11. METALLIC & NON METALLIC
THERMOWELL MATERIALS
Alumina and Mullite
Alumina (aluminum oxide) and mullite (a compound of alumina and silica) have been used for many years
as thermowells for chromel-alumel and platinum-rhodium thermocouples. They can be used for high tempe-
ratures: 3,450 °F (1,900 °C) for high purity alumina and 3,100 °F (1,700 °C) for mullite. One problem with
these materials: they are subject to thermal shock. They can crack if subjected to sudden or non-uniform,
localized heating or cooling.
Mullite has a co-effcient of thermal expansion of about 2/3 of alumina and a consequently somewhat bet-
ter resistance to thermal shock. Both materials are gas tight. Alumina, other than mullite, should be used
with platinum-rhodium thermocouples for any conditions rather than oxidizing. The reason: the silicon can
be reduced from the mullite and will contaminate platinum-rhodium thermocouples, thereby throwing them
out of calibration.
Typical applications for alumina and mullite protection tubes include heat treatment furnaces operating
at high temperatures, where little danger from thermal shock or from mechanical damage is involved.
This type of protection tube is also widely used in the glass industry.
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