1. THeRmOCOUpleS THeORy
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1. Wire type construction
The most basic thermocouple construction is the ‘wire type’.
It consists of two dissimilar metals, homogeneously joined at
one end to form the measuring junction.
A common factor inherent in all wire type constructions is the
fact that they all have an exposed junction: although it offers
good response time, it is subject to environmental restrictions.
In most cases the advantages are: a good response time, rug-
gedness and high temperature use. The disadvantage is the
exposed junction: which means that it is susceptible to the envi-
ronment (oxidizing and reducing atmospheres) and therefore it
must be protected.
2. Mineral insulated construction
In order to overcome the disadvantages of the ‘wire type con-
struction’, manufacturers developed the mineral insulation ther-
mocouple. The ‘mineral insulated construction’ consists of two
thermocouple material wires embedded in a ceramic insulation
and protected by a metallic sheath.
The two primary components of this construction are:
1. The mineral insulation material
2. The metallic sheath
Sheath material characteristics
The table on the next page shows just some of the many dif-
ferent materials which can be used to protect the minerally
insulated thermocouple. The two most important parameters
in selecting the sheath material are: the operating temperature
and the atmospheric environment. The atmospheric environ-
mental parameters are oxidizing, reducing, neutral and vacuum.
For example, SS 304 can be used in each type of atmosphere
with a maximum operating temperature of 890 °C (1,650 °F).