger insulation materials to preserve the integrity of the output.
According to The Engineering Toolbox, calibration (which refers to the correlation of readings between the two junctions) should be performed when the thermocouple is in use. It states that readings can be affected if the device is removed and placed in a calibration bath, for example. According to Capgo, the calibration bath has been replaced by the more useful system of cross-referencing the temperature at the junction with an alternative sensor, and then using software to correlate and interpret results. This method also corrects the nonlinear measurements produced by the thermocouple.