A thermocouple is a temperature sensor. In its most common form it consists of two wires of different composition. The two wires are joined together at two points which have different temperatures.
Measuring Junction Thermocouple Connection Head
One of the points is at a known temperature. This point is the reference junction. The reference junction is also often, but less preferably, called the “cold” junction. The tem- perature of the reference junction is held constant, or its variation is electrically compensated for in the associated measuring instrumentation.
The second junction is the measuring junction. The measuring junction is also often, but less preferably, called the “hot” junction. The measuring junction is often at an unknown temperature requiring measurement, or at a temperature at which control is required.
A thermocouple is useful for temperature sensing because a measurable electrical signal is produced. The signal is a function of the difference in temperature between the measuring and reference junctions. Numerous combinations of dissimilar metals are used as thermo- couples. Some of these combinations have become relatively standard and widely accepted for a large segment of industrial temperature measurements. A specific combination is generally referred to as a type, or calibration. Most of the common calibrations have American National Standards Institute (ANSI) letter codes. These letter codes were originally established by the Instrument Society of America.
The recommended temperature range for each type is that for which limits of error are established. No guarantee is made, or implied, regarding the successful use of any of the
above calibrations in their recommended temperature range.
range may adversely affect its
of a thermocouple outside its reliability over its recommended
Numerous factors combine to determine the successful application of a particular thermo- couple. Some of these factors are temperature, cycling, chemical exposure, degree of protection provided, and mechanical abuse given to the thermocouple.
Thermocouple calibrations are maintained by proper manufacturing control of each of the thermoelements. Elemental constituents are controlled to a high degree. Homogeneity must be maintained, and all wire must be properly annealed.
To reduce costs when long thermocouple lengths are required, especially with the noble metal calibrations, extension leadwire extends the reference junction of the thermocouple to the instrument. For the base metal calibrations the extension wire is nominally of the same composition as the thermocouple grade material. Control in manufacturing is not to the same degree as thermocouple grade wire. With lessening rigidity of manufacturing control considerable expense can be saved. There is a limitation on the maximum tempera- ture to which the junction of extension wire and thermocouple wire should be exposed. For the base metal calibration except Type T the maximum temperature is 400F (204C). For type T it is 200F (93C).
Barber-Colman Company, Loves Park, IL.