11. METALLIC & NON METALLIC
THERMOWELL MATERIALS
82
PROTECTION TUBE MATERIALS
Introduction
There are many applications across the industry where the temperature to be measured, is too high for the
standard stainless steel and nickel-based alloy thermowell materials. All of the more common stainless
steels and nickel-based alloys melt at/or before 2,550 °F/1,400 °C and become weak or soft at/or before
approx. 2,200 °F/1,200 °C. In these applications a different material must be utilized.
There are two metals available which have a much higher melting point than the stainless steels or nickel-
based alloys: tantalum 5,425 °F/2,996 °C and molybdenum 4,730 °F/2,610 °C. However, these metals
have inherent problems that limit their use in high temperature service:
- they oxidize rapidly (tantalum above 530 °F/276 °C and molybdenum above 930 °F/499 °C), there fore
they can’t be used for thermowell materials except in strictly non-oxidizing atmospheres;
- they are very expensive to be used as a thermowell or protection tube material.
The solution is to use a non-metallic or ceramic type of protection tube material. There are a number of
these type materials available for high temperature service, each with its own unique capabilities: fused
quartz, cermet, silicon carbide, boron nitride, mullite and alumina.
While these materials exhibit varying degrees of high temperature capabilities there are disavantages to
their use. Being almost completely made of ceramic, they are extremely brittle and can be broken quite
easily by a mechanical shock. Also, most of these materials have a very poor resistance to ‘thermal shock’.
If a fame is applied suddenly to one side, it expands. Since the other side is cooler, it doesn’t expand at
the same rate. This leads to stresses which, if severe enough, will crack the protection tube. The lower
co-effcient of thermal expansion’ these materials have, the more resistance they exhibit to this thermal
shock cracking.
The following is a discussion of each of the above referenced materials with some examples of their typical
uses in industry.
Fused quartz
Pure silica, fused quartz, has a very low co-effcient of thermal expansion, giving it excellent resistance to ther-
mal shock cracking. It is also a vey chemically inert material and resists attacks by many corrosive chemicals
and liquid metals. An unfortunate limitation of fused quartz is, that it is a super cooled ‘glass’. At about
2,000 °F (1,094 °C) it will devitrify so that it can’t be used for service above this temperature. Also, any
surface contamination will accelerate devitrifcation at high temperatures. (Devitrifcation refers to the fact
that fused quartz will re-crystallize and can’t be used above 2,000°F).
Because of fused quartz’ excellent thermal shock resistance, it is often used in the metal casting industry
as a ‘disposable’ thermocouple protection tube. The fused quartz tube is inserted into the melt and the
temperature (used for control of the pouring temperature) is read. Due to its excellent thermal shock resis-
tance, fuse quartz is able to withstand the sudden change from ambient to melt temperatures.