11. METALLIC & NON METALLIC
THERMOWELL MATERIALS
Cermet
Cermet is a mixture of 77% chromium oxide and 23% aluminum oxide. Made by the Union Carbide Company
it is a dense, abrasion resistant material with a high thermal conductivity and a good resistance to wetting
by many liquid metals. (Wetting is the degree at which a liquid metal will ‘adhere’ to a protection tube).
It resists sulphur gases under oxidizing conditions up to at least 2,000 °F (1,094 °C). Also, non-ferrous
alloys such as copper, brass, zinc and lead do not wet cermet. It is not recommended for use in carburizing
or nitrogen atmospheres, since the chromium in the cermet will form carbides or nitrides.
Cermet is somewhat sensitive to thermal shock and has a maximum service temperature in oxidizing con-
ditions of 2,500 °F (1,370 °C).
Cermet fnds use in copper and brass melting pots, and in abrasive atmospheres of elevated temperature
where particles might damage a metal thermowell operating near its softening point.
Silicon carbide
Silicon carbide is another very inert material which resists attacks from many aggressive environments,
such as sulphur gases. Having a low co-effcient of thermal expansion, it has an excellent resistance to
thermal shock and a good thermal conductivity. The material is made by the Carborundum Company.
Two types of silicon carbide are available:
- carbofrax ‘A’: about 90% silicon carbide with the balance being mainly silica;
- ‘KT’ silicon carbide: about 96% silicon carbide.
Thermowells of carbofrax are considerably less expensive than ‘KT silicon carbide, but are not gas tight. Ho-
wever, they give excellent service at high temperatures up to 3,000 °F (1,649 °C). An inner alumina ‘slee-
ve’ is used to protect a platinum-rhodium thermocouple from contamination when it is the sensor of choice.
‘KT’ silicon carbide is used for special applications when high density gas tight thermowells are needed.
Silicon carbide is often used in the steel industry, due to its good thermal shock resistance and elevated
temperature capabilities. It is used as a protection tube, which is inserted into a ladle to read the melt
temperature.
Boron nitride
Boron nitride is a synthetic material made by the Carborundum Company which can be used in oxidizing
atmospheres up to about 2,000 °F (1,094 °C) or in reducing of inert atmospheres up to about 5,000 °F
(2,760 °C). It has a very low co-effcient of thermal expansion and hense excellent resistance to thermal
shock. It is not wetted by many liquid metals. A big advantage is that it is machinable with ordinary too-
ling and has lubricating characteristics somewhat similar to graphite. Recent applications where boron
nitride has been used include as an intermittent thermowell with a ‘B’ calibration thermocouple to measure
pouring temperatures of cupro nickels.
83